Appendix A: The Elements of Exposure

If you haven’t used a dSLR before, then the options for controlling exposure on the 60D may seem foreign to you, and knowing when to use them to get specific results may seem overwhelming. But once you know how the elements of exposure work together, the camera will make much more sense. In this appendix, you learn how aperture (f-stop), shutter speed, and ISO affect your images, as well as how they work together. And if you are returning to photography after time away, the information in this appendix serves as a refresher on the elements of exposure.

The Four Elements of Exposure

Making a good picture begins with your creative eye and personal expression. The unique way that you see a subject or scene is the first step in making engaging and unique pictures. Then you use the camera to express how you see the scene or subject. While creativity is first and foremost in making images, you also need to have essential technical skills to use the camera effectively to express your vision.

While the technical aspects may not seem as exciting as the creative aspects of picture making, the more you understand what goes into making a photographic exposure, the more creative control you have. And creative control moves you out of the realm of getting a few happy-but-accidental images and into the realm of building a portfolio of beautiful-and-intentional images. A solid understanding of exposure also prepares you for when you need to find creative workarounds for challenging lighting, scenes, subjects, or limitations of the camera itself.

A.1 Learning to use the 60D exposure controls means that you can control the aperture, and, thus, the depth of field. In this image, I chose a wide f/4.5 aperture to bring the flower visually forward in the frame. Exposure: 1S0 200, f/4.5, 1/250 second using –1/3 stop of Exposure Compensation.

Photographic exposure is the combination of four elements:

Light. The starting point of exposure is the amount of light that is in the scene. Before making any image, the 60D first measures, or meters, the amount of light in the scene, and only then can it calculate its suggested exposure settings. Your range of creative control with the 60D is often determined by the amount of light you have to work with or the amount of light that you can add using a flash or other lights.

Sensitivity. Sensitivity refers to the amount of light that the camera’s image sensor needs to make an exposure or to the sensor’s sensitivity to light. Sensitivity is controlled by the ISO setting. At high ISO settings, the camera needs less light to make an image than it does at low ISO settings.

Intensity. Intensity refers to the strength or amount of light that reaches the image sensor. Intensity is controlled by the aperture, or f-stop. The aperture controls the lens diaphragm, an adjustable opening within the lens that opens or closes to allow more or less light into the camera.

Time. Time refers to the length of time that light is allowed to reach the sensor. Time is controlled by setting the shutter speed, which determines how long the camera’s shutter stays open to let light reach the sensor.

This language may sound unfamiliar, so it may be easier to think of exposure as filling the image sensor with light, just as you would fill a glass with water. The goal is to fill the image sensor with just the right amount of light to get a good exposure, as you would fill the glass with the perfect amount of water.

In photography, you start with the amount of light that’s in the scene and use that amount to determine the correct exposure. In the water glass analogy, that equates to the level to which you want to fill the glass with water. Then you have several choices. You can use a strong or weak stream of water, analogous to a large or a small aperture or f-stop. Then you can choose how long the water flows into the glass, analogous to setting the shutter speed. Finally, you can choose the size of the glass to fill. The size of the glass represents the ISO, or the sensitivity of the sensor. A small glass fills faster than a large glass, as a high ISO requires less light than a low ISO.

So starting with the goal of reaching the perfect level of water (the perfect amount of light reaching the sensor), you can choose to use a fast or slow water flow (aperture), and can let the water run a long or short time (shutter speed) depending on the size of the glass (the ISO). Further, there are many combinations of these variables that all result in getting just the right amount of water into the size glass that you’re filling.

The following sections look at each exposure element in more detail. As you read, know that every exposure element is related to the other elements. If one exposure element changes, one or all of the other elements must also change proportionally.

Light

The first element in any image is the light that you have to make the picture. And that’s the first thing the camera looks at — it first measures the light in the scene using the onboard light meter. Every time that you half-press the shutter button, the camera measures the light.

The 60D uses a reflective light meter that measures the amount of light that is reflected from the subject back to the camera.

The light meter reading is biased toward the active autofocus (AF) point. The active AF point tells the camera where the subject is so that the camera can evaluate the subject light relative to the rest of the light in the scene. The camera factors in the current ISO setting, and then calculates how much light (determined by the aperture) is needed and for how long (determined by the shutter speed setting) to make a good exposure. Then the camera gives you its recommended exposure.

