TCampbell wrote...
Keep in mind that when you use a "reflective" meter, several things can throw the metering.
(a) are you using full 'evaluative' metering, or have you switched to an alternate metering area? (e.g. spot metering, etc.)?
(b) have you dialed in exposure compensation without realizing it? (your EXIF data will tell you if you dialed in exposure compensation).
(c) are you metering subjects that are known to throw metering? (e.g. metering very dark things or very white things will throw the accuracy of the metering. That's normal for in-camera meters because of the nature of "reflected" light vs. "incident" light.)...
Tim is right, but he mixed up the reflective vs incident metering ininitially, so I corrected it above.
Your 5DIII uses a reflective metering system. All cameras with through-the-lens meterings systems have to use this type. As Tim mentions, reflected light is highly susceptible to being skewed by subject tonality.... so, for example, a mostly white snow scene would tend to underexpose badly, while a mostly black coal mine would want to overexpose heavily. The metering system is only able to interpret everything as "medium gray". This is the reason our cameras have Exposure Compensation, to correct these "errors" caused by extremes of subject tonality.
Separate handheld light meters are available in a variety of types... one of which is an incident meter. When used correctly, this type of meter measures the light falling onto the subject, instead of what's being reflected off of it. This makes for a much more accurate reading, in many cases, because it's not subject to the skewing by subject tonality.
Withouit seeing an example image with EXIF data intact, it's hard to say with much certainty. But, another possiblity is the wrong camera metering mode being used. You have choice of Evaluative, Center Weighted, Partial and Spot metering patterns .
Evaluative and Center Weighted similarly measure most of the image area seen in the viewfinder. They differ in that Evaluative places more emphasis right around the active AF point, sort of assuming that's your primary subject. Center Weighted simply puts a bit more emphasis on the center of the image area, as the name implies. Partial measures a somewhat smaller area and Spot meters just the very center of the image, as indicated by a circle seen in the viewfinder. All these have their purposes. Evaluative is probably the best to set as default, will usually give good results. Partial and especially Spot have to be used correctly, can cause problems if accidentally set and used wrong.
Sure, there is possibility that something is out of calibration in your camera. Something faulty like lens aperture that's not working correctly also might cause a lot of incorrectly exposed images. But a lot of the time it's simply something set wrong. We may be able to tell more, if we can see a sample "bad" image, as Tim requested.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & EXPOSUREMANAGER
