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One thing that the 5D Mark III doesn't do well is to take pictures of the backside of a lens cap when it is on the front of the lens and I am not kidding . You will get only mediocre results, at best.
The lens cap is the opaque protectant filter.
Try substantially increasing the exposure. A full day of sunlight at high ISO might allow sufficient light to creep in behind the opaque protectant filter to get a good exposure of the backside, with a macro lens manually focused.