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Sensor pixels are sensitive to the angle of light striking them. They usually are setup to only receive light within 15 degrees off vertical. The microlenses are designed to "straighten up the light".
This probably accounts for the drop off in performance with faster F ratio.
It can be an important factor. The Sony Nex 7 has an engineering fault where an ultra wide lens causes colour shift to magenta because of the above.
Greg.
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