Thanks to the tiny computers built into modern film and digital cameras, exposure can be determined accurately in a variety of situations. Most cameras have multiple metering modes which measure light in any of several different ways. This next section explains the most common modes and how they work.
Exposure Zones
Sophisticated electronic cameras divide the subject area into a few dozen different zones to more than 1,000 zones, and can measure the light for each separately. Then, your camera's programming can make decisions about how the differing illumination in the zones affects your photo and calculate an exposure based on those decisions. You can choose how the exposure is calculated yourself, too, if you understand how these zones work. Here are the most common metering schemes. Keep in mind that the illustrations greatly simplify how the zones look. In a real digital camera the zones aren't large, coarse blocks, but, rather, many different data points segregated into specific areas.
Averaging. In this mode, the camera's exposure meter figures the exposure based on the average of all the illumination falling on the entire sensor. This scheme works best with scenes that have a mixture of light and dark subjects. You might select the averaging mode when shooting photos that involve a variety of subjects, and you feel that the other exposure modes aren't suitable. For example, if you're taking pictures at an indoor soccer game, your main subjects might be scattered all over the frame, so an exposure based on the average of the frame can work for you. (Actually, because the lighting doesn't vary much, a manual exposure setting might be even better, but we'll explore that approach in the chapter on action photography later in this book.) With the camera set for averaging exposure, the entire subject area, shown in Figure 3.6, is used to calculate the exposure. (The bracket shown is not used for exposure calculation.)
Exposure Zones
Sophisticated electronic cameras divide the subject area into a few dozen different zones to more than 1,000 zones, and can measure the light for each separately. Then, your camera's programming can make decisions about how the differing illumination in the zones affects your photo and calculate an exposure based on those decisions. You can choose how the exposure is calculated yourself, too, if you understand how these zones work. Here are the most common metering schemes. Keep in mind that the illustrations greatly simplify how the zones look. In a real digital camera the zones aren't large, coarse blocks, but, rather, many different data points segregated into specific areas.
Averaging. In this mode, the camera's exposure meter figures the exposure based on the average of all the illumination falling on the entire sensor. This scheme works best with scenes that have a mixture of light and dark subjects. You might select the averaging mode when shooting photos that involve a variety of subjects, and you feel that the other exposure modes aren't suitable. For example, if you're taking pictures at an indoor soccer game, your main subjects might be scattered all over the frame, so an exposure based on the average of the frame can work for you. (Actually, because the lighting doesn't vary much, a manual exposure setting might be even better, but we'll explore that approach in the chapter on action photography later in this book.) With the camera set for averaging exposure, the entire subject area, shown in Figure 3.6, is used to calculate the exposure. (The bracket shown is not used for exposure calculation.)
Figure 3.6. An averaged exposure reading takes into account the illumination of the entire scene.
Center-weighting. In this mode, the exposure meter emphasizes a zone in the center of the frame to calculate exposure on the theory that, for most pictures, the main subject will be located there. Center-weighting works best for portraits, architectural photos, and other pictures in which the most important subject is located in the middle of the frame. (No surprise there!) As the name suggests, the light reading is weighted towards the central portion, but information is also used from the rest of the frame. If your main subject is surrounded by very bright or very dark areas, the exposure might not be exactly right. However, this scheme works well in most situations. In Figure 3.7, most of the exposure would be calculated from the mountains and the lake, but the darker trees and foreground would also be taken into account.
Figure 3.7. Center-weighting emphasizes the middle portion of the frame to calculate exposure.
Corner-weighting. You won't find this as an option in any digital camera. Corner-weighting is actually just a way of calculating exposure that your camera's matrix metering system might decide on, using its database of picture types. The exposure meter places the emphasis on the corners of the frame. One way of doing this is shown in Figure 3.8. If corner-weighted exposure were calculated for the example photo, the trees and foreground would receive more exposure. This scheme works well when there is a very bright or very dark object in the center of the frame.
Figure 3.8. Corner-weighting de-emphasizes the center of the frame when calculating exposure.
Top- or bottom-weighting. Again, this kind of metering isn't a setting you can choose, but rather, a weighting that might be assigned by your camera based on its exposure programming. These schemes weight the top or bottom of the frame, or both. Bottom-weighting can help you ignore, say, a very bright sky area that would fool the meter or cause underexposure. Top-weighting can be used when you want to expose for an area in the upper part of the frame. Using both top- and bottom-weighting lets you de-emphasize the middle of the frame. In the sample picture, shown in Figure 3.9, these modes would measure the foreground or trees (or both) in calculating the main exposure.
Figure 3.9. Top- and bottom-weighted metering emphasizes the upper and lower edges of the frame.
Matrix-metering. This is a sophisticated metering technique (also called evaluative metering) in which many different zones of the image are metered separately. Then, the camera evaluates the results to make an educated guess about what kind of picture you're taking, based on examination of thousands of different real-world photos. Eastman Kodak Company developed this sort of exposure control for its automated printing machines many years ago, when it found it was possible to determine whether a photo was a portrait, landscape, or other type of picture simply from the exposure matrix. For example, if the top sections of a picture are much lighter than the bottom portions, the algorithm can assume that the scene is a landscape photo with lots of sky. Figure 3.10 is a greatly simplified representation that shows typical zones that a matrix metering system might consider in calculating exposure.
