5
Mastering the Mysteries of Autofocus
One of the most useful and powerful features of modern digital SLR cameras is their ability to lock in sharp focus faster than the blink of an eye. Sometimes. Although autofocus has been with us for more than 20 years, it continues to be problematic. While vendors like Nikon are giving us faster and more precise autofocus systems, with many more options, it’s common for the sheer number of options to confuse even the most advanced photographers.
One key problem is that the camera doesn’t really know, for certain, what subject you want to be in sharp focus. It may select an object and lock in focus with lightning speed—even though the subject is not the one that’s the center of interest of your photograph. Or, the camera may lock focus too soon, or too late. This chapter will help you choose the options available with your Nikon D800 that will help the camera understand what you want to focus, when, and maybe even why.
How Focus Works
Although Nikon added autofocus capabilities in the 1980s, back in the day of film cameras, prior to that focusing was always done manually. Honest. Even though viewfinders were bigger and brighter than they are today, special focusing screens, magnifiers, and other gadgets were often used to help the photographer achieve correct focus. Imagine what it must have been like to focus manually under demanding, fast-moving conditions such as sports photography.
Focusing was problematic because our eyes and brains have poor memory for correct focus, which is why your eye doctor must shift back and forth between sets of lenses and ask “Does that look sharper—or was it sharper before?” in determining your correct prescription. Similarly, manual focusing involves jogging the focus ring back and forth as you go from almost in focus, to sharp focus, to almost focused again. The little clockwise and counterclockwise arcs decrease in size until you’ve zeroed in on the point of correct focus. What you’re looking for is the image with the most contrast between the edges of elements in the image.
The camera also looks for these contrast differences among pixels to determine relative sharpness. There are two ways that sharp focus is determined: phase detection (used when framing your image through the optical viewfinder) and contrast detection (used when shooting stills and movies with live view).
Phase Detection
The 51 autofocus sensors of Nikon’s Multi-CAM 3500FX autofocus module are located in the “floor” of the mirror box, just under the flip-up mirror, which is partially silvered so that most of the light reaching it from the lens is bounced upward to the viewfinder, while some light is directed downward toward the focus sensors. If you lock up the mirror of your camera (using the Lock Mirror Up for Cleaning option in the Setup menu), you can see where these sensors are located. The focus zones cover an area on the center of the viewing frame in FX mode, as shown in Figure 5.1.
Figure 5.1 The D800’s focus sensors cover an area in the center of the frame.
The outer rectangle in Figure 5.1 shows the actual frame area when you switch the D800 from FX to DX mode (as described in Chapter 8). One thing that jumps out at you is that, while in FX mode the focus points are concentrated only in the center of the frame, but when cropping the image using DX mode, the focus zones now virtually fill the frame, providing more AF area coverage.
This phenomenon is one more reason why you might want to use DX mode for some types of shots where the “expanded” AF area coverage is useful—such as sports. The D800 can more easily locate the important subject matter to zero in on, and you still have a 16MP image to work with even in cropped mode. As a bonus, you pick up a modest frame rate increase in either continuous shooting mode—from 4 fps boosted to 5 or 6 fps (with the MB-D12 pack). Note that you reap these gains only if you’re shooting in a crop mode—not if you shoot in FX mode and then crop down to DX size in your image editor. Score one for DX mode vs. post processing crops.
In Phase Detection mode, the autofocus sampling area for each autofocus sensor is divided into two halves by a prism-like optical component in front of the focus sensor, similar to the simplified diagram shown in Figure 5.2. Inside your D800, there are actually two sets of reflective surfaces tucked behind the main mirror. One directs the light that passes through the main mirror downward to the bottom of the mirror box, while the second bounces the light back toward the front of the camera, where the actual focus sensor resides.
Figure 5.2 The image is split in half before it reaches the AF sensor.
The two halves of the image areas are compared, much like (actually, exactly like) a two-window rangefinder used in surveying, weaponry—and non-SLR cameras like the venerable Leica M film models. The relative positions between the two images change as focus is moved in or out, until sharp focus is achieved when the images are “in phase,” or lined up. The process can be visualized roughly using Figure 5.3, which shows two image areas being “merged” as the D800 adjusts focus.
Figure 5.3 The AF sensor tries to match up the two halves of the image.
You can visualize how Phase Detection autofocus works if you look at Figures 5.4 and 5.5. (However, the action of your camera’s actual autofocus sensors don’t look anything like this; I’m providing a greatly simplified view just for illustration.) In Figure 5.4, a typical horizontally oriented focus sensor is looking at a series of parallel vertical lines in a weathered piece of wood. The lines are broken into two halves by the sensor’s rangefinder prism, and you can see that they don’t line up exactly; the image is slightly out of focus.
Fortunately, the rangefinder approach of phase detection tells the D800 exactly how out of focus the image is, and in which direction (focus is too near, or too far) thanks to the amount and direction of the displacement of the split image. The camera can quickly and precisely snap the image into sharp focus and line up the vertical lines, as shown in Figure 5.5. Of course, this scenario—vertical lines being interpreted by a horizontally oriented sensor—is ideal. When the same sensor is asked to measure focus for, say, horizontal lines that don’t split up quite so conveniently, or, in the worst case, subjects such as the sky (which may have neither vertical nor horizontal lines), focus can slow down drastically, or even become impossible.
Figure 5.4 When an image is out of focus, the split lines don’t align precisely.
Figure 5.5 Using Phase Detection mode, the D800 is able to align the features of the image and achieve sharp focus quickly.
Phase Detection is the normal mode used by the D800. As with any rangefinder-like function, accuracy is better when the “base length” between the two images is larger. (Think back to your high school trigonometry; you could calculate a distance more accurately when the separation between the two points where the angles were measured was greater.) For that reason, Phase Detection autofocus is more accurate with larger (wider) lens openings—especially those with maximum f/stops of f/2.8 or better—than with smaller lens openings, and may not work at all when the f/stop is smaller than f/8. As I noted, the D800 is able to perform these comparisons very quickly.
Improved Cross-Type Focus Point
One improvement that Nikon D800 owners sometimes overlook is the upgrade to a cross-type focus point at 15 of the 51 center positions, functional with lenses with maximum apertures of f/5.6 or larger; with lenses having an effective maximum aperture of f/8, 11 central sensors are still able to function in cross-type mode. Why is this important? It helps to take a closer look at the Phase Detection system when presented with a non-ideal subject.
Figure 5.6 shows the same weathered wood pictured earlier, except in this case we’ve chosen to rotate the camera 90 degrees (say, because we want a vertically oriented composition). In the illustration, the image within the focus sensor’s area is split in two and displaced slightly side-to-side, but the amount and direction of the misalignment is far from obvious. A horizontally oriented focus sensor will be forced to look for less obvious vertical lines to match up. Our best-case subject has been transformed into a worst-case subject for a horizontal focus sensor.
