5d mark 3 viewfinder light leak question
All canon's I've owned, in fact all dslr's film or digital do this.
It's why on the 1d series cameras you have a eyepiece shutter lever that blocks the light in the viewfinder if you're not going to be looking through it.
And the 5d series cameras come with a rubber eyepiece cover to put over in place of the eye-cup to block light from the viewfinder.
Not only can this change the metering, but the stray light that comes in through the eyepiece can 'fog' the image in long exposures as the light coming in this reverse route is not focused light like comes through the front lens.
Work harder, you'll make it happen on all cameras.
John Mason - Lafayette, IN