Today, modern digital cameras like the Z50 can identify potential subject matter, lock in on human faces, if present, and automatically focus faster than the blink of an eye. Usually. Of course, sometimes a camera’s AF will zero in on the wrong subject, become confused by background pattern, or be totally unable to follow a fast-moving target like a bird in flight. While autofocus has come a long way in the last 30-plus years, it’s still a work-in-progress that relies heavily on input from the photographer. Your Nikon Z50 can calculate and set focus for you quickly and with a high degree of accuracy, but you still need to make a few settings that provide guidance on three Ws of autofocus: what, where, and when. Your decisions in how you apply those choices supplies the fourth W: why. This chapter will provide you with everything you need to put all four to work.
Simply put, focus is the process of adjusting the camera so that parts of our subject that we want to be sharp and clear are, in fact, sharp and clear. We allow the camera to focus for us, automatically, or we can rotate the lens’s focus ring manually to achieve the desired focus. Manual focusing is especially problematic because our eyes and brains have poor memory for correct focus. That’s why your eye doctor conducting a refraction test must shift back and forth between pairs of lenses and ask, “Does that look sharper—or was it sharper before?” in determining your correct prescription. Too often, the slight differences are such that the lens pairs must be swapped multiple times.
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 Nikon Z50’s autofocus mechanism, like all such systems found in modern cameras, also evaluates these increases and decreases in sharpness, but 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 camera’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? Using autofocus effectively involves telling the Z50 exactly what it should be focusing on.
Learning to use the Z50’s modern autofocus system is easy, but you do need to fully understand how the system works to get the most benefit from it. Once you’re comfortable with autofocus, you’ll know when it’s appropriate to use the manual focus option, too.
As the camera collects focus information from the sensors, it then evaluates it 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.
The Z50 has a hybrid autofocus system, using two technologies called contrast detection autofocus (CDAF) and phase detection autofocus (PDAF). I’m going to provide a quick overview of contrast detection first, and then devote much of the rest of this chapter to optimizing your camera’s use of its phase detection features.
This is a slower, but potentially more accurate mode, best suited for static subjects, and was originally the only kind of autofocus available for mirrorless cameras and for dSLRs when shooting in their live view and movie modes. The recent innovation of adding phase detection abilities to the sensor itself (as I’ll describe shortly) made contrast detection a fine-tuning option for hybrid autofocus systems that combined CDAF and PDAF.
Contrast detection is very easy to understand, and is illustrated by Figure 5.1, a close-up of some weathered wood. 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. Whether the edges are horizontal, vertical, or diagonal doesn’t matter in the least; the focus system looks only for contrast between edges, and those edges can run in any direction at all.
At the bottom of Figure 5.1, 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, it can be used to fine-tune focus on a subject that has already been brought into general focus using phase detection (described next). There are several advantages—and disadvantages—to contrast detection:
Phase detection is easily much more rapid than contrast detection. The challenge is to make its operation as accurate as possible. Digital SLRs, like the Z50’s sibling the Nikon D850, have always used PDAF. Such dSLR systems include an array of tiny autofocus sensors, located in the “floor” of the mirror box, with a small portion of the illumination directed downward to the autofocus sensor array.
In the Z50, that separate AF sensor is replaced by 209 phase detect autofocus points embedded in the camera’s imaging sensor. These points are located in the center of the boxes shown in Figure 5.2. The boxes represent the positions you can individually select as your AF area in Single-point AF area mode—not the size of the PDAF pixels themselves, which are much smaller.
The phase detection pixels in the Z50’s sensor split incoming photons arriving from opposite sides of the lens into two parts, forming a pair of images, exactly like the rangefinders used for surveying and in rangefinder-focusing cameras like the venerable Leica M series. The dual images are separated when out of focus, and then gradually brought together to achieve sharp focus, as shown from top to bottom in Figure 5.3.