The value of the cross-type focus sensor, which can interpret contrast in both horizontal and vertical directions, can be seen in Figure 5.7. The horizontal lines are still giving the horizontal portion of the cross sensor fits, but the vertical bar can easily split and align the subject to achieve optimum focus. Cross-type sensors can handle horizontal and vertical lines with equal aplomb and, if you think about it, lines at any diagonal angle as well. In lower light levels, with subjects that were moving, or with subjects that have no pattern and less contrast to begin with, the cross-type sensor not only works faster but can focus subjects that a horizontal- or vertical-only sensor can’t handle at all.
So, you can see that having a center cross-type focus sensor that is extra sensitive with faster lenses is a definite advantage. The location of the D800’s cross-type focus sensors is shown in Figures 5.8 and 5.9. With lenses having an effective maximum aperture (that is, taking into account any light loss from teleconverters, automatic extension tubes, etc.) of f/5.6 or faster, 15 central focus points function as cross-type sensors; the rest act as line sensors. If you’re using a lens with a maximum aperture a tad smaller than f/5.6 and not quite as small as f/8 (in other words, f/5.7 to f/7.9), nine central AF points function as cross-type sensors, while the six sensors flanking them still act as line sensors. With lenses having an effective maximum aperture of f/8, only the central focus point acts as a cross-type sensor, but the remaining focus points function as line sensors.
Figure 5.6 A horizontal focus sensor doesn’t handle horizontal lines very well.
Figure 5.7 Cross-type sensors can evaluate contrast in both horizontal and vertical directions, as well as diagonally.
Figure 5.8 With lenses having a maximum aperture of f/5.6 or larger, the 15 central AF sensors, shown here in green, function as cross-type sensors. The remaining 36 sensors (shown in blue) function as line sensors.
Figure 5.9 With lenses having maximum apertures smaller than f/5.6, but no smaller than f/8, the nine cross-type sensors are located in the middle of the frame (shown in green). The remaining six sensors (shown in blue) function as line sensors.
Figure 5.10 If you mount a lens having a maximum aperture of f/8, only the central focus point functions as a cross sensor; the six remaining sensors act as line sensors. If your lens has a maximum aperture that’s smaller than f/8, autofocus is not possible.
Contrast Detection
This is a slower mode, suitable for static subjects, and used by the D800 in Live View and Movie modes. It’s a bit easier to understand, and is illustrated by Figure 5.11. At top in the figure, the transitions between the edges found in the image are soft and blurred because of the low contrast between them. Although the illustration uses the same vertical lines used with the phase detection example, the orientation of the features doesn’t matter. The focus system looks only for contrast between edges, and those edges can run in any direction.
At the bottom of Figure 5.11, the image has been brought into sharp focus, and the edges have much more contrast; the transitions are sharp and clear. Although this example is a bit exaggerated so you can see the results on the printed page, it’s easy to understand that when maximum contrast in a subject is achieved, it can be deemed to be in sharp focus. Although achieving focus with contrast detection is generally quite a bit slower, there are several advantages to this method:
Works with more image types. Contrast detection doesn’t require subject matter rotated 90 degrees from the sensor’s orientation to operate optimally. Any subject that has edges will work.
Focus on any point. While phase detection focus can be achieved only at the points that fall under one of the nine autofocus sensors, with contrast detection, any portion of the image can be used. Focus is achieved with the actual sensor image, so focus point selection is simply a matter of choosing which part of the sensor image to use. As you’ll learn in Chapter 14, you can move the focus frame around on the screen when working with live view.
Potentially more accurate. Phase detection can fall prey to the vagaries of unco-operative subject matter: if suitable lines aren’t available, the system may have to hunt for focus or achieve less than optimal focus. Contrast detection is more clear-cut. The camera can clearly see when the highest contrast has been achieved, as long as there is sufficient light to allow the camera to examine the image produced by the sensor. (The focus assist lamp can help when shooting subjects close enough to the camera for the focus assist illumination to provide extra contrast.)
The D800’s autofocus mechanism, like all such systems found in SLR cameras, evaluates the degree of focus, but, unlike the human eye, it is able to remember the progression perfectly, so that autofocus can lock in much more quickly and, with an image that has sufficient contrast, more precisely. Unfortunately, while the D800’s focus system finds it easy to measure degrees of apparent focus at each of the focus points in the viewfinder, it doesn’t really know with any certainty which object should be in sharpest focus. Is it the closest object? The subject in the center? Something lurking behind the closest subject? A person standing over at the side of the picture? Many of the techniques for using autofocus effectively involve telling the Nikon D800 exactly what it should be focusing on, by choosing a focus zone or by allowing the camera to choose a focus zone for you. I’ll address that topic shortly.
Figure 5.11 Focus in Contrast Detection mode evaluates the increase in contrast in the edges of subjects, starting with a blurry image (top) and producing a sharp, contrasty image (bottom).
Adding Circles of Confusion
But there are other factors in play, as well. You know that increased depth-of-field brings more of your subject into focus. But more depth-of-field also makes autofocusing (or manual focusing) more difficult because the contrast is lower between objects at different distances. So, autofocus with a 200mm lens (or zoom setting) may be easier than at a 28mm focal length (or zoom setting) because the longer lens has less apparent depth-of-field. By the same token, a lens with a maximum aperture of f/1.8 will be easier to autofocus (or manually focus) than one of the same focal length with an f/4 maximum aperture, because the f/4 lens has more depth-of-field and a dimmer view. That’s why lenses with a maximum aperture smaller than f/8 can give your D800’s autofocus system fits.
To make things even more complicated, many subjects aren’t polite enough to remain still. They move around in the frame, so that even if the D800 is sharply focused on your main subject, it may change position and require refocusing. An intervening subject may pop into the frame and pass between you and the subject you meant to photograph. You (or the D800) have to decide whether to lock focus on this new subject, or remain focused on the original subject. Finally, there are some kinds of subjects that are difficult to bring into sharp focus because they lack enough contrast to allow the D800’s AF system (or our eyes) to lock in. Blank walls, a clear blue sky, birds-in-flight, or other subject matter may make focusing difficult.
If you find all these focus factors confusing, you’re on the right track. Focus is, in fact, measured using something called a circle of confusion. An ideal image consists of zillions of tiny little points, which, like all points, theoretically have no height or width. There is perfect contrast between the point and its surroundings. You can think of each point as a pinpoint of light in a darkened room. When a given point is out of focus, its edges decrease in contrast and it changes from a perfect point to a tiny disc with blurry edges (remember, blur is the lack of contrast between boundaries in an image). (See Figure 5.12.)
Figure 5.12 When a pinpoint of light (left) goes out of focus, its blurry edges form a circle of confusion (center and right).
If this blurry disc—the circle of confusion—is small enough, our eye still perceives it as a point. It’s only when the disc grows large enough that we can see it as a blur rather than a sharp point that a given point is viewed as out of focus. You can see, then, that enlarging an image, either by displaying it larger on your computer monitor or by making a large print, also enlarges the size of each circle of confusion. Moving closer to the image does the same thing. So, parts of an image that may look perfectly sharp in a 5 × 7-inch print viewed at arm’s length, might appear blurry when blown up to 11 × 14 and examined at the same distance. Take a few steps back, however, and it may look sharp again.