Figure 3.10. Matrix metering tries to guess what kind of photo you're taking by metering separate zones and evaluating the results.
Spot metering. This mode confines the reading to a limited area, indicated in the center of the viewfinder. Some digital cameras let you move the meter spot to any location you like in the frame. If you're lucky, the process will be simple enough to use easily. One camera I use switches to spot metering when the Spot button is pressed; moving the spot to a different location is accomplished by continuing to hold the Spot button while pressing the cursor keys. Spot metering is a great tool when your subject is relatively small and surrounded by dark or light areas that would confuse any other metering method. Figure 3.11 shows the spot metering zone of a typical digital camera.
Figure 3.11. Spot metering can be used to calculate exposure from a limited area of the frame.
Aperture Priority
Sometimes you want to use a particular lens opening to achieve a desired effect. Perhaps you'd like to use the smallest f-stop possible to maximize depth-of-field in a close-up picture. Or, you might want to use a large f-stop to throw everything except your main subject out of focus, as shown in Figure 3.12. In this shot, the background, which did have some texture, is completely out of focus because of the shallow depth-of-field.
Figure 3.12. Choose your own aperture for maximum (or minimum) depth-of-field.
However, there's no need to switch to full manual exposure if all you want to do is specify a particular aperture. Most digital cameras can be set for Aperture Priority mode. In that mode, you select the f-stop and your camera's automatic exposure system will determine the correct shutter speed. This mode is a good compromise between fully automatic and manual exposure when you really need to use a particular aperture. It's fast to use, too, because you don't have to worry about selecting the shutter speed.
One pitfall to keep in mind is that you might select an f-stop that is too small or too large to allow an optimal exposure with the available shutter speeds. For example, if you choose f2.8 as your aperture and the illumination is quite bright (say, at the beach or in snow), even your camera's fastest shutter speed might not be able to cut down the amount of light reaching the sensor to provide the right exposure. Or, if you select f8 in a dimly lit room, you might find yourself shooting with a very slow shutter speed that can cause blurring from subject movement or camera shake. Aperture priority is best used by those with a bit of experience in choosing settings.
Shutter Priority
Shutter priority is the inverse of aperture priority: You choose the shutter speed you'd like to use, and the camera's metering system selects the appropriate f-stop. Perhaps you're shooting action photos and you want to use the absolute fastest shutter speed available with your camera, as was the case for Figure 3.13. Or, you might want to use a slow shutter speed to add some blur to an otherwise static photograph.
Shutter priority is the inverse of aperture priority: You choose the shutter speed you'd like to use, and the camera's metering system selects the appropriate f-stop. Perhaps you're shooting action photos and you want to use the absolute fastest shutter speed available with your camera, as was the case for Figure 3.13. Or, you might want to use a slow shutter speed to add some blur to an otherwise static photograph.
Figure 3.13. Choose your own shutter speed for maximum action-freezing capability.
Shutter priority mode gives you some control over how much action-freezing capability your digital camera brings to bear in a particular situation. Keep in mind that your camera might have a more flexible program mode (described in the next section) that does an even better job. You'll also encounter the same problem as with aperture priority when you select a shutter speed that's too long or too short for correct exposure under some conditions. I've shot outdoor soccer games on sunny Fall evenings and used shutter priority mode to lock in a 1/1,000th second shutter speed, only to find my camera refused to shoot when the sun dipped behind some trees and there was no longer enough light to shoot at that speed, even with the lens wide open.
Program Modes and Scene Modes
Many digital cameras have specific user-selectable exposure programs specifically designed for certain kinds of picture-taking situations. These can do an even better job than semiautomatic modes like shutter- and aperture-priority, and can even do as well as an experienced photographer selecting settings manually.
One common mode is called Program, and it selects the correct f-stop and shutter speed using a database of picture information that tells it whether it's better to use a high shutter speed and wide-open aperture, or just the reverse. In most digital cameras, Program mode still allows the photographer to dial in some changes (as opposed to Auto mode, which locks you out and obstinately insists on making all the settings itself).
Other program modes are commonly called "scene" modes, because they are designed to work with particular types of scenes, such as portraits, night shots, fireworks, landscapes, or other situations. I've used digital cameras with as many as two dozen scene modes. For example, a digital camera I use has an Action program setting. In that mode, it will try to use the highest shutter speed possible appropriate for the lighting conditions. However (unlike shutter priority mode), if there simply isn't enough light, rather than prevent the camera from taking a picture, it will drop down to the next slowest shutter speed. The process is much like the one a veteran shooter will use to choose the optimal settings, but is much faster.
Your digital camera might have other program modes suitable for close-ups, telephoto shots, portraiture, night portraiture, sunsets, and other common shooting situations.
Many digital cameras have specific user-selectable exposure programs specifically designed for certain kinds of picture-taking situations. These can do an even better job than semiautomatic modes like shutter- and aperture-priority, and can even do as well as an experienced photographer selecting settings manually.