Once the exposure settings are calculated, the 60D applies the aperture, shutter speed, and the ISO automatically in the Basic Zone modes, such as Portrait, Landscape, Sports, and so on. But in the semiautomatic shooting modes such as Shutter-priority AE (Tv) and Aperture-priority AE (Av), you have more control over the exposure by setting the shutter speed in Tv shooting mode, or the aperture in Av shooting mode. When you do that, the camera takes your aperture or shutter speed into account and calculates the aperture in Tv mode, or the shutter speed in Av mode based on the ISO that you’ve set.

Sensitivity: The role of ISO

In broad terms, the ISO setting determines how sensitive the image sensor is to light. ISO settings work this way: The higher the ISO number, the less light that’s needed to make a picture; and the lower the ISO number, the more light that’s needed to make a picture. In the analogy of filling a glass with water, the size of the glass corresponds to the ISO. If the glass is small (more sensitive to light), less water is needed, and vice versa.

In practical terms, high ISO settings such as ISO 800 to 1600 give you faster shutter speeds. That’s important to know because if you’re handholding the camera, shooting at slow shutter speeds in low light results in blurry pictures, caused by the slight shake of your hands. Fast shutter speeds in low light increase the chance that you can handhold the camera and get a sharp image. On the other hand, in bright to moderately bright scenes, low ISO settings from 100 to 400 provide fast shutter speeds because there is enough light in the scene that the camera can give you a fast-enough shutter speed to handhold the camera and get a sharp image.

Each higher ISO setting is twice as sensitive to light as the previous setting. For example, ISO 800 is twice as sensitive to light as ISO 400. As a result, the sensor needs half as much light to make an exposure at ISO 800 as it does at ISO 400 and vice versa.

In P, Tv, Av, and M shooting modes, the ISO sequence encompasses Auto (ISO 100 to 6400, which is set automatically by the camera, although you can set an upper limit) and ISO 100, 200, 400, 800, 1600, 3200, and 6400. The ISO can also be expanded to include ISO 12800 using Custom Function (C.Fn) I-3. In the automated Basic Zone modes, the 60D automatically sets the ISO between 100 and 3200, depending on the light. In P, Tv, Av, and M shooting modes, the Auto ISO setting ranges from 100 to 6400.

The International Organization for Standardization (ISO) measures, tests, and sets standards for many photographic products, including the rating or speed for film, which has, in turn, been applied to equivalents for the sensitivity of digital-image sensors.

But ISO does more than affect the amount of light that’s needed to make a good exposure. ISO also impacts the overall image quality in several areas, including sharpness, color saturation, contrast, and digital noise or lack thereof.

In situations where you must use a high ISO setting, you can use Custom Function (C.Fn) II-2: High ISO speed noise reduction to minimize digital noise. The higher levels of noise reduction can reduce the fine detail in images, so you may want to test each level at high ISO settings to see which works best for your photography. You can also use C.Fn II-1: Long exposure noise reduction to reduce digital noise in exposures of 1 second or longer. This option slows down shooting, but it is very effective in reducing the level of noise in the image.

See Chapter 5 for details on setting Custom Functions.

Intensity: The role of aperture

The lens aperture (the size of the lens diaphragm opening) determines the intensity of light that reaches the image sensor. Going back to the water glass analogy, aperture represents the strength or size of the water flow. A strong flow, or a large aperture (f-stop), fills the glass (the image sensor) faster than a weak stream of water.

Aperture is indicated as f-stop numbers, such as f/2.8, f/4.0, f/5.6, f/8, and so on. Small aperture numbers such as f/5.6, f/4, and f/2.8 correspond to a strong water flow because the lens opening, or diaphragm, is large. Large aperture numbers such as f/8, f/11, f/16, and so on correspond to a weak water flow because the lens diaphragm is small.

A.2 The left image shows the lens diaphragm at f/22, the smallest or minimum aperture for this lens. The right image shows the lens opened up to f/5.6. On a lens that has a maximum aperture of f/2.8, the diaphragm opens completely letting in the maximum amount of light.

When you increase or decrease the aperture by a full f-stop, it doubles or halves the exposure, respectively. For example, f/5.6 provides twice as much light as f/8 provides, while f/5.6 provides half as much light as f/4.0.

The apertures that you can choose depend on the lens that you’re using. And with a variable aperture lens such as the Canon EF-S 18-55mm f/4.5-5.6 lens, the maximum aperture of f/4.5 can be used when the lens is zoomed to 18mm and f/5.6 can be used when the lens is zoomed to 55mm.

