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Exposure Modes

Digital cameras usually provide several exposure modes for your photographing pleasure. Although most cameras do a pretty good job when left on autopilot, you gain more technical control and opportunity for creative image making when you explore the other exposure modes. Although the modes vary, some are common to nearly every camera. Typically, these include a semiautomatic mode where you make certain exposure choices and the camera handles the rest; a full manual mode; and possibly a selection of scene modes that are designed for specific situations. Before considering what each exposure mode is and what it offers you, let's look at the basics of photographic exposure.

What Is Exposure?

In photography exposure is the moment when the light strikes the film or sensor and the image is recorded. Three factors combine to determine the correct exposure for a digital image: the amount of light in the scene that strikes the image sensor (controlled by the lens aperture); the length of time that the sensor is exposed to the light (controlled by the shutter speed); and the sensitivity of the sensor (determined by the ISO setting). The importance of a good exposure cannot be overstated. If an image is overexposed, the highlights will be completely white without any tonal information (Figure 4.14). If an image is grossly underexposed, the image will be dark and lack shadow detail (Figure 4.15).

Figure 4.14

Figure 4.14 When an image is grossly overexposed, the brightest highlights are rendered as white with no detail. In the histogram for the image, the vertical bars representing the tonal values for the highlights are pushed up against the right side of the graph, indicating lost highlight detail.

Figure 4.15

Figure 4.15 When an image is severely underexposed, the darkest shadows are rendered as black with no detail. In the histogram for the image, the vertical bars representing the tonal values for the darker parts of the image are pushed up against the left side of the graph, indicating lost shadow detail.

Although it is possible to improve moderate exposure mistakes using image-editing software, no amount of digital darkroom magic can save a picture that is extremely overexposed or underexposed. Fortunately, camera light meters are very sophisticated instruments and do an excellent job of determining the settings for a proper exposure in most common photographic situations. However, by changing any of the three main settings, you can gain control over certain characteristics that can influence the look of the image. We'll examine in great detail how light meters see the world in the next chapter.

Aperture

Aperture refers to the opening of the iris, or diaphragm, in the lens that can be adjusted to let more or less light hit the image sensor. One way to think of aperture is to imagine a funnel in which the large end is the lens gathering the light and the small end is the aperture that controls how much light reaches the image sensor in a given period of time. Aperture is measured in f-stops, and each full stop represents a factor of two in the amount of light admitted. Thus, "opening up" a lens from f5.6 to f4 will admit twice as much light, and "stopping down" from f11 to f16 will cut the amount of light in half (see the sidebar "What's in an f-stop?").

Apart from controlling how much light passes through the lens, the aperture is also one factor that affects depth of field (the others are the focal length of the lens, the size of the image sensor, and the distance between objects in the scene). Depth of field is the area of the image that appears in focus from foreground to background and is one of the main ways that you can change the appearance of an image (Figure 4.17). We'll cover depth of field in more detail later in this chapter.

Figure 4.17

Figure 4.17 You can achieve a shallow depth of field (less area in focus) by using a larger lens aperture and deeper depth of field (more area in focus) by using a smaller aperture.

Shutter speed

If we continue with the same funnel analogy that was used to explain the aperture, the shutter is the valve that controls how long the light flows through the lens and onto the image sensor. The smaller the aperture, the longer it will take a given amount of light to flow through the funnel and the longer the required exposure will be. Shutter speeds are measured in extremely small fractions of a second, and speeds on DSLRs range from 30 seconds up to 1/8000 of a second. Every camera is different, of course, and your mileage may vary. The range of shutter speeds on compact cameras is not as large as those found in DSLRs. The function of the shutter is similar to aperture in that each successive change in the shutter speed either halves or doubles the exposure time. Using standard shutter speeds as an example, 1/125 of a second is half as much exposure as 1/60 but twice as much as 1/250.

In addition to controlling how long the light is exposed to the sensor, the shutter speed also impacts how motion is rendered in a scene. Speeds below 1/40 of a second are likely to result in motion blur when moving subjects are photographed, and very fast shutter speeds of 1/1000 or higher can do an excellent job of freezing even very fast movement in a scene. Just as depth of field can be used creatively to affect the look of an image, shutter speed is also an important creative control (Figure 4.18).