This process tells the camera when the image pair are “in phase” and aligned. The rangefinder approach of phase detection tells the Z50 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 match the lines. If necessary, the Z50’s contrast-detect feature can follow up to perfect the plane of focus. This hybrid AF process is available in all cases (except in Pinpoint AF Area mode, which I’ll explain shortly).
The PDAF sensors in the Z50 are all line sensors, which means they work best with features that transect the sensor either perpendicularly or at an angle, as visualized in Figure 5.4, left. It’s easy to detect when the two halves of the vertical lines of the weathered wood are aligned. (See Figure 5.4, center.) However, 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. One such scenario is pictured in Figure 5.4, right. Fortunately, as I mentioned earlier, the Z50’s contrast detection AF will come to the rescue and take over where phase-detect AF is stymied.
As with any rangefinder-like function, phase detection 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 Z50 is able to perform these comparisons very quickly.
But there are other factors in play, as well. You know that increased depth-of-field, a by-product of shorter focal lengths and/or smaller f/stops, brings more of your subject into focus, at the same time making it more difficult to focus on a subject with, say, a 24mm zoom setting than a 200mm zoom setting. By the same token, a lens with a maximum aperture of f/1.8, such as the Z-mount 50mm f/1.8 and 35mm f/1.8 optics, will be easier to autofocus (or manually focus) than one of the same focal length with an f/4 maximum aperture, such as the 24-70mm kit zoom, because the f/4 lens has more depth-of-field and a dimmer view at a particular focal length.
Of course, many subjects move around in the frame, so that even if the Z50 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 Z50) 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 Z50’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.
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. 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. (See Figure 5.5.)
Autofocus can sometimes be frustrating for the new digital 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 digital camera, even when photographing the same subjects. Most point-and-shoot digital cameras and smartphones have sensors that are much tinier than the sensor in the Z50. Those smaller sensors require shorter focal length lenses and zoom settings, which have, effectively, more depth-of-field.
The bottom line is that with the average point-and-shoot camera or smartphone, 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 Z50, 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:
Manual focus requires judgment and fast reflexes (if your subject is moving). On the one hand, autofocus 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 Z50’s autofocus system most effectively also 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. Your Z50 includes some useful features that help you focus manually—including magnification and focus peaking, a way of highlighting out-of-focus areas that may need correction just as “blinkies” show you what parts of the image are overexposed. I’ll show you how to use these manual focus aids later in this chapter.
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 photographer, correct focus can be one of the trickiest parts of the technical and creative process.
I’ve explained individual bits and pieces of the Nikon Z50’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 first introduced 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 Photo/Movie shooting modes. For now, we’re going to concentrate on the system’s most important features:
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 Z50 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 Z50’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).
Focus Modes (labeled AF-S, AF-C, AF-A, and Manual in your camera’s settings) are another key to focusing success. (Your Z50 also has an additional focus mode, AF-F—full-time autofocus in Movie mode, as explained in Chapter 12.) To save battery power, when shooting stills, your Z50 doesn’t start to focus the lens until you partially depress the shutter release or press an AF-ON button you’ve defined using Custom Controls, as explained in Chapter 10. 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 Z50 to AF-S, AF-C, AF-A, or manual focus.
You can easily specify any of the three automatic modes plus manual: just press Fn2 and rotate the main command dial (unless you’ve redefined the button for some other behavior). Or press the i button and access Focus Mode, located by default at the far right in the bottom row of the i menu. If you have a lot of time on your hands, use the Focus Mode entry in the Photo or Movie Shooting menus. The AF mode appears at the top of the viewfinder or on the monitor screen as you make the adjustments.
This choice determines when your Z50 starts to autofocus, and what it does when focus is achieved. In still photo mode, automatic focus is not something that happens all the time when your camera is turned on. As I mentioned, to save battery power, your Z50 generally doesn’t start to focus the lens until you partially depress the shutter release or a defined custom AF-ON button.
In this mode, also called Single Autofocus or 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. AF-S is available in both still photo and movie shooting modes.