To a lesser extent, the viewer also affects the apparent size of these circles of confusion. Some people see details better at a given distance and may perceive smaller circles of confusion than someone standing next to them. For the most part, however, such differences are small. Truly blurry images will look blurry to just about everyone under the same conditions.
Technically, there is just one plane within your picture area, parallel to the back of the camera (or sensor, in the case of a digital camera), that is in sharp focus. That’s the plane in which the points of the image are rendered as precise points. At every other plane in front of or behind the focus plane, the points show up as discs that range from slightly blurry to extremely blurry (see Figure 5.13). In practice, the discs in many of these planes will still be so small that we see them as points, and that’s where we get depth-of-field. Depth-of-field is just the range of planes that include discs that we perceive as points rather than blurred splotches. The size of this range increases as the aperture is reduced in size and is allocated roughly one-third in front of the plane of sharpest focus, and two-thirds behind it. The range of sharp focus is always greater behind your subject than in front of it.
Figure 5.13 Only the blossoms in the foreground are in focus—the area behind them appears blurry because the depth-of-field is limited.
Using Autofocus with the Nikon D800
Autofocus can sometimes be frustrating for the new digital SLR photographer, especially those coming from the point-and-shoot world. That’s because correct focus plays a greater role among your creative options with a dSLR, even when photographing the same subjects. Most non-dSLR digital cameras have sensors that are much tinier than the sensor in the D800. Those smaller sensors require shorter focal lengths, which have, effectively, more depth-of-field.
The bottom line is that with the average point-and-shoot camera, everything is in focus from about one foot to infinity and at virtually every f/stop. Unless you’re shooting close-up photos a few inches from the camera, the depth-of-field is prodigious, and autofocus is almost a non-factor. The D800, on the other hand, is a full-frame camera that uses longer focal length lenses to achieve the same field of view with its larger sensor, so there is less depth-of-field. That’s a good thing, creatively, because you have the choice to use selective focus to isolate subjects. But it does make the correct use of autofocus more critical. To maintain the most creative control, you have to choose three attributes:
How much is in focus. Generally, by choosing the f/stop used, you’ll determine the range of sharpness/amount of depth-of-field. The larger the DOF, the “easier” it is for the autofocus system’s locked-in focus point to be appropriate (even though, strictly speaking, there is only one actual plane of sharp focus). With less depth-of-field, the accuracy of the focus point becomes more critical, because even a small error will result in an out-of-focus shot.
What subject is in focus. The portion of your subject that is zeroed in for autofocus is determined by the autofocus zone that is active, and which is chosen either by you or by the Nikon D800 (as described next). For example, when shooting portraits, it’s actually okay for part of the subject—or even part of the subject’s face—to be slightly out of focus as long as the eyes (or even just the nearest eye) appear sharp.
When focus is applied. For static shots of objects that aren’t moving, when focus is applied doesn’t matter much. But when you’re shooting sports, or birds in flight, or children, the subject may move within the viewfinder as you’re framing the image. Whether that movement is across the frame or headed right toward you, timing the instant when autofocus is applied can be important.
Autofocus Simplifies Our Lives… Doesn’t It?
Manual focus is tricky, requires judgment, and fast reflexes. So, we’re all better off now that autofocus has become almost universal, right? On the one hand, AF does save time and allows us to capture subjects (particularly fast-moving sports) that are difficult to image sharply using manual focusing (unless you have training and know certain techniques). On the other hand, learning to apply the Nikon D800’s autofocus system most effectively requires a bit of study and some practice. Then, once you’re comfortable with autofocus, you’ll know when it’s appropriate to use the manual focus option, too.
The important thing to remember is that focus isn’t absolute. For example, some things that look in sharp focus at a given viewing size and distance might not be in focus at a larger size and/or closer distance. In addition, the goal of optimum focus isn’t always to make things look sharp. Not all of an image will be or should be sharp. Controlling exactly what is sharp and what is not is part of your creative palette. Use of depth-of-field characteristics to throw part of an image out of focus while other parts are sharply focused is one of the most valuable tools available to a photographer. But selective focus works only when the desired areas of an image are in focus properly. For the digital SLR photographer, correct focus can be one of the trickiest parts of the technical and creative process.
The D800 now uses Nikon’s 51-zone Multi-CAM 3500FX autofocus system. As mentioned previously, the D800 deploys an intelligent array of cross-type sensors and horizontal sensors in the viewing system. All 51 autofocus sensors can be used individually or in groups of 9, 21, or all 51 focus zones. The AF system uses the color and light values, as measured by the 91,000 pixel RGB exposure sensor to accurately track moving objects, and to classify subjects.
Like all camera autofocus sensors, those in your D800 require a minimum amount of light as well as a minimum aperture size to operate, which is why autofocus capabilities are possible only with lenses having an f/8 or larger maximum aperture. While there’s not a lot you can do to “fix” a lens that has a maximum aperture that’s too small, if your subject’s focus is difficult to evaluate because of waning light levels, the AF assist beam built into the D800 (usually of minimal aid because it is relatively weak) and the assist beams of Nikon’s dedicated flash units provide additional light that helps assure enough illumination for autofocus under some circumstances.
SUPERIOR FACE TRACKING
If you’re using Auto-area AF (discussed later in this chapter), the D800 has enough resolution in its exposure system to identify faces, and use that information to track your human subjects. That’s a powerful capability for anyone shooting sports or other fast-moving subjects.
Bringing the Multi-CAM 3500FX AF System into Focus
I’ve explained individual bits and pieces of the Nikon D800’s autofocus system earlier in this book, particularly in the “roadmap” sections that showed you where all the controls were located, and the “setup” chapters that explained the key autofocus options. Now it’s time to round out the coverage as we tie everything together. There are three aspects of autofocus that you need to understand to use this essential feature productively. They apply—in slightly different ways—to both autofocus when using the optical viewfinder, and in live View/Movie modes Live View. For now, we’re going to concentrate on the optical viewfinder/Phase Detection system’s most important features:
Autofocus point selection. This aspect controls how the D800 selects which areas of the frame are used to evaluate focus. Point selection allows the camera (or you) to specify a subject and lock focus in on that subject.
Autofocus mode and priority. This governs when during the framing and shooting process autofocus is achieved. Should the camera focus once when activated, or continue to monitor your subject and refocus should the subject move? Is it okay to take a picture even if sharp focus isn’t yet achieved, or should the camera lock out the shutter release until the image is sharp?
Autofocus activation. When should the autofocus process begin, and when should it be locked? This aspect is related to the autofocus mode, but uses controls that you can specify to activate and/or lock the autofocus process.
