11-09-2011, 08:26 AM
Most lenses perform more or less the same with smaller f-values. So it is not all that exciting to do MTF measurements for f-values smaller than f11.
Light gets "bent" (diffracted) around edges, and the smaller the hole the light has to pass through, the bigger precentage of the light will diffracted and end up in neighbouring pixels.
Of course, longer focal lengths have bigger apertures with the same f-value, so less light gets to be diffracted. But the apertures with longer lenses are further away too, so the diffracted light will hit further away pixels too.
You will find that with lenses of similar focal length diffraction will more or less always take a similar toll on the resolving power of the system, and even lenses with different focal lengths will show similar behavior.
There are some slight differences (due to factors I do not understand), but they are quite insignificant if sharpness id paramount to you (because then you have to shoot at f5.6-f8 or something).
Light gets "bent" (diffracted) around edges, and the smaller the hole the light has to pass through, the bigger precentage of the light will diffracted and end up in neighbouring pixels.
Of course, longer focal lengths have bigger apertures with the same f-value, so less light gets to be diffracted. But the apertures with longer lenses are further away too, so the diffracted light will hit further away pixels too.
You will find that with lenses of similar focal length diffraction will more or less always take a similar toll on the resolving power of the system, and even lenses with different focal lengths will show similar behavior.
There are some slight differences (due to factors I do not understand), but they are quite insignificant if sharpness id paramount to you (because then you have to shoot at f5.6-f8 or something).