Doug Pardee
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stklaw wrote: It asks if I want average or evaluative when using flash.
Evaluative flash exposure mode is a "smart" mode that attempts to determine what your subject is, then expose that subject at midtone. As a general rule, it will take the brightest returns from the preflash as being your subject. That means that usually the closest object is considered to be your subject, although a white object a little farther away than a dark subject might be picked.
Evaluative generally works pretty well as long as you understand its limitations. Shooting past, over, or through something will tend to cause problems. If you shoot a subject through a doorway, the flash will expose for the doorway. If you shoot a subject down a hall, the flash will expose for the near parts of the hallway. And so on.
Evaluative flash exposure has special processing to try to ignore point reflections from an unbounced flash when trying to determine what the subject is. This didn't work very well in previous incarnations, but Canon claims that big advances were made in this processing starting with the 1DmkIII. If you bounce your flash, this isn't an issue anyway.
Because evaluative flash exposure takes the brightest return as being your subject, and exposes that at midtone, almost the entire histogram will be left of center. Some people freak out about that, but if you think about it it's perfectly reasonable.
In contrast, the average flash exposure mode is a "dumb" mode that simply blasts out enough light to bring the entire scene up to midtone. If the background is far away or non-existent (a subject standing outside in the open), the flash may need to nuke the subject until it glows in the dark in order to get the desired average results. This is how older flash exposure systems—thyristor autoflash, for example—work and many people have come to think of this as "correct" flash exposure. Those people tend to think that evaluative flash exposure is "too dark" because the subjects aren't blown out and glowing.
Edited to add: The histogram from the average flash exposure mode looks more "normal", too, although for the typical case where the subject is closer to the flash than anything else is, the subject is way up on the right side of the histogram. Still, many people value a balanced histogram over a properly exposed subject.
For fill flash, evaluative flash metering is (IMO) the clear method of choice. Also note that in daylight conditions, evaluative flash metering will automatically reduce the flash to prevent overexposure from the combined effects of ambient and flash, while average flash metering will not… with average flash metering you must adjust FEC yourself.
A final note: with Flash Exposure Lock the flash metering mode is overridden to Spot (if available on the particular camera model) or Partial. The subject is presumed to be at the center of the scene when the FEL button is pressed, and that subject is exposed at midtone. When using fill flash in daylight conditions, it is up to the photographer to adjust FEC to prevent overexposure when using FEL.
Edited by Doug Pardee on Mar 31, 2008 at 01:10 PM GMT (Reason: Added comment on histogram in average mode)
Edited on Mar 31, 2008 at 09:10 PM
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