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.

By The Editors

Labels: How-To, Camera Technique

This Article Features Photo Zoom

Evaluative Metering The DSLR divides the image area into many segments to generate accurate exposures in a wide range of situations. It's best for general landscapes.

Nikon D300S Meter-Mode Switch

Let's start with the basics. An exposure meter measures the amount of light that strikes it and, based on this data and the ISO you've programmed into it, provides you with a series of shutter-speed/ƒ-stop combinations that will produce proper exposure for the metered subject or scene. So goes the theory.

"Proper" exposure is the one that produces the results you want in your photograph. If you want your image to look like the scene you photographed, with detail from the brightest important area to the darkest important area and a natural range of tones, the camera's built-in exposure meter will deliver that—assuming the scene has an "average" mix of tones and its brightness range doesn't exceed the film's or sensor's capabilities. And, of course, artistic expression may cause you to vary from a literal rendering of the scene.

If you give a scene too much exposure, the resulting photo will be too light—the darkest tone will be gray instead of black and the brighter areas will be blown out. If you give a scene too little exposure, the resulting photo will be too dark—the middle tones will be dark gray or black and the brightest areas will be muddy gray instead of white. So learning how your camera's metering system handles various scenes and exposure situations is an important part of being a good photographer.

Partial Metering Unique to Canon DSLRs, this mode reads only the central portion of the image area. It's especially useful when the background is much brighter than the subject.

The main idea behind reflected-light exposure meters (which includes those built into cameras) is that they're calibrated so that whatever you take a reading from will be reproduced as a medium tone in the photo if you expose according to the meter reading. If you take a reading from a medium-toned gray card and expose accordingly, the card will appear medium gray in the resulting photo. If you take a reading of a black card, less light will reach the meter, so it will call for more exposure, and the result will be the black card appearing as a medium-gray tone in the photo. If you meter a white card, more light will reach the meter, so it will call for less exposure, and the white card will appear medium gray in the resulting photo.

If all three cards appear in the photo, what's the correct exposure? The one from the gray card. If you expose for that (by metering it), the gray card will appear medium gray in the resulting photo, the black card will appear darker and the white card lighter. If you meter the black card and expose accordingly, the black card will appear medium gray in the resulting photo, and the gray and white cards will be too light. If you meter the white card and expose accordingly, the white card will appear medium gray in the resulting photo, and the gray and black cards will be too dark.

TTL Metering
The meters in DSLRs (and most film SLRs) read the light through the lens. This provides a number of advantages over an external exposure meter. First, the meter reads only the light from the image area, regardless of lens focal length. Second, you don't have to carry an external meter. Third, the built-in TTL meter is connected to the camera's controls, so exposure automation is possible. The camera can set the shutter speed and/or aperture automatically based on the meter reading. While it can be "fooled" by particularly bright or dark subjects or scenes, TTL metering is a good thing. It speeds up shooting, makes things easier for the less experienced photographer and gives even the experienced shooter a good starting point.

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