Figure 4.18

Figure 4.18 In the photo on the left, the shutter speed was 1.3 seconds, resulting in the water in the stream being blurred. For the photo on the right, the shutter speed was 1/800th of a second, freezing the movement of the bicyclists racing down the hill.

The beauty of reciprocity

Aperture and shutter speed work together to create a proper exposure in a given lighting situation. Because of the way they function, you could take several shots, each with a different aperture and shutter speed, and produce several images that all had equal exposure. You could use a wider aperture for a shorter amount of time, for instance, or a smaller aperture for a longer amount of time to admit equivalent amounts of light. Another way to look at this is that opening up the lens aperture by one stop is exactly the same as decreasing the shutter speed by one setting; each doubles the amount of light for the exposure. And increasing the shutter speed by one setting has the same effect on exposure as stopping down a stop to a smaller aperture. This give-and-take nature of the aperture-shutter speed relationship is known as reciprocity, and it's one of the most effective exposure tools available to photographers (Figure 4.19).

Figure 4.19

Figure 4.19 Reciprocity in action: Four photos of the same scene, each with a very different shutter speed and aperture, yet all have the same exposure (in other words, the same amount of light reached the sensor). The only noticeable difference in these images is greater or less depth of field, depending on the aperture. All images were photographed using a tripod.

The benefits of reciprocity come into play when your camera meter recommends a certain exposure, but you need to change either aperture or shutter speed to produce a desired creative effect. Let's say you're photographing a flower, and the camera's light meter indicates that it will use a shutter speed of 1/2 of a second and an aperture of f11. Although this might yield a correctly exposed image, an aperture of f11 would produce too much depth of field, making the background details too distinct and distracting. If you were using a manual exposure mode, you could take advantage of the reciprocity principle to quickly (well, reasonably quickly) calculate an equivalent exposure that would give you a wider aperture and throw the background out of focus. If you decided that an aperture of f2.8 would produce the desired shallow depth of field, you'd increase the aperture by 4 stops (f8, f5.6, f4, f2.8); that would require an equivalent adjustment of the shutter speed. Opening the lens aperture to f2.8 lets in more light (16 times as much in this case); so to balance out the exposure you would need to shorten the amount of time the shutter is open by 4 stops—to 1/30 of a second. For the final exposure of the flower, the shutter speed is at 1/30 and the aperture is at f2.8. This produces exactly the same exposure (in other words, the same amount of light reaches the sensor) as the initial camera meter's suggestion of 1/2 at f11, but the differences between the two images is significant (Figure 4.20).

Figure 4.20

Figure 4.20 In the photo on the left the exposure is a 1/2 second at f11. In the photo on the right, the exposure is 1/30 of a second at f2.8. The amount of light reaching the image sensor is exactly the same for both images.

Fortunately, unless you're operating on full manual mode or you just enjoy the intellectual challenge, when used in Aperture or Shutter Priority mode, cameras will automatically calculate the reciprocal aperture and shutter speed values for you. This makes it easier to concentrate on the image and choose the settings that will give you the right creative look. We'll cover exposure considerations in greater depth later in this chapter and also in Chapter 5.

Full Auto Mode

Nearly all modern cameras provide a fully automatic mode that does everything for you but compose the shot and decide when to press the shutter button. Full Auto mode evaluates the lighting; selects the ISO, white balance, aperture, and speed settings; and even decides whether the scene needs a little extra light from the built-in flash. This is a good mode to use if you're new to digital photography and you still don't know much about your new camera but you want to take pictures right away—or when you need to hand the camera to someone else to take a picture of you.

Keep in mind that some camera features, such as the abilities to change the ISO, adjust the exposure with exposure compensation, and shoot in RAW format, may not be available in Full Auto mode. To gain an extra level of control and customization while enjoying the ease of automatic operation, you may have to use another automatic mode that is commonly called Program.