One common mode is called Program, and it selects the correct f-stop and shutter speed using a database of picture information that tells it whether it's better to use a high shutter speed and wide-open aperture, or just the reverse. In most digital cameras, Program mode still allows the photographer to dial in some changes (as opposed to Auto mode, which locks you out and obstinately insists on making all the settings itself).
Other program modes are commonly called "scene" modes, because they are designed to work with particular types of scenes, such as portraits, night shots, fireworks, landscapes, or other situations. I've used digital cameras with as many as two dozen scene modes. For example, a digital camera I use has an Action program setting. In that mode, it will try to use the highest shutter speed possible appropriate for the lighting conditions. However (unlike shutter priority mode), if there simply isn't enough light, rather than prevent the camera from taking a picture, it will drop down to the next slowest shutter speed. The process is much like the one a veteran shooter will use to choose the optimal settings, but is much faster.
Your digital camera might have other program modes suitable for close-ups, telephoto shots, portraiture, night portraiture, sunsets, and other common shooting situations.
Manual Exposure
Part of being an experienced photographer comes from knowing when to rely on your camera's automation, when to go semiautomatic, and when to set exposure manually. With all the automated features built into modern cameras, you should find that you'll rarely need to set an exposure manually. After all, built-in bracketing can create several shots at different settings for you, or you can use shutter-or aperture-priority when a particular shutter speed or f-stop is required.
Manual exposure can come in handy in some situations. You might be taking a silhouette photo and find that none of the exposure modes or exposure correction features give you exactly the effect you want. Set the exposure manually to use the exact shutter speed and f-stop you need. Or, you might be working in a studio environment using multiple flash units. The additional flash are triggered by slave devices (gadgets that set off the flash when they sense the light from another flash, or, perhaps from a radio or infrared remote control). Your camera's exposure meter might not compensate for the extra illumination (although some do), so you need to set the aperture manually.
Because you might not need to set exposure manually very often, you should make sure you understand how it works. Some cameras force you to switch to full manual mode, or wend your way through menus to activate manual exposure. I like my camera's procedure: Choose manual exposure, then set shutter speed or aperture with a command dial next to the shutter release button while holding down either a shutter speed or f-stop control button. Other cameras have separate command dials for shutter speed and aperture.
Manual exposure lets you apply special effects, such as the one shown in Figure 3.14. For that photo, I used a slow shutter speed but still froze the action by panning the camera in the direction of the movement during the exposure. Because the background is blurred, the emphasis is on the soccer players, who are relatively sharp. You'll learn how to pan action photos later in this book.
Part of being an experienced photographer comes from knowing when to rely on your camera's automation, when to go semiautomatic, and when to set exposure manually. With all the automated features built into modern cameras, you should find that you'll rarely need to set an exposure manually. After all, built-in bracketing can create several shots at different settings for you, or you can use shutter-or aperture-priority when a particular shutter speed or f-stop is required.
Manual exposure can come in handy in some situations. You might be taking a silhouette photo and find that none of the exposure modes or exposure correction features give you exactly the effect you want. Set the exposure manually to use the exact shutter speed and f-stop you need. Or, you might be working in a studio environment using multiple flash units. The additional flash are triggered by slave devices (gadgets that set off the flash when they sense the light from another flash, or, perhaps from a radio or infrared remote control). Your camera's exposure meter might not compensate for the extra illumination (although some do), so you need to set the aperture manually.
Because you might not need to set exposure manually very often, you should make sure you understand how it works. Some cameras force you to switch to full manual mode, or wend your way through menus to activate manual exposure. I like my camera's procedure: Choose manual exposure, then set shutter speed or aperture with a command dial next to the shutter release button while holding down either a shutter speed or f-stop control button. Other cameras have separate command dials for shutter speed and aperture.
Manual exposure lets you apply special effects, such as the one shown in Figure 3.14. For that photo, I used a slow shutter speed but still froze the action by panning the camera in the direction of the movement during the exposure. Because the background is blurred, the emphasis is on the soccer players, who are relatively sharp. You'll learn how to pan action photos later in this book.
Figure 3.14. Even slow shutter speeds work if you pan in the direction of the movement.
Locking Exposure
Another control to master is your camera's exposure lock control. As the subject matter seen through your viewfinder changes, the correct exposure might change, too. That's fine and dandy in most cases, because you want the camera to base its exposure on what's actually in the frame. Other times, however, you'll want to lock the exposure at a certain point. For example, you're shooting a portrait of someone outdoors; the sky behind her is very, very bright; the subject is located to one side of the frame; and you don't want a silhouette. Rather than switch to another exposure mode for just that one picture, compose the photo so the main subject is in the center of the picture. Then lock in the exposure (usually by pressing the shutter release button down partway). Then, reframe the picture with the subject located as you please. The photo will be taken using the exposure you locked in.
Some cameras have a spot exposure lock, too. Press the Spot button to switch to spot metering, keep holding the button down to lock the exposure, then reframe to take the picture. Others can lock the exposure using any metering mode. Using the exposure lock capabilities is generally a lot faster than switching to a special mode or specifying which exposure zone(s) to use.
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