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Point-and-shoot cameras don't give you much input into adjusting exposure. You point, you shoot. You have a dozen or two Scene modes that can adjust exposure and some other settings based on the kind of picture you're taking.
As you'd expect, digital SLRs (dSLRs) have the same roster of exposure controls found in even the most advanced non-SLR digital cameras. But dSLRs give you a lot more to work with, too, because, while getting a good exposure is relatively easy, getting the best exposure can be tricky.
Metering modes
All digital cameras have built-in number-crunching routines that let the camera read the lightness and darkness values from a range of locations (or matrix) in the scene and calculate the best compromise in terms of exposure. So, your camera can offer the following metering modes:
- Matrix metering: This is usually the default mode. The camera collects information from multiple locations in the scene, which can range from a dozen or so to more than 1,000 locations, as shown in Figure 1.
Figure 1: Exposure info may be gathered from a few locations or, as in this figure, from more than 1,000 invisible points arranged in a matrix.
- Center-weighted metering: In this mode, the camera puts the most emphasis on the information in the center of the scene, but takes into account the illumination in the rest of the frame.
- Spot metering: In spot mode, the camera derives its exposure setting exclusively from an area in the center of the frame.
Correcting exposure
Digital SLRs have an adjustment system called EV (exposure value) compensation. With dSLRs, this adjustment is easy to apply: You simply press the EV button on your camera (usually it's on the top panel) and use a command dial or left/right cursor key to add more exposure (+EV) or reduce exposure (–EV) using 1/2- or 1/3-value increments.
You can determine whether to add or subtract EVs by reviewing an image you've just taken on the LCD or by reviewing a picture's histogram, which can be displayed on the LCD along with the image itself.
Your goal is to produce a histogram in which the entire "mountain" is solidly in the center of the chart, with none of the dark tones lost at the left side or light tones at the right. If you look at a histogram of your most recent shot carefully, you can learn to add a little exposure (+EV) when the dark tones are cut off at the left side or subtract a little (–EV) when the light tones are cut off on the right side. (See Figure 2.)
Figure 2: An underexposed image (top), correctly exposed image (middle), and overexposed image (bottom).
Other exposure adjustments
You have several other options for changing your camera's exposure setting:
- Scene modes: Scene modes are the modes offered on many digital SLRs that are similar to the scene options found in other digital cameras.
- Exposure lock: The exposure settings are fixed when you press a special exposure lock button or simply press the shutter release down halfway.
- Bracketing: Bracketed pictures are a series of photos taken at a particular setting and then taken at a "plus" and "minus" setting.
- Shifting equivalent exposures: Most dSLRs allow you to choose equivalent exposures without changing the actual amount of light that reaches the sensor.
Shooting modes
Metering modes are the options for collecting exposure information and calculating an exposure. Shooting modes tell your camera how to apply that information. The following shooting modes are the most used by dSLR owners:
- Auto: Full auto mode handles all the settings for you, with (often) no options for adjustment.
- Programmed: Your dSLR uses its built-in smarts to analyze your scene and come up with a recommended exposure.
- Shutter priority: You choose the shutter speed, and the camera locks it in, which makes this a good choice for sports shots like the one shown in Figure 3. The autoexposure system keeps the shutter speed you've chosen and varies only the aperture to achieve correct exposure.
Figure 3: Shutter priority lets you lock in a high shutter speed to freeze action.
- Aperture priority: This mode lets you choose the f-stop, letting the camera select the shutter speed needed for correct exposure.
- Manual exposure: Use manual exposure when you want to set both the shutter speed and aperture yourself. Manual exposure is a good choice if you're using a nonautomatic flash unit or an older lens that doesn't allow automatic exposure with your camera.
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