I was wondering whether and how much the distance from the subject affects sharpness of the image taken by different lenses (on same sensor). Is there any website or source where such information may be available for Canon lenses?
This isn't a rumor. What do you mean by sharpness? If you mean the ability to resolve two lines a certain distance apart, then, all things being equal (sharp lens, not diffraction limited etc), the answer is simple. If you use lenses of different focal lengths, then the resolving power is directly proportional to focal length - a 400mm lens is twice as good as a 200mm. If you mean the object is at different distances, then the degree of detail resolved is inversely proportional to the distance away.
You can calculate the resolving power yourself if you know the width of a pixel, d
. Two lines in the image have to span 2 pixels to be resolved, ie 2d
. If those two lines are a distance D
apart in real life and they are a far-away distance S
from the lens, their separation in the image from a lens of focal length f
. So, those two lines can be resolved if (f/S