When sharp focus is achieved, the selected focus point will turn green. 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 your AE-L/AF-L button. If the camera is unable to focus, the focus point will flash red. Because of the small delay while the camera zeroes in on correct focus, you might experience slightly more shutter lag. This mode uses less battery power.
You can select any of the 209 focus areas in this AF mode when your Autofocus Area setting (described shortly) is set to Single-point AF. AF-S is the mode to use if you want to place the focus point within your frame with even more accuracy. And, AF-S is the autofocus mode in which you can specify Pinpoint-area AF as your focus area, and place a tiny focus point virtually anywhere in the frame encompassed by the boxes in Figure 5.2.
LOW-LIGHT AF
All digital cameras find autofocusing in low-light environments a challenge. The Z50’s abundant PDAF pixels slightly reduce the area of the sensor available to capture photons, and some report the bands or stripes that can appear in darker areas of an image. You may be able to improve autofocus performance for subjects that are within the range of about three feet to nearly 10 feet (1 to 3 meters) with the built-in AF-assist illuminator lamp on the front of the camera, which is activated by default but may be disabled using Custom Setting a8. You can also activate Custom Setting a7: Low-Light AF, which works in any still photo shooting mode other than Auto. Autofocus may take longer in Low-Light mode, so a warning appears on the display. Both settings operate only when using the AF-S focus mode.
This mode, also known as AF-C, is the mode to use for sports and other types of photography with 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, even if sharp focus has not quite been achieved. It also uses the most battery power, because the autofocus system operates as long as the shutter release button is partially depressed. AF-C is also available in both still photo and movie shooting modes.
Continuous-servo autofocus uses a technology called predictive tracking AF, which allows the Z50 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 10, you can set AF-C mode to use release-priority (the default) or focus-priority using Custom Setting a1. You can temporarily lock the focus point by partially depressing the shutter release, or pressing the AE-L/AF-L button.
This mode, also known as AF-A, is available only in Photo mode. The Z50 starts out in AF-S mode, but will switch to AF-C mode if your subject starts to move. It’s useful for unpredictable subjects, say, a pet canine that appears to be in repose, but which may perk up and dash off when it’s spotted a squirrel.
This mode, also known as AF-F, is available only in Movie mode. You don’t need to activate focus with a button: the camera adjusts focus continually as your subject moves. Focus locks only when the shutter release is pressed halfway. Obviously, this mode uses the most juice.
Set manual focus by sliding the switch on the lens to the M position (if the lens has one) or by using the Fn2 button, i menu, or Photo/Movie Shooting menu options. 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 depend so much on autofocus that the viewfinders 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. I’ll tell you more about manual focus later in this chapter.
If your Z50 isn’t focusing on the correct subject, autofocus speed and activation are pretty much wasted effort. As you’ve learned, the Z50 has up to 209 different points on the screen that can be individually selected to determine the active focus zone. You can choose which of those points is used by selecting an AF-area mode. As with AF mode, you can select the AF-area mode from the i menu, Photo/Movie Shooting menus, or by holding down the Fn2 button and rotating the sub-command dial. The Z50 has six different focus point selection modes. I’m going to describe each of the modes, and explain how to use them.
This mode is available only in Single AF (AF-S) focus mode. As I mentioned earlier, Pinpoint AF is the only AF-area mode in which contrast-detect autofocus (CDAF) is used exclusively. The 209 phase-detect pixels are ignored, and the Z50 evaluates a tiny area of the sensor frame to determine correct focus. As you might expect, this mode is slower than other AF-area modes, especially Single-point AF (described next), but highly accurate.
In Pinpoint AF mode, you always select the focus point manually, using the multi selector directional controls (which, helpfully, will respond to your thumb presses not only in the left/right and up/down directions, but diagonally, as well). Return the focus point to the center at any time by pressing the OK button.