Autofocus Point Selection Overview
I’m discussing this aspect of autofocus first, because, in many ways, it is the most important. If your D800 isn’t focusing on the correct subject, autofocus speed and activation are pretty much wasted effort. As you’ve learned, the D800 has up to 51 different points on the screen that can be individually selected by you or the camera as the active focus zone.
The number and type of autofocus sensors in use can affect how well the system operates. The focus sensors can consist of lines of pixels, cross-shapes, and/or a mixture of these types within a single camera, as with the D800. The more AF points available, the more easily the camera can differentiate among areas of the frame, and the more precisely you can specify the area you want to be in focus if you’re manually choosing a focus spot.
But, there’s another side of the coin. There is such a thing as too many focus zones for some types of subjects. For example, when using 51 focus points to select a zone for large, evenly illuminated subjects, you can waste a lot of time thumbing the multi selector among the four-dozen (plus) focus points. That’s why CSM #a7 lets you switch the D800 to a more widely spaced set of 11 focus zones that you can select quickly. Figure 5.14 shows the distribution of the active focus points when in 51-point mode (top) or 11-point mode (bottom).
As the camera collects contrast information from the sensors, it then evaluates the data to determine whether the desired sharp focus has been achieved. The calculations may include whether the subject is moving, and whether the camera needs to “predict” where the subject will be when the shutter release button is fully depressed and the picture is taken. (Predictive focus tracking kicks in when the camera is set to AF-C continuous autofocus. I’ll explain these modes in more detail in the next section.)
Figure 5.14 The D800 can use a dense set of 51 focus points (top), or a wider array of 11 points (bottom).
The speed with which the camera is able to evaluate focus and then move the lens elements into the proper position to achieve the sharpest focus determines how fast the autofocus mechanism is. Although your D800 will almost always focus more quickly than a human, there are types of shooting situations where that’s not fast enough. For example, if you’re having problems shooting sports because the D800’s autofocus system manically follows each moving subject, a better choice might be to switch autofocus modes or shift into manual and prefocus on a spot where you anticipate the action will be, such as a goal line or soccer net. At night football games, for example, when I am shooting with a telephoto lens almost wide open, I often focus manually on one of the referees who happens to be standing where I expect the action to be taking place (say, a halfback run or a pass reception). I also use trap focus, which is a technique discussed in a sidebar later in this chapter.
Choosing Autofocus Point Selection Mode
The D800 has only three different focus point selection modes. I’m going to describe each of the three modes, and explain how to use them. To set any of the three point selection modes rotate the focus mode selector switch, located on the front of the camera (under the lens release button) to the AF position. Then, hold down the button in the center of that switch (seen in Figure 5.15) and rotate the sub-command dial. The selected AF mode will appear in the viewfinder briefly as you make your selection, and on top of the camera in the control panel. It will also be shown on the back-panel LCD when the shooting settings screen is active. (See Figure 5.16.)
Figure 5.15 Autofocus mode selector switch is used to choose autofocus settings
Figure 5.16 Autofocus point selection modes.
Single-Point AF
In this mode, you always select the focus point manually, using the multi selector button (which, helpfully, will respond to your thumb-presses not only in the left/right and up/down directions, but diagonally, as well). The D800 evaluates focus based solely on the point you select, making this a good choice for subjects that don’t move much. As I mentioned earlier, you can use CSM #a7 to choose whether the selected focus point resides in an 11-point widely spaced distribution, or within the full 51-point array. In either Single-point AF or Dynamic-area AF, if you want to lock the focus point you’ve selected for a series of shots, rotate the focus selector lock lever surrounding the multi selector back to the L position. You can also temporarily lock the focus point by partially depressing the shutter release, or pressing the AE-L/AF-L button (unless you’ve redefined this behavior to some other controls in the Custom Settings menu).
Dynamic-Area AF
In this mode, you still select the focus point yourself using the multi selector button, and when using Single-servo autofocus (AF-S, discussed later), the D800 will evaluate focus solely based on that point. In that respect, the D800 behaves exactly as it does in Single-point AF mode. However, if you have chosen Continuous-servo autofocus (AFC), the D800’s “smarts” spring to life if your subject leaves the selected focus zone. When that happens, the camera re-evaluates focus based on the other focus points surrounding the one you chose. To recap, you can set the D800 so that it will use 9, 21, or 51 points. You can view what pattern is currently being used by pressing the Info button and viewing the autofocus array representation in the screen that pops up.
9 points. Only eight focus points surrounding the selected point will be used, allowing the D800 to respond quickly to subjects that are moving in a predictable way.
21 points. Should the subject leave the selected focus point, the D800 will refocus based on information from 20 surrounding focus points. This mode is best for subjects that are moving erratically. Autofocus may take slightly longer because more points are considered. I prefer this mode most of the time.
Note
The active focus points surrounding the red highlighted focus zone will shift around the array as you move the manually chosen focus point. The highlighting shown in the figure is an example that applies to the 9-point and 21-point options only when the center point is selected as the active focus point.
51 points. Should the subject leave your selected focus point, the camera will refocus based on information from all 50 surrounding focus points, which may be best for fast moving subjects.
3D-Tracking
All 51 points are used, and you can select the focus point as described for the other modes. Should the subject leave your selected focus zone, the D800 uses distance information to calculate the path of the subject and select a new focus point. Nikon recommends using this setting to focus subjects that move erratically from side to side (say, a child at play or a basketball player moving around the court on defense), because the camera can use the distance information to differentiate the original subject from objects that are closer or farther away. Tracking will abort if your subject leaves the viewfinder entirely; in that case, release the shutter button and reframe your image with your subject in the selected focus point.
Automatic-Area AF
In this mode, autofocus point selection is out of your hands; the D800 performs the task for you using its own intelligence. If you are using a type G or D lens, the camera can even work with the supplied distance information to distinguish humans from their background, so a person standing at the side of the frame will be detected and used to evaluate focus, while the camera ignores the background area in the frame.
The D800 tends to keep the active focus point somewhat of a mystery (although it will be displayed during picture review if you’ve activated that option). In AF-S mode, the active focus point is highlighted in the viewfinder for about one second after focus is achieved. In AF-C mode, the active focus point is not shown. This is the mode that allows face-tracking using the 91,000 point RGB exposure sensor. The D800 will find faces in your image and use the sensor’s data to track focus on that subject as it moves within the frame.
Autofocus Mode and Priority
Choosing the right autofocus mode (AF-S, AF-C, or Manual) is another key to focusing success. To save battery power, your D800 doesn’t start to focus the lens until you partially depress the shutter release or press the AE-L/AF-L button on the back of the camera or on the MB-D12 battery pack/grip (unless you’ve reprogrammed the button for some other function or have specified another control to activate autofocus, as described in Chapter 12). But, autofocus isn’t some mindless beast out there snapping your pictures in and out of focus with no feedback from you after you press that button. There are several settings you can modify that return at least a modicum of control to you. Your first decision should be whether you set the D800 to AF-S, AF-C, or Manual.
