Monday, October 1, 2007
Best Techniques For Digital Exposures
Setting everything on full auto isn't always the ideal solution. Try these tips to get your best shots every time.
Exposure meters measure the amount of light that strikes them, then provide you (or your camera, with built-in meters) with a series of shutter-speed/ƒ-stop combinations that will result in a proper exposure for the ISO speed in use. Simple enough, right?
Well, that depends on at what you point the meter. Exposure meters are calibrated to recommend exposures that will properly reproduce a medium-toned subject. How well brighter and darker subjects reproduce depends on how much brighter and darker they are than a medium-toned subject, and the dynamic-range capabilities of the film or digital camera’s image sensor, A/D converter and imaging engine.
There are two basic types of exposure meters: reflected light and incident light. Reflected light meters measure the light reflected from (or emanating from, in the case of light sources) whatever you point them at; incident light meters measure the light incident (falling) on the subject. Both types are calibrated to reproduce a medium tone as a medium tone in the resulting photograph.
If you expose according to a reflected-light reading, whatever you take the reading from will reproduce as a medium tone in the resulting photograph. The problem with reflected light meters (and the meters built into SLR cameras are reflected meters) is that not everything you read with them is a medium tone. Problems occur if you meter a subject that’s brighter or darker than a middle tone.
More problems can occur when you meter a scene containing a bright object against a dark background. The meter can be influenced by the dark background, calling for too much exposure and blowing out the bright subject. Conversely, when you meter a scene containing a dark object against a bright background (as when photographing a backlit subject), the meter can be influenced by the bright background, calling for too little exposure and rendering the dark subject as a silhouette with no detail.
In order to deal with these problems, camera manufacturers have come up with multi-segment metering systems that break up the image area into a number of parts, then take into consideration such things as the overall brightness, brightness distribution, brightness of the subject (the AF system provides the metering system data on the subject’s position and distance), contrast and, in some cases, even color. The camera’s onboard computer then uses complex algorithms (and with some cameras, an extensive onboard library of actual scenes) to come up with a good exposure in an amazingly wide range of exposure situations. I use my cameras’ multi-segment metering most of the time, overriding it (via exposure compensation) when experience has shown that I must.
Another way to overcome the problems of reflected meters being "fooled" is to use a handheld spot meter (or the camera’s spot-metering mode, if it has one) to read specific areas of the scene. Read the main subject, and if it’s of medium brightness, expose according to the meter’s recommendation. If the subject is particularly bright and you want it to appear that way in the image, give a stop or two more exposure than the spot reading suggests. If the subject is particularly dark and you want it to appear that way in the photo, give a stop or two less exposure than the spot reading suggests. You can also use the spot meter to check the brightness range of the scene, reading the brightest and darkest important areas to see how much they differ. With a little experience, you’ll learn how much brightness range your film or digital camera can handle, and how to expose the subjects you regularly shoot to get them to appear the way you want them to in your photos.
Probably the most accurate way to expose with a spot meter is Ansel Adams’ famous Zone System. We don’t have room to go into that here, but Adams’ book The Negative lays it all out beautifully.
Another way to avoid problems that can fool a reflected light meter is to use an incident light meter. Incident meters read the light incident (falling) upon the subject, rather than the light reflected from it. Thus, they can’t be "fooled" by particularly bright or dark subjects or backgrounds. But incident meters must be positioned so the light striking them is the same as the light striking the subject (so you can’t use one here in the shade to determine the exposure for that glorious sunlit Kodiak bear just across the river). They can’t determine the correct exposure for light sources, such as fires and sunsets, and don’t tell you anything about the relative brightness of the subjects in your scene.
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