google will turn up lots of pages (many from engineering school labs ;-)
try for example
http://zone.ni.com/devzone/conceptd....e?OpenDocument

basically, one interpretation is that you have to sample at least twice
the rate of the maximum frequency you want to characterize. For us, it
means that a fixed sensor with say 100 pixels per mm can only respond at a
50 lpmm or lower rate.

So when a sensor that has say 124 pixels per mm is in a DSLR, it can
respond to 124/2 or 62 lpmm. Claims that it is recording 75 lpmm or any
higher value have to be challenged on the basis of the Nyquist theorem -
you aren't sampling fast enough to "see" a 75 lpmm image.

Even the maximum 62 lpmm would be hard to support, because the effects of
other system elements like the anti-aliasing low-pass filter and optics
and so on would likely reduce the system response below that of the sensor
alone. Most anti-aliasing cutoff filters are very sharp, and made just at
or above the Nyquist limit for the given sensor, another reason to doubt
higher lpmm (frequency) claims. This is done to prevent aliasing and moire
patterns and other image faults...

hth bobm

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* Robert Monaghan POB 752182 Southern Methodist Univ. Dallas Tx 75275 *
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