To set any of the three modes, with the AF mode switch in the AF position, hold down the center button of the focus mode selector switch on the front of the camera (under the lens release button, and seen earlier in Figure 5.15) and rotate the main command dial. The selected AF mode will appear in the viewfinder briefly as you make your selection, and on top of the camera in the control panel. It will also be shown on the back-panel LCD monitor when the shooting settings screen is active.
Autofocus Mode
This choice determines when your D800 starts to autofocus, and what it does when focus is achieved. Automatic focus is not something that happens all the time when your camera is turned on. To save battery power, your D800 generally doesn’t start to focus the lens until you partially depress the shutter release. (You can also use the AE/AL Lock button to start autofocus, as described in Chapter 12.)
Single-Servo Autofocus (AF-S)
In this mode, also called AF-S, focus is set once and remains at that setting until the button is fully depressed, taking the picture, or until you release the shutter button without taking a shot. For non-action photography, this setting is usually your best choice, as it minimizes out-of-focus pictures (at the expense of spontaneity). The drawback here is that you might not be able to take a picture at all while the camera is seeking focus; you’re locked out until the autofocus mechanism is happy with the current setting. As described in Chapter 12, you can set AF-S mode to use either focus priority (the default) or release priority using CSM #a2.
When sharp focus is achieved, the selected focus point will flash red in the viewfinder, and the focus confirmation light at the lower left will flash green. If you’re using Matrix metering, the exposure will be locked at the same time. By keeping the shutter button depressed halfway, you’ll find you can reframe the image while retaining the focus (and exposure) that’s been set. You can also use the AE-L/AF-L button if you’ve defined that button to lock focus when pressed. Because of the small delay while the camera zeroes in on correct focus, you might experience slightly more shutter lag. AF-S mode uses less battery power.
Continuous-Servo Autofocus (AF-C)
This mode, also known as AF-C, is the mode to use for sports and other fast-moving subjects. In this mode, once the shutter release is partially depressed, the camera sets the focus but continues to monitor the subject, so that if it moves or you move, the lens will be refocused to suit. Focus and exposure aren’t really locked until you press the shutter release down all the way to take the picture. You’ll find that AF-C produces the least amount of shutter lag of any autofocus mode when set to release priority: press the button and the camera fires. It also uses the most battery power, because the autofocus system operates as long as the shutter release button is partially depressed.
Continuous-servo autofocus uses a technology called predictive tracking AF, which allows the D800 to calculate the correct focus if the subject is moving toward or away from the camera at a constant rate. It uses either the automatically selected AF point (in Auto-area AF mode) or the point you select manually to set focus. As described in Chapter 12, you can set AF-C mode to use release priority (the default), or focus priority using CSM #a1.
If you want to lock the focus point you’ve selected for a series of shots, rotate the focus selector lock lever up to the L position. You can also temporarily lock the focus point by partially depressing the shutter release, or pressing the AE-L/AF-L button (unless you’ve redefined this behavior to some other controls in the Custom Settings menu).
Manual Focus
With manual focus activated by sliding the switch on the lens to the M position, or the rotating switch on the camera body near the lens mount set to M, your D800 lets you set the focus yourself. (Both the camera and lens settings must agree if you want to use an autofocus mode; when either is set to Manual, then only manual focus is possible.) There are some advantages and disadvantages to this approach. While your batteries will last longer in manual focus mode, it will take you longer to focus the camera for each photo, a process that can be difficult. Modern digital cameras, even dSLRs, depend so much on autofocus that the viewfinders of models that have less than full-frame-sized sensors are no longer designed for optimum manual focus. Pick up any film camera and you’ll see a bigger, brighter viewfinder with a focusing screen that’s a joy to focus on manually.
WARNING
Do not use a lens marked AF (rather than AF-S) when the focus mode switch on the lens is set to M and the switch on the camera is set to AF. As you’ll learn in Chapter 7, non-AF-S lenses do not have a built-in focus motor, and are focused by a motor in the camera body instead. When you set up the lens/camera in this conflicting configuration, it’s possible the camera’s internal motor may try to focus the lens (which has been set for manual focus only) and, with some lenses, damage can occur to the camera’s focus motor. With AF-S lenses, the lens and body switches should agree, but no damage should occur if you have a mismatch; if either switch is set to M, then manual focus is what you will get.
If the lens mounted on your camera has a maximum f/stop of f/5.6 or larger, you can use the D800’s electronic rangefinder feature to assist in focusing manually. Just follow these steps:
1. Activate manual focus. Set the lens and camera focus mode to M (see the warning above). If you happen to be using an old non-autofocus (manual focus) lens, you’re all set—it won’t autofocus under any circumstances.
2. Select a focus point. With the focus point selector lock off, select one of the 51 focus points using the multi selector.
3. Press the shutter release button halfway. This activates the D800’s autofocus system, which continues to function even though the camera’s ability to adjust the focus of the lens has been disabled.
4. Rotate the lens’s focus ring. Adjust until the in-focus indicator at the lower left of the viewfinder display illuminates continually.
5. Confirm. Visually evaluate whether you’re actually in sharp focus—the in-focus indicator is sometimes wrong, because it doesn’t really know the exact plane you want to be in focus!
Autofocus Activation… and More
The final considerations in using autofocus are the control or controls used to activate and lock autofocus, plus a few odds and ends. I’ll cover them in ample detail in Chapter 12, which explains all the options, but here are some cross references if you feel you need some review. I’m going to continue the discussion of focusing in Chapter 14, which explains live view and movie making, and some special focus options the D800 has when using those two modes.
Focus tracking with lock on. Intervening subjects passing in front of your main area of interest can interfere with autofocus. Set a delay time before the camera refocuses using CSM #a3, as described in Chapter 12.
AF point illumination. Do you want the active autofocus point illuminated when that is an option? Look up CSM #a5 in Chapter 12.
Focus point wrap around. Do you want the focus point to wrap around to the opposite side during manual selection? Use CSM #a6.
AF-assist illuminator. Need to turn off the autofocus assist illuminator on your camera, flash, or SC-29 connecting cable? Use CSM #a8.
Which controls activate/lock autofocus. You can use a half-press of the shutter release or a press of the AE-L/AF-L button (or both), or another button. See the instructions for CSM #f4, f5, and f6 in Chapter 12 for your options.
Center/show focus point. You can program the multi selector center button to either jump the active focus point to the center or to highlight the active focus point, using CSM #f2, as described in Chapter 12.
Focusing in Live View
When you’re not using the optical viewfinder, and instead using the D800’s Live View mode on the back panel color monitor, available modes differ slightly. Instead of using phase detection autofocus (or the human eye’s contrast detection system when focusing manually), the D800 puts contrast detection to work full-time. The camera evaluates the focus of the image as seen by the sensor, and makes adjustments from there.
This section will explain your live view focus options.
