Re: Michael Reichmann reasoning for AA filters?

In article <1192839046.272956.35020@xxxxxxxxxxxxxxxxxxxxxxxxxxx>, acl <achilleaslazarides@xxxxxxxxxxx> writes

Actually a step function causes ringing (thus power in higher
frequencies).

It causes ringing in the fourier domain (time in audio, space in imaging) not in the frequency domain. There is NO power at higher frequencies - that is the point of the stop filter.

Anyway, physically I can't see how to construct a step function filter
for optical transmission (not that you said such a thing is feasible,
I'm just wondering).

You can't, and it is fairly obvious why. If you look at the fourier terms required to create a brick wall type filter, the even terms are positive, but the odd terms are negative. That's easy enough to implement in an electronic signal, such as an audio signal. However there is no such thing as negative light intensity, black (ie. zero intensity) is as low as it gets. Consequently it is impossible to create an optical brick wall spatial filter, ie. an ideal anti-aliasing filter and all practical optical AA filters lose some MTF below the critical Nyquist frequency, whilst having significant residual MTF above it. They are, however, better than no AA filter at all.
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