I'm using a Canon EF 100mm f2.8L Macro IS USM on a Canon 600D (T3i) with firmware version 1.0.2.
When using live view, the camera's exposure simulation seems to automatically account for a decrease in effective aperture as you focus the lens to a close distance. For example, if you set the aperture to f2.8 (or any aperture wider than about f5.6) when focused at infinity (keeping exposure time and ISO fixed), and then you turn the focus ring so that you reach the closest (1:1) focus distance, the live view gradually makes the exposure preview (including the histogram) darker as you approach 1:1 focus distance, such that at 1:1 the exposure appears as though the aperture were set to about f5.6, even though the nominal aperture setting is fixed at f2.8.
This makes sense in principle because we know that closer focus distances decrease the effective aperture, and it seems that the live view is attempting to account for this decrease in effective aperture by showing a darker preview. In fact, the lens instruction manual mentions this decrease in effective aperture with close focus distances, as follows:
The aperture displayed by the camera assumes that the focus is set to infinity. The actual aperture (effective f-number) becomes darker (effective f-number increases) at closer focusing distances (magnification increases).
The problem is that the live view seems to be overcompensating by making the preview (including the histogram) significantly darker that it actually should be at close focus distances. For example, if I set the aperture to f2.8 and focus on a subject at 1:1 distance, the live view shows a rather dark preview, but when I actually take the photo, the image is much brighter than the preview indicated. We're not talking just a minor difference due to the preview not being able to perfectly predict the exposure; we're talking at least a whole stop brighter.
This means that if you are focusing at a close distance and you set the exposure so that the preview and histogram look properly exposed, the actual photo turns out overexposed. In order to compensate for this, you would have to set the exposure so that the preview looks underexposed (often making it difficult to see anything), and then it will turn out properly exposed.
If the live view were to compensate approximately correctly for the decrease in effective aperture, that would be a great, but since it overcompensates and makes the preview much darker than what you actually get when you take the photo, this seems to be a problem. Or perhaps I'm misunderstanding something about the situation.
Does this overcompensation occur with any other Canon body (such as the 5D Mark III) that is paired with the 100mm f2.8L Macro IS USM? Just to clarify, I'm wondering whether overcompensation (as described above) occurs on other bodies, as opposed to correct compensation.
If not, it would lead me to think that this issue might be limited to the 600D, or perhaps to 1.6x cropped sensor bodies, for one reason or another.
I have tried to search extensively on the internet for any mention of this behavior, and I have found nothing except for a single post from several years ago, which sounds like it's describing the same behavior with the 7D (in the second half of the post): https://www.dpreview.com/forums/thread/3016953
Thank you for your input!