Focus Mode
Activate live view for still or movie mode by rotating the Live View switch to either Still or Movie modes, and then pressing the LV button on the back of the D800. Then press the information edit button to view the information edit screen. You can then adjust the focus mode by pressing the Focus Mode button and rotating the main command dial. The available modes differ slightly from those possible when not shooting in Live View mode, and your selection will appear at the top of the live view screen (highlighted in yellow when initially set):
AF-S. This single autofocus mode, which Nikon calls single-servo AF, locks focus when the shutter release is pressed halfway. This mode uses focus priority; the shutter can be fully released to take a picture only if the D800 is able to achieve sharp focus.
AF-F. This mode is roughly the equivalent of AF-C. Nikon calls it full-time servo AF. The D800 focuses and refocuses continually as you shoot stills in Live View modes or record movies. Unlike AF-C, this mode also uses focus priority. You can’t release the shutter unless the camera has achieved sharp focus.
Focus Area
With the Focus Mode button depressed, you can also choose the D800’s AF-area mode for live view by rotating the sub-command dial. Your choices are as follows:
Face-priority AF. The camera automatically detects faces, and focuses on subjects facing the camera, as when you’re shooting a portrait. You can’t select the focus zone yourself. Instead, a double yellow border will be displayed on the LCD monitor when the camera detects a face. You don’t need to press the shutter release to activate this behavior. (Up to five faces may be detected; the D800 focuses on the face that is closest to the camera.) When you press down the shutter release halfway, the camera attempts to focus the face. As sharp focus is achieved, the border turns green (see Figure 5.17). If the camera is unable to focus, the border blinks red. Focus may also be lost if the subject turns away from the camera and is no longer detectable by Face-priority.
Wide-area AF. This is the mode to use for non-portrait subjects, such as landscapes, as you can select the focus zone to be used manually. It’s good for shooting hand-held, because the subjects may change as you reframe the image with a hand-held camera, and the wide-area zones are forgiving of these changes. The focus zone will be outlined in red. You can move the focus zone around the screen with the multi selector buttons. When sharp focus is achieved, the focus zone box will turn green. (See Figure 5.18.)
Normal-area AF. This mode uses smaller focus zones, and so is best suited for tripod-mounted images where the camera is held fairly steady. As with Wide-area AF, the focus zone will be outlined in red. You can move the focus zone around the screen with the multi selector buttons. When sharp focus is achieved, the focus zone box will turn green. (See Figure 5.19.)
Figure 5.17 Face-priority AF attempts to focus on the face that’s closest to the camera.
Figure 5.18 Wide-area AF is best for landscapes and other subjects with large elements.
Figure 5.19 Normal-area AF allows you to zero in on a specific point of focus.
Subject-tracking AF. This mode allows the camera to “grab” a subject, focus, and then follow the subject as it moves within the frame. You can use this mode for subjects that don’t remain stationary, such as small children. When using Subject-tracking AF, a white border appears in the center of the frame, and turns yellow when focus is locked in (as described in the section that follows). To activate focus or refocus, press the multi selector up button. I’ll explain Subject-tracking in more detail next. (See Figure 5.20.)
Manual focus. In this non-automatic focus mode, which you have to select by setting the Focus Mode switch on the camera and lens to M, you can select the focus zone to use with the multi selector buttons, press the shutter release halfway, and then adjust focus manually by rotating the focus ring on the lens. When sharp focus is achieved, the focus confirmation indicator at the lower left of the viewfinder will turn a steady green.
Introducing Subject-Tracking
The useful Subject-tracking autofocus feature is one of those features that can be confusing at first, but once you get the hang of it, it’s remarkably easy to use. Face-priority, in comparison, is almost intuitive to learn. Here’s the quick introduction you need to Subject-tracking.
Ready, aim… When you’ve activated Subject-tracking, a white border appears in the center of the frame. Use that border to “aim” the camera until the subject you want to focus on and track is located within the border.
…Focus. When you’ve pinpointed your subject, press the OK button to activate the D800’s contrast detection autofocus feature. The focus frame will turn yellow and the camera will emit a beep (unless you’ve disabled the beep within the Setup menu) when locked in.
Reframe as desired. Once the focus frame has turned yellow, it seemingly takes on a life of its own, and will “follow” your subject around on the monitor as you reframe your image. (See Figure 5.20.) (In other words, the subject being tracked doesn’t have to be in the center of the frame for the actual photo.) Best of all, if your subject moves, the D800 will follow it and keep focus as required.
Tracking continues. The only glitches that may pop up might occur if your subject is small and difficult to track, or is too close in tonal value to its background, or if the subject approaches the camera or recedes sufficiently to change its relative size on the monitor significantly.
Grab a new subject. If you want to refocus or grab a new subject, press the OK button again.
Figure 5.20 Subject-tracking can keep focus as it follows your subject around in the frame.
Focus Stacking
If you are doing macro (close-up) photography of flowers, or other small objects at short distances, the depth-of-field often will be extremely narrow. In some cases, it will be so narrow that it will be impossible to keep the entire subject in focus in one photograph. Although having part of the image out of focus can be a pleasing effect for a portrait of a person, it is likely to be a hindrance when you are trying to make an accurate photographic record of a flower, or small piece of precision equipment. One solution to this problem is focus stacking, a procedure that can be considered like HDR translated for the world of focus—taking multiple shots with different settings, and, using software as explained below, combining the best parts from each image in order to make a whole that is better than the sum of the parts. Focus stacking requires a non-moving object, so some subjects, such as flowers, are best photographed in a breezeless environment, such as indoors.
For example, see Figures 5.21 through 5.23, in which I took photographs of three colorful crayons. As you can see from these images, the depth-of-field was extremely narrow, and only a small part of the subject was in focus for each shot.
Now look at Figure 5.24, in which the entire subject is in reasonably sharp focus. This image is a composite, made up of the three shots above, as well as 10 others, each one focused on the same scene, but at very gradually increasing distances from the camera’s lens. All 13 images were then combined in Adobe Photoshop using the focus stacking procedure. Here are the steps you can take to combine shots for the purpose of achieving sharp focus in this sort of situation:
1. Set the camera firmly on a solid tripod. A tripod or other equally firm support is absolutely essential for this procedure.
2. Attach a remote release, such as the MC-30. You want to be able to trigger the camera without moving it.
3. Set the camera to manual focus mode. Use the procedure described in the previous section to activate manual focus.
4. Set the exposure, ISO, and white balance manually. Use test shots if necessary to determine the best values. This step in the Shooting menu will help prevent visible variations from arising among the multiple shots that you’ll be taking. You don’t want the D800 to change the ISO setting or white balance between shots.
5. Set the quality of the images to NEF (RAW) + JPEG FINE. Use the Shooting menu to make this adjustment. Having both formats will give you flexibility when combining the images.
6. Turn off image stabilization. Navigate to Vibration Reduction in the Shooting menu. You don’t need it when the camera is securely mounted on a tripod, and disabling VR will keep the D800 from making any sort of image adjustment between exposures.
7. Focus manually on the very closest point of the subject to the lens. Rotate the multi selector dial to change the focus.