Wide aperture

Smaller f-stop numbers, such as f/5.6, f/4, f/3.5, and f/2.8, set the lens diaphragm to a large opening that lets more light reach the sensor. A large lens opening is referred to as a wide aperture. Based on the ISO and in moderate light, a wide aperture (a large diaphragm opening) such as f/5.6 delivers sufficient light to the sensor so that the amount of time the shutter has to stay open to make the exposure decreases. Thus you get a faster shutter speed than you would by using a narrow aperture of f/8 or f/11. In general terms, this means that wide apertures of f/5.6 to f/1.2 enable you to shoot in lower-light scenes with a reasonably fast shutter speed (depending on the existing light and the ISO setting). And that combination helps you get sharp handheld images in lower light.

Narrow aperture

Larger f-stop numbers, such as f/8, f/11, f/16, and narrower, set the lens diaphragm to a small opening that allows less light to reach the sensor. A small lens opening is referred to as a narrow aperture. Based on the ISO and in moderate light, a small diaphragm opening such as f/11 delivers less light to the sensor — or fills the glass slower — so the amount of time that the shutter has to stay open increases, resulting in a slower shutter speed. Because narrow apertures of f/8 to f/32 require longer shutter speeds, you need a lot of light to shoot with narrow apertures, or you need to use a tripod and have a scene or subject that remains stock still.

A.3 In this image of a barn in the shadow of Mt. Baker in Washington State, a narrow f/11 aperture keeps acceptably sharp focus through the entire frame. Exposure: ISO 100, f/11, 1/200 second.

Aperture also plays a starring role in the depth of field of images.

Choosing an aperture

In everyday shooting, photographers most often select an aperture based on how they want the background and foreground to look — either showing acceptably sharp detail or blurred detail. This is called controlling the depth of field, discussed next. But you may also need to choose a specific aperture for practical or creative reasons. For example, if the light is low and you want to avoid blur from camera shake, then choosing a wide aperture (smaller f-number) provides faster shutter speeds. Or if you want to use selective focus, where only a small part of the image is in sharp focus, choose a wide aperture. But if you’re shooting a group of people or a landscape, then choose a narrow aperture to render sharper detail throughout the entire frame.

You can control the aperture by switching to Av or M shooting mode. In Av shooting mode, you set the aperture, and the camera automatically sets the correct shutter speed based on the selected ISO. In M shooting mode, you set both the aperture and the shutter speed based on the reading from the camera’s light meter. The Exposure level indicator is displayed in the viewfinder as a scale and it indicates over-, under-, and correct exposure based on the aperture, shutter speed, and ISO.

You can use Program AE (P) mode to make one-time changes to the aperture and shutter speed from the camera’s recommended exposure. Unlike Av mode where the aperture you choose remains in effect until you change it, in P mode, changing the aperture is temporary. After you take the picture, the camera reverts to its suggested aperture and shutter speed.

What is depth of field?

Depth of field is the zone of acceptably sharp focus in front of and behind a subject. In simple terms, depth of field determines if the foreground and background are rendered as a soft blur or with distinct detail. Depth of field generally extends one-third in front of the plane of sharp focus and two-thirds behind it. Aperture is the main factor that controls depth of field, although camera-to-subject distance and focal length affect it as well.

Depth of field is both a practical matter — based on the light that’s available in the scene to make the picture — and a creative choice to control the rendering of background and foreground elements in the image. For example, if you are shooting at a music concert and you’ve set the ISO where you want it, you might creatively prefer to shoot using f/8 so that the midstage props are in acceptably sharp focus. But on a practical level, you might glance at the f/8 shutter speed and quickly decide that you need to use a wide aperture to get as fast a shutter speed as possible. A goodly number of scenes involve practical and creative tradeoffs, and the more you know about exposure, the better prepared you are to make judicious decisions.

Shallow depth of field

Images where the background is a soft blur and the subject is in sharp focus have a shallow depth of field. As a creative tool, shallow depth of field is typically preferred for portraits, some still-life images, and food photography. As a practical tool, choosing a wide aperture that creates a shallow depth of field is necessary when shooting in low light. To create a shallow depth of field, choose a wide aperture ranging from f/5.6 to f/1.2 depending on the lens. The subject will be sharp, while the background and foreground will be soft and less distracting. Lenses also factor into depth of field, with a telephoto lens offering a shallower depth of field than a normal or wide-angle lens. Figure A.1 shows a shallow depth of field.

Extensive depth of field

Extensive depth of field maintains acceptably sharp focus in front of and behind the plane of sharpest focus. It is preferred for images of landscapes, large groups of people, architecture, and interiors. When you want an image with extensive depth of field, choose a narrow aperture, such as f/8, f/11, f/16, f/22, or smaller. Figure A.3 shows an extensive depth of field.