The focus point is represented by the tiny box shown in Figure 5.6. You have an extraordinary amount of freedom in placing the focus area, which is half the size of that used for Single-Point AF. Because Pinpoint AF uses contrast detection within the selected area, it’s not limited by the position of the fixed PDAF pixels represented by the larger boxes.
Obviously, selecting an AF point so precisely can be time-consuming, so Pinpoint AF is best suited for subjects that don’t move. It’s especially good for macro work and any photography undertaken with the camera mounted on a tripod.
As with the other user-selectable area modes, you can select the focus point manually, using the multi selector. The Z50 evaluates focus based solely on the point you select, making this another good choice for subjects that don’t move much. You can choose Custom Setting a3: Focus Points Used, to select whether the focus point resides within the 209-area array by choosing ALL (see Figure 5.7) or a more widely spaced distribution (Every Other Point) which activates only one-quarter of the focus areas (see Figure 5.8). The alternate point mode allows much faster positioning of your focus zone. I use Every Other Point a lot for sports, because I plan on filling as much of the frame as possible with players, and can usually estimate where they will appear in the frame. (That’s usually off center, closer to the edge where I expect them to enter the frame.) Single-Point AF is excellent for achieving focus on a subject that might otherwise blend in with its background.
TIP The Every Other Point setting does not affect the number of focus points used with Wide Area AF (Small or Large), which I’ll describe shortly.
In this mode, available only when AF-C is active (or the camera has switched to AF-C in AF-A mode), you still select the primary focus point yourself from among the 209 available using the multi selector. The camera will focus on that point, but if the subject departs from the selected area, the Z50 uses information from the surrounding areas. The red-highlighted points in the middle of Figure 5.9 show the active focus points when the dynamic area is centered in the frame. You can move the grouping to any of the other points, but when the grouping reaches the top border or corners (as shown in the corners of the figure), some of the potential points are outside the array.
DOT’S THE POINT
In all Focus Area Modes except Auto-area AF (discussed shortly), a dot appears within the focus point when it is located in the center of the frame, as you can see in Figure 5.9.
Dynamic-area AF is a more sophisticated version of Single-Point AF. You can select the initial focus point, and then trust the Z50’s smarts to continue focusing on a moving subject. That frees you to concentrate on framing your composition rather than worrying about whether your subject remains in focus.
This setting is excellent for slow-moving subjects, as seen in Figure 5.10, left, but flexible enough to follow 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, especially human subjects (see Figure 5.10, right). However, many photographers also use this setting for birds in flight. Very rapid motion may call for the Z50’s tracking feature, described shortly.
TIP In Pinpoint AF, Single-point AF, or Dynamic-area AF, if you want to lock the focus point you’ve selected for a series of shots, you can temporarily lock the focus point by partially depressing and holding the shutter release, or pressing and holding the AE-L/AF-L button. Press the multi selector center button to move the single focus point back to the center of the frame quickly.
Uses the focus points within zones you can move around the display. You can choose the smaller of the wide-area zones (shown in Figure 5.11, or even larger zones, seen in Figure 5.12). I don’t use these much, especially Wide-area (L), because they each encompass so much of the frame that they’re best suited for large subjects that will reside in a predictable area, and those are often handled quite efficiently with Single-point AF.
In this mode, autofocus point selection is out of your hands; the Z50 performs the task for you using its own intelligence. Red brackets will be displayed to show the area the camera will use to establish focus (see Figure 5.13). The camera can even work with the distance information supplied by the lens and its face detection technology 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.
In AF-S mode, the active focus points are highlighted with an array of green boxes that illuminate as you hold the shutter release down halfway. In AF-C mode, a dancing array of red boxes appears around the focused area, highlighted in that color to indicate that the current focus point is only tentative and won’t be locked in until you press the shutter release down all the way. Birds in flight—one of the most difficult of all autofocus targets—can often be grabbed using Automatic-area AF.
This mode’s face-detection and tracking features merit a more detailed discussion of their own, which I’ll provide in the next section.