Program Mode

Program mode is similar to Full Auto mode in that the camera selects the appropriate aperture and shutter speed to deliver the correct exposure for the scene you're photographing. You also have the ability to modify the settings the camera has chosen by shifting the aperture–shutter speed combination to select a mix that better serves your creative goals (reciprocity in action). On DSLRs you usually make this adjustment by dialing a control wheel until you arrive at a desired aperture or shutter speed, something you can do without taking your eye away from the viewfinder. On compact cameras or deluxe point-and-shoot models, the procedure may be more cumbersome: You usually have to manipulate a series of buttons, requiring you to take your eye away from the camera. Program modes also offer access to more advanced features of the cameras, such as shooting in RAW format, exposure compensation, higher ISO settings, and choosing a custom white balance. Because it offers the convenience of being fully automatic with the flexibility of changing some of the settings, you may find that Program mode works well for many situations.

Aperture Priority Mode

Aperture Priority can be thought of as a semiautomatic mode because it relies on you to decide which aperture to choose while the camera supplies the appropriate shutter speed. Once you select a given aperture, the camera constantly adjusts the shutter speed in response to changing exposure conditions, but the aperture remains the same. This mode is an excellent choice for images where depth of field issues take precedence over shutter speed. A wider aperture causes the background to be more out of focus, and a smaller aperture yields a photo with more areas of the image in focus. Aperture Priority is excellent for portraits where you want only the subject in focus (use a smaller f number for a larger aperture) and for scenic shots where you want good depth of field throughout the scene (use a larger f number for a smaller aperture).

Shutter Priority Mode

Like Aperture Priority, Shutter Priority is a semiautomatic mode. You decide what shutter speed you want to shoot with, and the camera chooses the correct aperture. Shutter Priority is ideal for situations where exposure time is more important than depth of field. If you need to freeze motion, such as with sports or birds in flight, using this mode allows you to select an appropriately fast shutter speed. If your aim is to use motion blur creatively, such as the classic rendition of moving water in a stream, you can also use Shutter Priority to choose a slow shutter speed. Depending on the speed of the object you're trying to blur, you may need to use a tripod so that stationary elements in the image remain sharp.

Manual Mode

With Manual mode you have to do all the work. Well, maybe not all the work. The camera does provide a light meter to tell you if your settings will give you a properly exposed image, but you have to turn the dials or push the buttons and make sure that aperture and shutter speed are set correctly.

Although a Manual mode is essential for photographic control geeks (like the three of us) and those who want as many creative options as possible, it's not as spontaneous as some of the other modes, and realistically you may only need to control either aperture or shutter speed to achieve the effect you want. For some situations, however, such as night photography and in the studio, having a Manual mode is critical.

Scene Modes

Scene modes are preset configurations that are designed for you to use under specific shooting conditions to achieve good results without having to think about the optimal camera settings. They're not exposure modes you would use all the time. You'll find these modes on many digital cameras, from compact point-and-shoot models all the way up through advanced DSLRs. The actual names and modes vary from camera to camera (other terms we've heard include Best Shot and Creative Assist modes), and depending on their features, some cameras may offer more sophisticated interpretations. But here's a rundown of some of the most common scene modes:

  • Portrait. The main feature of this mode is that it will try to soften the focus of the background while keeping the main subject sharp. The degree to which the background is thrown out of focus depends on a number of factors, including the amount of light available, the distance between the subject and the background, the maximum aperture and focal length of the lens, as well as the size of the image sensor (compact cameras cannot create the same shallow depth of field that is possible from a DSLR). Some cameras may also use a Center-Weighted metering pattern to give emphasis to the center portion of the frame. Center-Weighted metering is common in portrait situations. See Chapter 5 for detailed coverage of how a camera's light meter works.
  • Night portrait. This mode is for portraits of people or any photo where the subject is relatively close to the camera, at twilight or at night. If such a scene is photographed normally, the flash will fire and the camera will expose for the immediate foreground subject, leaving the background very dark and underexposed. In Night Portrait mode, the camera uses the flash and also chooses a slower shutter speed, creating a balanced exposure between the main subject and the darker background. The exposure for the main subject and the background will look good. Some cameras have a mode that is similar to this called Slow-Sync Flash (Figure 4.21).
    Figure 4.21

    Figure 4.21 Night Portrait mode uses a longer exposure (slower shutter speed) to create a good exposure for a dark background, combined with a fill flash to properly expose the foreground. The photo on the left uses a regular auto flash and shows the dark background; on the right is the same scene photographed using Night Portrait mode.

