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Tuesday, July 26, 2011

The Perfect Exposure

The various in-camera metering modes and how they’re used are mysteries for most photographers. OP guides you through the alphabet soup and confusing monikers so you can use your meter to the fullest and get perfect exposures.

This Article Features Photo Zoom

Digital Exposure Advantages
Digital cameras offer two big exposure advantages over film cameras: instant playback and histograms. With a digital camera, you can check the image right after you shoot it on the camera's LCD monitor. If it's over- or underexposed, you'll know at a glance and can reshoot it then and there.

You also can display a histogram for any image. A histogram is just a graph showing the distribution of tones in the image, from darkest at the left end to brightest at the right end. Each histogram will be different, as each image is different. A low-key image containing mainly dark tones will have most of the histogram toward the left side, while a high-key image consisting mostly of bright tones will have the histogram loaded to the right.

The main thing to keep in mind about the histogram is that you don't want it running off either end, especially the right end. If it runs off the right end, you've blown out the highlights. If it runs off the left end, you've lost the shadows. A quick glance at the histogram will tell you instantly if either has happened. If the histogram runs off the right side, make another shot, giving less exposure. Generally, you'll want the histogram to run near to, but not off, the right edge. If the histogram runs off both edges, the scene's brightness range is too great to record detail throughout, and you'll have to choose whether you want to lose highlight or shadow detail (generally, it's better to save highlight detail and let the darkest areas go black, but that's up to you, the artist, to decide). (Note that the histogram is for a camera-processed JPEG image, even if you're shooting RAW, and thus isn't necessarily an accurate representation of the RAW file data. Likewise, the image you play back on the LCD monitor is an in-camera JPEG, even if you're shooting RAW.)

Spot Metering Although unforgiving, spot metering is the best choice for situations where you need to base the exposure on a particular portion of a scene or subject. It's also ideal for checking the overall range in a scene and applying the Zone System. With most cameras, spot metering can be linked to the active AF point; it needn't always be the center point.

In the early days of built-in TTL metering, it was all center-weighted average. Today, multiple metering modes and exposure algorithms provide for greater accuracy and control, as well as higher success rates with a range of subjects and scenes. It's difficult to apply hard-and-fast rules to one metering mode versus another. By understanding how each mode works, you can determine the best choice for any scene. Generally, evaluative metering is a good all-purpose mode. The other modes come into play where the light or subject presents some challenges. Always keep in mind that meters are tools; they don't think and they don't know what you've previsualized. Our spot-metering example shows a scene where you need to use the meter to evaluate the range and then set the appropriate exposure for the effect that you want.

You can try using the camera's range-expanding tools (Canon's Auto Lighting Optimizer and Highlight Tone Priority, Nikon's Active D-Lighting, Sony's D-Range Optimizer, etc.) or built-in HDR, if available, to retain more detail from highlight through shadow. You also can shoot a bracketed series of exposures (using a tripod to hold the camera in position), then merge the highlight data from the underexposed image and the shadow data from the overexposed image in Photoshop or other image-editing software, or using HDR software.

Exposure Compensation
Exposure compensation lets you direct the camera to give more or less exposure than the built-in meter suggests. This is handy if your camera consistently under- or overexposes slightly, or when shooting a particularly difficult scene that "fools" the built-in meter. If your image appears too dark or too light when you check it on the LCD monitor, dial in some exposure compensation and shoot another image. (Keep in mind that some metering systems break down the scene into many segments, and if you move the camera slightly between the original shot and the compensated one, that can change the metered exposure; with such cameras, it's best to use a tripod to avoid this possibility.)

Auto-Exposure Modes
All DSLRs offer four basic exposure modes, and some offer additional ones. The basic exposure modes are programmed AE, shutter-priority AE and aperture-priority AE, plus manual.


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