The Auto-area AF feature has two great features that allow you to identify human targets and track any kind of moving subjects (birds-in-flight, people-in-flight, and others). Face detection allows the camera to locate potential portrait subjects when the visage of a human being appears in the frame. Subject tracking with lock-on directs the Z50 to follow the movement of a subject you specify and continue to focus on it as it roams around the frame. Just keep in mind that you must be using Auto-area AF to access either of these features.
Use this mode for portraits and people pictures. The camera automatically detects faces, or both eyes and faces (your choice), and focuses on subjects facing the camera, as when you’re shooting a portrait. Within this mode, the Z50 really does assign finding faces and autofocusing on them its number-one priority.
To use this feature, you must first enable Custom Setting a2: Auto-Area AF Face/Eye Detection. You can choose from Face and Eye Detection On, Face Detection On (the camera will not seek out eyes), and Off. (Eye detection is not available in Movie mode.)
Once enabled, you can’t directly select the focus zone yourself. Instead:
The useful Subject-tracking autofocus feature of Auto-area AF is one of those capabilities that can be confusing at first, but once you get the hang of it, it’s remarkably easy to use. In fact, it’s always available any time you’re using Auto-area AF.
Here’s the quick introduction you need to Subject Tracking.
As I’ve noted several times, some subjects lend themselves to manual focus, especially close-up or macro photography, in which AF can be a creative hindrance. If you’re photographing a flower, why let the camera decide which leaf or petal should be sharpest—particularly if you are using a relatively large aperture and selective focus as an effect? A mirrorless camera’s live view is especially useful when focusing manually, because you have extra tools at your disposal to focus precisely and quickly.
Here are the basic steps for quick and convenient setting of focus manually with the Z50:
The Z50 doesn’t have a Pv (Preview) button by default, but you can define one for, say, Fn1 or Fn2 in Custom Setting f2: Custom Control (Shooting). Then, when you press the defined Pv button, the lens will stop down to the aperture selected by you (in Manual mode) or by the camera (in Aperture-priority mode). (This doesn’t work in Program or Shutter-priority mode, because the aperture isn’t determined in either mode until you lock exposure or take a photo.) To open the aperture to maximum again, release the button.
Once you’ve been using your camera for awhile, you’ll invariably encounter the terms back focus and back button focus, and wonder if they are good things or bad things. Actually, they are two different things, and are often confused with each other. Back focus is a bad thing, and occurs when a particular lens consistently autofocuses on a plane that’s behind your desired subject. This malady may be found in some of your lenses, or all your optics may be free of the defect. The good news is that if the problem lies in a particular lens (rather than a camera misadjustment that applies to all your lenses), it can be fixed. I’ll show you how to do that at the end of this chapter.
Back button focus, on the other hand, is a tool you can use to separate two functions that are commonly locked together—exposure and autofocus—so that you can lock in exposure while allowing focus to be attained at a later point, or vice versa. It’s a good thing, although using back button focus effectively may require you to unlearn some habits and acquire new ways of coordinating the action of your fingers.
As you have learned, the default behavior of your Nikon Z50 is to set both exposure and focus (when AF is active) when you press the shutter release down halfway. When using AF-S mode, that’s that: both exposure and focus are locked and will not change until you release the shutter button, or press it all the way down to take a picture and then release it for the next shot. In AF-C mode, exposure is locked and focus set when you press the shutter release halfway, but the Z50 will continue to refocus if your subject moves for as long as you hold down the shutter button halfway. Focus isn’t locked until you press the button down all the way to take the picture.
What back button focus does is decouple or separate the two actions. You can retain the exposure lock feature when the shutter is pressed halfway, but assign autofocus start and/or autofocus lock to a different button. So, in practice, you can press the shutter button halfway, locking exposure, and reframe the image if you like (perhaps you’re photographing a backlit subject and want to lock in exposure on the foreground, and then reframe to include a very bright background as well).