  • Landscape. Whereas Portrait mode chooses as wide an aperture as possible for shallow depth of field, Landscape mode uses a small aperture to produce the deep depth of focus commonly associated with scenic images. Be aware, however, that some cameras may be doing a bit more behind the scenes than simply choosing an aperture for good depth of field. The manual for one of Seán's compact digital cameras claims that the Landscape mode will "enhance outlines, colors, and contrast in subjects such as skies and forests." This suggests that the camera is actually applying more aggressive sharpening, contrast, and saturation adjustments when it processes the image—and subsequent testing proved this to be true. Before you rely on any scene mode, it's a good idea to run some tests and see how it affects image quality.
  • Night Landscape. This is useful when you want to photograph cityscapes at night or twilight views of grand vistas. It cancels any flash operation, sets the focus distance to infinity, and uses slow shutter speeds to gradually build up an exposure of a night scene. Due to the slower shutter speeds, a tripod or other stabilizing surface may be necessary.
  • Beach/Snow and Backlight. These two modes are very similar and are designed to compensate for photographing very bright subjects.When a camera light meter tries to evaluate a scene such as a beach or a snowy field on a sunny day, the brightness reflected from the sand or snow can confuse the meter and lead to an image that is too dark. A mode designed for photographs of beach and snow scenes adjusts the exposure so that the scene will be properly exposed (Figure 4.22). The Backlight mode does essentially the same thing but is used for situations where the light is coming from behind your main subject or the background is brightly lit. The Backlight mode chooses a shutter speed–aperture combination that creates a proper exposure for the foreground subject. On some cameras, the flash may fire in Backlight mode to fill in the shadows on a person's face. Although the name Beach/Snow Mode may suggest that it should only be used for those type of scenes, it can be effective in any situation where you are photographing a bright subject that reflects a lot of light.
    Figure 4.22

    Figure 4.22 The darker photo is a result of the camera's meter being fooled by the bright reflected light on the snow. When the Beach/Snow scene mode was used, a correct exposure was captured that better represented the real appearance of the scene.

  • Close-Up/Macro. This scene mode is typically offered on compact or deluxe point-and-shoot cameras (macro photography with DSLRs involves using a special lens that is designed for close focusing). Depending on the camera, this mode selects a range of settings designed to produce a better close-up photo. On compact cameras, all this may amount to is extending the zoom lens all the way and adjusting the AF (auto focus) sensors for close focusing. On some cameras the flash will go off; on others it won't. Still other cameras may employ some form of camera-shake reduction to help with handheld shots. Even though the close-up capabilities of compact cameras are limited by the camera's built-in lens, the macro features on many compact models can be quite impressive (Figure 4.23).
    Figure 4.23

    Figure 4.23 This iris was photographed using the Super Macro scene mode on an entry-level Canon Powershot A480.

  • Sports. This mode is optimized for photographs where you want to freeze the action at sporting events and on fast-moving subjects. The actual functionality that this mode offers greatly depends on the capabilities of the camera. Features such as auto focus speed, AF servo focus (the ability to track a moving subject), variable-focus sensors, and the speed of the continuous-shooting drive are all put to work when you use this mode on a DSLR. Compact cameras and deluxe point-and-shoots, which don't have the advanced focus and drive features of DSLRs, generally offer an exposure mode that is biased toward faster shutter speeds and a drive setting for taking a series of multiple shots.
  • Black and White/Monochrome. For those times when you want to create a black and white photo with no hassle, this is the mode to use. On some cameras the Black and White mode may only be available when shooting JPEG files. If you are shooting in RAW+JPEG, the JPEG file will be black and white and the RAW file will still retain all color and editing properties. But we prefer to take our images in color and convert to black and white using an image-editing program, since that gives us the most flexibility to creatively interpret the file The only reasons we can think of to use a Black and White or Monochrome mode is if you don't have the time or experience to make the conversion in another program, or if you're new to viewing the world in gray tones and actually seeing the image in black and white on the LCD helps with the composition and subject matter you've chosen. Otherwise, we don't recommend shooting in Black and White mode.
  • Other modes. The variety of scene modes is limited only by the imagination of camera manufacturers. Some of the modes are useful in relatively limited circumstances, such as a Fireworks mode or a mode designed to copy documents (espionage mode?).
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