Modulation transfer function (MTF)

Hi,

I'm going to evaluate the MTF of an imaging system. I have a MatLab code and it works fine and my question is not related with programming. I would like to know why spatial resolution is evaluated at the 50% of the MTF. In the web it is written that it is because it correlates well with our perception of sharpness, but other web site (wikipedia) says that is it raleted with the best focus position... Is there any good reference that explains properly this point? Thank you

[Francesco]

I forgot to mention that sometimes it is evaluated at 30% of MTF... but I can not find any clue so far

Answer 1

[Image Analyst]

I suggest you ask in the sci.optics or sci.image.processing newsgroup. My guess is that is just the convention, though some people have different conventions. Or do a search for "MTF cutoff frequency"