8. Trip the shutter. Use the remote.
9. Carefully refocus. Gently rotate the focus ring on the lens to focus on a point slightly farther away from the lens and trip the shutter again.
10. Continue taking photographs in this way until you have covered the entire subject with in-focus shots.
Figures 5.21, 5.22, 5.23 These three shots were all focused on different distances within the same scene. No single shot could bring the entire subject into sharp focus.
Figure 5.24 Three partially out-of-focus shots have been merged, along with ten others, through a focus stacking procedure in Adobe Photoshop, to produce a single image with the entire subject in focus.
The next step is to process the images you’ve taken in Photoshop. Transfer the images to your computer, and then follow these steps:
1. In Photoshop, select File > Scripts > Load Files into Stack. In the dialog box that then appears, navigate on your computer to find the files for the photographs you have taken, and highlight them all.
2. At the bottom of the next dialog box that appears, check the box that says, “Attempt to Automatically Align Source Images,” then click OK. The images will load; it may take several minutes for the program to load the images and attempt to arrange them into layers that are aligned based on their content.
3. Once the program has finished processing the images, go to the Layers panel and select all of the layers. You can do this by clicking on the top layer and then Shift-clicking on the bottom one.
4. While the layers are all selected, in Photoshop go to Edit > Auto-Blend Layers. In the dialog box that appears, select the two options, Stack Images and Seamless Tones and Colors, then click OK. The program will process the images, possibly for a considerable length of time.
5. If the procedure worked well, the result will be a single image made up of numerous layers that have been processed to produce a sharply focused rendering of your subject. If it did not work well, you may have to take additional images the next time, focusing very carefully on small slices of the subject as you move progressively farther away from the lens.
Although this procedure can work very well in Photoshop, you also may want to try it with programs that were developed more specifically for focus stacking and related procedures, such as Helicon Focus (www.heliconsoft.com), PhotoAcute (www.photoacute.com), or CombineZM (www.hadleyweb.pwp.blueyonder.co.uk).
Trap (Auto) Focus
This technique comes in handy when you know where the action is going to take place (such as at the finish line of a horse race), but you don’t know exactly when. The solution is to prefocus on the point where the action will occur, and then tell your D800 not to actually take the photo until something moves into the prefocus spot (see Figure 5.25). It’s a good technique for sports action when you know that, say, a runner is going to pass by a certain position. You can also use trap focus for hand-held macro shots—set focus for a particular distance and then move the camera toward your subject. The shutter will trip automatically when your subject comes into focus.
Trap focus isn’t as difficult as you might think. The key is to decouple the focusing operation from the shutter release function. Just follow these steps:
1. Set CSM #a4 to AF-ON Only. At that setting, pressing the shutter release halfway down does not activate autofocus. That happens only when you press the AF-ON button.
2. Set CSM #a2 to Focus Priority. The shutter will trip only when your subject is in focus.
Figure 5.25 By prefocusing on one of the hurdles, trap focus captured this athlete the instant he moved into the point of focus.
4. Set your point selection mode to Single Area.
5. Make sure your lens is set to autofocus (either A or M/A).
6. Prefocus on the spot where the action will occur, or an equivalent distance. You can use the AE-L/AF-L button to activate focus.
7. Reframe your picture, if necessary, so that nothing is at the prefocused distance. (If an object occupies that spot, the D800 will take the photo immediately when you press the shutter release.)
8. Press and hold down the shutter release all the way. The camera will not refocus, because you’ve disconnected the autofocus function from the shutter release.
9. The picture will be taken when a subject moves into the prefocused area.
DANGER, WILL ROBINSON!
If you don’t use trap focus often, or don’t work with the AF-ON button as your primary autofocus start control regularly, don’t make the mistake I did. One time I forgot that I had followed the steps above, and used a different camera (my Nikon D4) in the interim. The next time I picked up my D800, it “refused” to autofocus—at least when I pressed the shutter release halfway. Much consternation followed until I remembered what I had done a few days earlier, pressed the AF-ON button, and found that the AF function was just fine. I had simply “turned off” the shutter release as an AF start control.
Fine-Tuning the Focus of Your Lenses
In this section, I’ll show you how to calibrate your lenses using the D800’s AF Fine Tune feature.
Why is the focus “off” for some lenses in the first place? There are lots of factors, including temperature effects and humidity. A very slight variation in your lens’s mount can cause focus to vary slightly. A camera can even be miscalibrated at the factory, which was the case with the D800’s “left focus” defect (requiring a trip to a Nikon repair shop to fix). You’ll find more information in Appendix D. With any luck (if you can call it that) a lens that doesn’t focus exactly right will at least be consistent. If a lens always focuses a bit behind the subject, the symptom is back focus. If it focuses in front of the subject, it’s called front focus.
As I noted, you’re almost always better off sending such a lens in to Nikon to have them make it right. But that’s not always possible. Perhaps you need your lens recalibrated right now, or you purchased a gray market lens that Nikon isn’t willing to fix. If you want to do it yourself, the first thing to do is determine whether or not your lens has a back/front focus problem (or left-focus problems: use the procedure described in Appendix D).
For a quick-and-dirty diagnosis (not a calibration; you’ll use a different target for that), lay down a piece of graph paper on a flat surface, and place an object on the line at the middle, which will represent the point of focus (we hope). Then, shoot the target at an angle using your lens’s widest aperture and the autofocus mode you want to test. Mount the camera on a tripod so you can get accurate, repeatable results.
If your camera/lens combination doesn’t suffer from front or back focus, the point of sharpest focus will be the center line of the chart, as you can see in Figure 5.26. If you do have a problem, one of the other lines will be sharply focused instead. Should you discover that your lens consistently front or back focuses, it needs to be recalibrated. Unfortunately, it’s only possible to calibrate a lens for a single focusing distance. So, if you use a particular lens (such as a macro lens) for close-focusing, calibrate for that. If you use a lens primarily for middle distances, calibrate for that. Close-to-middle distances are most likely to cause focus problems, anyway, because as you get closer to infinity, small changes in focus are less likely to have an effect.
Lens Tune-up
The key tool you can use to fine-tune your lens is the AF Fine Tune entry in the Setup menu, shown in Figure 5.27. You’ll find the process easier to understand if you first run through this quick overview of the menu options:
AF fine tune (On/Off). This option enables/disables AF fine-tuning for all the lenses you’ve defined using the menu entry. If you discover you don’t care for the calibrations you make in certain situations (say, it works better for the lens you have mounted at middle distances, but is less successful at correcting close-up focus errors) you can deactivate the feature as you require. You should set this to On when you’re doing the actual fine-tuning.
Saved value. This setting lets you tune the autofocus calibration for the current CPU-chipped lens (virtually all Nikon-brand autofocus lenses) mounted on the D800. When you first fine-tune a lens, the saved value will be 0 (zero). You can press the multi selector up/down buttons to choose a value between +20 and –20. Positive numbers move the focal point farther from the camera, and would be used if your lens consistently suffers from front focus problems. Negative numbers move the focal point closer to the camera, and would be used if your lens is plagued with consistent back focus. The value is relative, and doesn’t correlate to any particular distance or percentage.
