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- Aug 16, 2012
A 2000mm f/15 with a 133mm front element will greatly outresolve an 800mm f/11 which has a 73mm front element. For an extremely high resolution sensor, the resolution of the 2000/15 will be 133/73 times that of the 800/11, and this will tend to 2000/800 as the sensor resolution goes down. Briefly, the size of the diffraction disc from the f/15 will be 15/11 times that from the f/11 but the magnification is 2000/800 times that of the 800mm. The size of the diffraction disc becomes unimportant for low resolution sensors as the pixels become larger than the diffraction disc, and the resolution then depends on the focal length of the lens. For very high resolution sensors, the resolution of a lens depends on the diameter of its front element (entrance pupil).Doesn't diffraction at f/15 spoil the resolution too much? Would it be any sharper than 800mm at f/11?