But, in this same scenario, you don’t want autofocus locked at the same time. Indeed, you may not want to start AF until you’re good and ready, say, at a sports venue as you wait for a ballplayer to streak into view in your viewfinder, or when you’re photographing a garden and expect a butterfly to alight somewhere nearby. With back button focus, you can lock exposure on the spot where you expect the athlete or insect to be, and activate AF at the moment your subject appears. The Z50 gives you a great deal of flexibility, both in the choice of which button to use for AF, and the behavior of that button. That’s where the learning of new habits and mind-finger coordination comes in. You need to learn which back button focus techniques work for you, and when to use them.
Back button focus lets you avoid the need to switch from AF-S to AF-C when your subject begins moving unexpectedly. You retain complete control. It’s great for sports photography when you want to activate autofocus precisely based on the action in front of you. It also works for static shots. You can press and release your designated focus button, and then take a series of shots using the same focus point. Focus will not change until you once again press your defined back button. (See Figure 5.19.)
Want to focus on a spot without moving the focus point within your frame? Use back button focus to zero in focus with the current focus area—no need to move it—then reframe. Focus will not change. Don’t want to miss an important shot at a wedding on a photojournalism assignment? If you’re set to focus-priority your camera may delay taking a picture until the focus is optimum; in release-priority there may still be a slight delay. With back button focus you can focus first, and wait until the decisive moment to press the shutter release and take your picture. The Z50 will respond immediately and not bother with focusing at all. Back button focus can also save battery power. Constantly refocusing in AF-C mode can consume a lot of power.
Here’s how to set up back button focus on your Z50. Just follow these steps:
Theoretically, at least, there should be no need at all to fine-tune the focus of autofocus lenses, native Z-mount lenses in particular. With a traditional dSLR, the autofocus mechanism is a separate component located in the floor of the mirror box. That module can be slightly misaligned in relationship to the lens and the dSLR’s optical manual focusing screen. AF fine-tune can correct for that error. The Z50, in contrast, calculates autofocus using the actual sensor image using its CDAF and PDAF features, and so should “see” any misalignment and correct for it automatically.
However, Nikon, in its wisdom, has included an AF fine-tune feature in the Z50. You can use it with both native Z-mount lenses as well as F-mount lenses attached using the FTZ adapter. In this section, I’ll show you how to calibrate your lenses using the Z50’s AF Fine-tune feature.
Why is the focus “off” for some lenses in the first place? There are lots of factors, including the age of the lens (an older lens may focus slightly differently), temperature effects on certain types of glass, humidity, and tolerances built into a lens’s design that all add up to a slight misadjustment, even though the components themselves are, strictly speaking, within specs. A very slight variation in your lens’s mount can cause focus to vary slightly. 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.
If it’s the lens that’s at fault, or if your camera consistently mis-focuses, you’re almost always better off sending your lens or Z50 to Nikon to have them make it right. But that’s not always possible or advised. Perhaps you need your lens recalibrated right now, or you purchased a gray market lens that Nikon isn’t willing to fix. You may have a problem only with one or two lenses, while the rest focus with more accuracy. If you want to do the fine-tuning yourself, the first thing to do is determine whether your lens has a back focus or front focus problem.
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 at top in Figure 5.20. 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.
There are products available, such as Reikan FoCal ($135) a tool for both PCs and Macs that comes with software and a focusing target. Also available is the LensAlign MkII Focus Calibration system ($85). You can fine-tune your camera’s focus manually using the instructions that follow.
To manually adjust your camera/lens AF setting, just follow these steps. The key tool you can use to fine-tune your lens is the AF Fine-Tune entry in the Setup menu, shown in Figure 5.21. You’ll find the process easier to understand if you first run through this quick overview of the menu options:
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 5.22, 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 autofocus. 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:
Adjust the fine-tuning for the lens you have mounted on the camera by following these steps:
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 5.26.
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