Once again, you may have to make creative and practical tradeoffs when your goal is to create an extensive depth of field. Narrow apertures require a lot of light because the lens diaphragm opening is very small. This is analogous to using a weak water stream to fill the water glass. In photography, narrow apertures require longer shutter speeds than wide apertures at the same ISO.

Other factors affecting depth of field

While aperture is a key factor in determining the depth of field — range of acceptably sharp focus in a picture — other factors also affect depth of field:

Camera-to-subject distance. At any aperture, the farther you are from a subject, the greater the depth of field is and vice versa. Additionally, the farther the subject is from the background the greater the background blur and vice versa.

Focal length. Focal length, or angle of view, is how much of a scene the lens “sees.” From the same shooting position, a wide-angle lens produces more extensive depth of field than a telephoto lens.

Time: The role of shutter speed

Shutter speed controls how long the shutter stays open to let light from the lens strike the image sensor. The longer the shutter, or curtain, stays open, the more light reaches the sensor (at the aperture and ISO that you’ve set). In the water glass example, the amount of time that you let the water flow into the glass is analogous to shutter speed.

When you increase or decrease the shutter speed by one full setting, it doubles or halves the exposure. For example, twice as much light reaches the image sensor at 1/30 second as at 1/60 second. Shutter speed is also related to the following:

The ability to handhold the camera and get sharp images, particularly in low light. The general rule for handholding a non-image stabilized lens is that you need a minimum shutter speed that is the reciprocal of the focal length, or 1 over the focal length. For example, if you’re shooting at 200mm, then the slowest shutter speed at which you can handhold the lens and get a sharp image is 1/200 second.

The ability to freeze motion or show it as blurred in a picture. For example, you can set a fast shutter speed to show a basketball player’s jump in midair with no blur. As a general rule, set the shutter speed to 1/125 second or faster to stop motion. Or set a slow shutter speed to show the motion of water cascading over a waterfall as a silky blur. To show motion as a blur, use a 1/30 second or slower shutter speed and use a tripod.

You can control the shutter speed in Tv or M shooting mode. In Tv shooting mode, you set the shutter speed, and the camera automatically sets the correct aperture. In M shooting mode, you set both the shutter speed and the aperture based on the reading from the camera’s light meter and the ISO. The light meter is displayed in the viewfinder as a scale — the Exposure level indicator — and it shows over-, under-, and correct exposure based on the shutter speed, aperture, and ISO.

Equivalent Exposures

As you have seen, after the camera meters the light and factors in the selected ISO, the two remaining factors determine the exposure — the aperture and the shutter speed. Just as when filling the glass with water, you can fill it slowly over a longer time, quickly, or any variation between.

Likewise, many combinations of aperture (f-stop) and shutter speed produce exactly the same exposure given the same ISO setting. For example, f/22 at 1/4 second is equivalent to f/16 at 1/8 second, as is f/11 at 1/15, f/8 at 1/30, and so on. This is based on the doubling and halving effect discussed earlier. For example, if you are shooting at f/8 and 1/30 second, and you change the aperture (f-stop) to f/5.6, then you have doubled the amount of light reaching the image sensor, so the time that the shutter stays open must change proportionally to 1/60 second.

While these exposures are equivalent, the way the image looks and your shooting options change noticeably. An exposure of f/22 at 1/4 second produces extensive depth of field in the image, but the shutter speed is slow, so your ability to handhold the camera and get a sharp image is dubious and a tripod becomes a necessity. However, if you switch to an equivalent exposure of f/5.6 at 1/60 second, you are more likely to be able to handhold the camera, but the depth of field will be shallow.

As with all aspects of photography, evaluate the tradeoffs as you make changes to the exposure. And again, it all comes back to light. Your creative options for changing the exposure setting are ultimately limited by the amount of light in the scene.

Putting It All Together

ISO, aperture, shutter speed, and the amount of light in a scene are the essential elements of photographic exposure. On a bright, sunny day, you can select from many different f-stops and still get fast shutter speeds to prevent image blur. You have little need to switch to a high ISO for fast shutter speeds at small apertures.

As it begins to get dark, your choice of f-stops becomes limited at ISO 100 or 200. You need to use wide apertures, such as f/4 or wider, to get a fast shutter speed. Otherwise, your images will show some blur from camera shake or subject movement. Switch to ISO 400 or higher, however, and your options increase and you can select narrow apertures, such as f/8 or f/11, for greater depth of field. The higher ISO enables you to shoot at faster shutter speeds to reduce the risk of blurred images, but it also increases the chances of digital noise.