I don't believe there are any endemic issues with metering on the 5D III. The 5D III, incidentally, actually has the SAME dual-layer Canon iFCL (intelligent Focus Color Luminance) metering system as the 7D.
I cannot say for certain whether or not there is an issue with your specific Canon 5D III, but I can say for certain that my Canon 5D III is bang-on accurate (and I know this... I've carefully tested it and built a callibration profile using software to check it.)
But in fairness, I must also point out that the difference between "incident" metering and "reflected" metering as well as things such as metering modes (which zones it uses and how it uses those zones), and the use (even if unintentional use) of exposure compensation features in the camera (and depending on the camera mode... selecting the wrong focusing screen type. That doesn't apply to the 5D III since it doesn't have swappable focusing screens) can all factor in to someone assuming there's a problem ... even if the problem is user knowledge (the camera was only doing what it was told to do.)
I have EVEN seen exposure issues blamed on the camera when really the issue was a dim LCD screen (always use the histogram to check the exposure and know how a histogram works.)
I have a Sekonic L-758DR hand-held light meter. It's the flagship meter by a company who pretty much only makes light meters -- that's their entire business. So their meters tend to be pretty good (and they're a top-rated name in light meters.) I normally advise caution when comparing the meter reading of an "incident" meter to the reading of a camera's built-in "reflected" meter because of the differences in how the metering systems work. But in this case, the Sekonic L-758 happens to also include a built-in reflected meter that includes 1º spot metering (an extremely tight angle of view -- much tighter than the camera's spot metering mode.)
I've tested the 5D III against the Sekonic and find that when both are in spot-metering mode and I carefully select the target, they are bang-on accurate in that they both precisely agree on the meter reading. But note that as I sweep-around the subject area with the spot, the meter reading will change (and you don't have to move far to get it ot change). That means you do need to be careful to ensure you really are metering the same subject with both meters.
If you've dialed in exposure compensation (even if by accident) that will show up in your EXIF data (you should see a row for "exposure compensation" and it should read "0". If it reads any other value (on the negative side) then the underexposure is the photographer's fault (they told the camera to underexpose the shot... it was doing as it was told.)
If the photographer is metering a largely "white" scene (or spot metering on a "white" -- or near-white part of a scene) then the underexposure is the photographer's fault.
I obviosly cannot know if your camera is working problem or if the issues are caused by your own mistakes... I can only offer mistakes that are commonly made so that you can check to make sure "you" aren't the cause of the underexposure.
You can use a gray card to check your meter. Just be careful because nearly all gray cards are designed to provide 18% gray and most cameras are callibrated to be closer to 12% gray. That's a 1/2 stop difference.
This means that if you point your camera at the gray card and move in close to get the gray card to "fill the frame" (that's usually not necessary as long as the camera's metering coverage area is all on the surface of the gray card), take that shot, and then inspect the histogram. You SHOULD notice the peak on the histogram is left of the center line (if it were a 12% gray card it would be "on" the center line of the histogram). But since the difference between an 18% gray and 12% gray card is only about 1/2 stop, if you deliberately over-expose by 1/2 stop above whatever the meter suggests (or dial in +1/2 stop worth of exposure compensation) then the histogram peak sould be "on" the centerline of the histogram. Of course the camera typically allows exposure compensation in 1/3rd stop increments, not 1/2 stop increments, but if I do this with my camera it does nail the exposure on my 18% gray card (and it also perfectly agrees with my Sekonic L-758 meter.)
Notice I mentioned comparing the camera to a known-accurate meter (an independent source to validate my results) and also included the use of a gray-card (although it's an 18% gray-card and not a 12% gray-card but you can compensate for that if you understand how gray-cards work)? This is because this provides a reasonable and, more importantly, a specifically measurable basis upon which to form a conclusion.
I should re-assert something I mention in threads from time to time: If you want to TEST your camera's accuracy (in this case for metering accuracy), then do it scientifically. YOU MUST CONTROL THE CIRCUMSTANCES OF THE TEST!!! I cannot stress this enough. What you absolutely MUST NOT do is take a photo of a typical subject and use that to conclude there is a problem with the "camera" (anytime someone does this, I immediately recognize that the real problem is the 12" behind the camera.) You can use your normal photographic results to "suspect" a problem with the camera, but you must be scientifically measureable and accurate methods to "validate" that suspicion.
Tim Campbell
5D III, 5D IV, 60Da
