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.
Most cameras offer three basic metering modes: multi-segment, center-weighted and spot.
With multi-segment metering, the system divides the image area into a number of segments (Nikon's D7000 uses a 2,016-pixel RGB metering sensor; more common are double-digit numbers of segments), then uses brightness data from those segments, subject-position data from the AF system and, with some systems, even color data to calculate the best exposure for the subject/scene. While it's not right all the time, multi-segment metering is a good one to use for general shooting, when you're shooting quickly or when you don't want to think about metering—it will give the best exposures for the greatest number of scenes/situations.
Center-weighted metering reads the entire image area, but places most of the emphasis on the central portion (where the subject often is), helpful when the subject is against a brighter or darker background. Center-weighted metering was popular in the early days of TTL metering, but multi-segment produces better exposures more often, and spot metering provides more control when you want it. Some cameras let you adjust the size of the center-weighted area. Center-weighted metering is good for portraits.
Spot metering reads a small area of the scene (generally indicated by a small circle in the center of the viewfinder) and allows knowledgeable users to control exposure more precisely, based on the fact that whatever you take a spot reading from will be reproduced as a medium tone in the photograph. Meter an important portion of the subject or scene, decide how you want it to appear in the photo (medium, darker or brighter), and adjust the exposure accordingly. If you meter a darker area and want it to appear dark, give less exposure than the spot reading calls for; if you meter a bright area and want it to appear bright, give more exposure than the reading calls for.
How much more or less exposure should you give? That depends on how much brighter or darker than medium tone you want the metered area to appear. The Zone System provides a very precise way of controlling exposures using a spot meter when shooting film and can be adapted to digital to a degree, as well. You could start by metering a medium-gray card, exposing per the meter reading, then shooting additional images giving more exposure and less exposure in 1/3- or 1/2-stop increments out to 4 or 5 stops beyond the initial exposure. That will give you an idea of how your camera responds to changes in exposure.
You can use the spot meter to determine the approximate brightness range of the scene before you—just meter the brightest important part and note the reading, then read the darkest important part and note that reading, and see how far apart the readings are. For example, if the meter called for a shadow exposure of 1/15 sec. at ƒ/8 and a highlight exposure of 1/2000 sec. at ƒ/8, the range is 7 stops.
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