Figure 5.26 Correct focus (top), front focus (middle), and back focus (bottom).
Default. This entry often confuses. It is a value that is applied to every lens mounted on the camera that doesn’t already have a customized saved value associated with it. That is, if your D800 has consistent front or back focus problems with all lenses, you can enter a value here, and the camera will apply the correction to each CPU lens you use. This default setting can be overridden by saved values you’ve stored for particular lenses. So, you can change the default focal plane for all lenses, and still further fine-tune specific lenses that need more autofocus correction. My recommendation is that if your camera and lenses are so out of whack that you need global correction and individual fine-tuning, you really ought to consider shipping the whole kit off to Nikon for proper calibration.
List saved values. This screen shows you the saved fine-tuning values for all lenses. If the currently mounted lens has a stored value, it will be marked with a solid black box icon. You can delete lenses from this list by highlighting them and pressing the Trash button. You can also change the number (a two-digit number from 00-99) used to identify a particular lens.
Figure 5.27 The AF Fine Tune menu.
Evaluate Current Focus
The first step is to capture a baseline image that represents how the lens you want to fine-tune autofocuses at a particular distance. You’ll often see advice for photographing a test chart with millimeter markings from an angle, and the suggestion that you autofocus on a particular point on the chart. Supposedly, the markings that actually are in focus will help you recalibrate your lens. The problem with this approach is that the information you get from photographing a test chart at an angle doesn’t actually tell you what to do to make a precise correction. So, your lens back focuses three millimeters behind the target area on the chart. So what? Does that mean you change the Saved Value by –3 clicks? Or –15 clicks? Angled targets are a “shortcut” that don’t save you time.
Instead, you’ll want to photograph a target that represents what you’re actually trying to achieve: a plane of focus locked in by your lens that represents the actual plane of focus of your subject. For that, you’ll need a flat target, mounted precisely perpendicular to the sensor plane of the camera. Then, you can take a photo, see if the plane of focus is correct, and if not, dial in a bit of fine-tuning in the AF Fine Tuning menu, and shoot again. Lather, rinse, and repeat until the target is sharply focused.
You can use the focus target shown in Figure 5.28, or you can use a chart of your own, as long as it has contrasty areas that will be easily seen by the autofocus system, and without very small details that are likely to confuse the AF. Download your own copy of my chart from www.nikonguides.com/FocusChart.pdf. (The URL is case-sensitive.) Then print out a copy on the largest paper your printer can handle. (I don’t recommend just displaying the file on your monitor and focusing on that; it’s unlikely you’ll have the monitor screen lined up perfectly perpendicular to the camera sensor.) Then, follow these steps:
1. Position the camera. Place your Nikon D800 on a sturdy tripod with a remote release attached, positioned at roughly eye-level at a distance from a wall that represents the distance you want to test for. Keep in mind that autofocus problems can be different at varying distances and lens focal lengths, and that you can enter only one correction value for a particular lens. So, choose a distance (close-up or mid range) and zoom setting with your shooting habits in mind.
2. Set the autofocus mode. Choose the autofocus mode (AF-C or AF-S) you want to test.
3. Level the camera (in an ideal world). If the wall happens to be perfectly perpendicular, you can use a bubble level, plumb bob, or other device of your choice to ensure that the camera is level to match. Many tripods and tripod heads have bubble levels built in. Avoid using the center column, if you can. When the camera is properly oriented, lock the legs and tripod head tightly. Using an actual level will give you more accurate results than the D800’s built-in virtual horizon feature.
Figure 5.28 Use this focus test chart, or create one of your own.
4. Level the camera (in the real world). If your wall is not perfectly perpendicular, use this old trick. Tape a mirror to the wall, and then adjust the camera on the tripod so that when you look through the viewfinder at the mirror, you see directly into the reflection of the lens. Then, lock the tripod and remove the mirror.
5. Mount the test chart. Tape the test chart on the wall so it is centered in your camera’s viewfinder.
6. Photograph the test chart using AF. Allow the camera to autofocus, and take a test photo, using the remote release to avoid shaking or moving the camera.
7. Make an adjustment and rephotograph. Make a fine-tuning adjustment and photograph the target again. Follow the instructions in the next section. I’ve separated the fine-tuning adjustments from these steps because some people may want to just tweak the focus at a later time without going through all these evaluation steps. Repeat steps 1-8 in the section that follows this one to make evaluation images at a range of corrections, say, –5 through +5.
8. Evaluate the image. If you have the camera connected to your computer with a USB cable and Camera Control Pro or other linkup software such as Nikon Transfer, or through a Wi-Fi connection, so much the better. You can view the image after it’s transferred to your computer. Otherwise, carefully open the camera card door and slip the memory card out and copy the images to your computer.
9. Evaluate focus. Which image is sharpest? That’s the setting you need to use for this lens. If your initial range doesn’t provide the correction you need, repeat the steps between –20 and +20 until you find the best fine-tuning.
Changing the Fine-Tuning Setting
Adjust the fine-tuning for the lens you have mounted on the camera by following these steps:
1. If you haven’t been running the test described previously, mount the CPU-equipped lens you want to fine-tune on the Nikon D800. The camera will automatically recognize the lens you are using during the “calibration” process and display its name on the screen.
2. If you haven’t already done so, choose AF Fine Tune (On/Off) and turn it ON.
3. Select Saved Value.
4. Press the multi selector up/down buttons to tell the D800 to adjust the autofocus from +20 (move the focal point away from the camera to fix front focus problems) to –20 values (move the focal point toward the camera to fix back focus). (See Figure 5.29.)
5. Press OK when the value you want is entered. You may have to run the test described above several times and use some trial and error to determine the correct adjustment.
6. Choose List Saved Values to see the lenses you’ve fine-tuned. (See Figure 5.30.)
7. Assign a lens identifier from 00 to 99 to the lens you’ve just calibrated. This identifier can be used to differentiate a particular lens from other lenses of the same type, if you own, say, some duplicate lenses. That’s not as far-fetched as you might think. Some organizations, such as newspapers, allow their photographers to use favorite lenses exclusively, but may need to share other optics among several photographers. If you don’t know which of the pooled AF-S Nikkor 600mm f/4G ED VR lenses you’ll be using on any particular day, you can calibrate your camera separately for each of them. (See Figure 5.31.)
8. Press MENU to exit.
Set Default Value
If you want to set a default value for lenses that aren’t in your Saved Values list (say, because your camera always back or front focuses slightly), choose the Default setting from the AF Fine Tune menu, and adjust as shown in Figure 5.32.
Figure 5.29 Change a Saved Value for a particular lens.
Figure 5.30 List the Saved Values you’ve stored.
Figure 5.31 Assign a lens number, if necessary, to differentiate a particular lens from other lenses of the same type you may use.
Figure 5.32 Choose a default value to be applied to all lenses not already fine-tuned.
