Tests and additional commentary can be found here:
cameratown.com/reviews/canon5dMKIII/light_leak.cfm

4/11/2012 Update: I confirmed that the 5D Mark II also will change exposure even with just ambient ceiling light when you pull your eye away from the viewfinder. My EOS 60D will not, so this seems to have something to do with the positioning of the light meter inside the 5D series.

While it is a recommended practice to cover up the viewfinder if you plan to shoot without your eye against the viewfinder, it is virtually never done. However, light can enter the viewfinder, bounce off the mirror and fool the meter into thinking that the light is coming in from the lens. The side-effect of this would be an incorrect meter reading and ultimately an improperly exposed shot.

Light entering through the viewfinder has always been a concern when shooting in a situation where the sun is behind or above & behind the camera - as well as when shooting long exposures at night with brighter lights (street lights, etc) located behind the camera, but it has never really been an issue when shooting normal exposures inside under standard house lights - yet the 5D Mark III seems to be super sensitive even to ambient light entering through the rear viewfinder.

This video demonstrates that even with just a ceiling light on in the my home office, putting my eye to the viewfinder resulted in almost a full stop difference in metering, compared to when I moved my eye away from the camera.

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