4 DSLR Questions you were afraid to ask--Part 41.) What's the difference between evaluative, centerweighted and spotmetering?
All camera meters try to make some part f the frame appear as a midtone---right between very light and very dark. Evaluative metering divides the frame up into segments, compares the reading in each section, and decides on an appropriate pattern for determining exposure. Centerweighted takes into account all the light, but puts a heavy emphasis on the middle of the frame. Spotmetering uses a small percentage of the frame--often less than 5 percent and typically in the center, though some cameras let you link it to the active AF point while evaluative is best for most situations, centerweighted works well for subjects such as group portraits. For the most control, use spotmetering to choose the midtone.
2.) Why use a handheld lightmeter when I have all those choices in the camera?
Because most handheld meters let you set your exposure based on the light falling on your subject, called "incident" light. They're great when your subject is unusually dark or light which camera meters tend to over or underexpose. They also help get consistent exposures in scenes with a wide range of tones. For an incident reading, hold the meter at the point on your subject where exposure is most critical, such as the face, and point it at the camera position. Many handheld meters can be set to read reflected light ad flash intensity, too.
3.) How do I tell how far my flash will reach?
By the Guide Number, assigned to almost all built in and shoe mount flashes and listed in feet, meters, or both in the back of you camera or flash manual. The GN tells you how far your flash will carry at a given aperture or what aperture to set for a given distance. Just divide the GN by either distance or f-number. For example, if a flash has a GN of 80 at ISO of 100, it can reach up to 20 feet with a lens set to f/4. If you want to shoot at 40 feet at the same ISO, you'd have to open your lens to f/2.
4.) My flash has TTL and Auto settings. What's the difference?
The TTL setting controls flash output by measuring how much flash reflects back through the lens (TTL) to the camera's built in lightmeter. It's usually the most accurate means of determining flash exposure, and it lets you aim your flash in any direction to bounce the light. It's accurate enough for most scenes and usually will function on many different cameras, while flash units that offer just TTL control work only with compatible cameras.
Dave Berry shares his Photography Tips at Computer School for Seniors (www.cs4seniors.com).