>From: Joe Gwinn <joegwinn@xxxxxxxxx>
>
>Aliasing is a consequence of the loss of information due to the resulting
>undersampling (too few samples to fully capture the image).
Someone has probably already jumped on you for this, but it is actually the
opposite: sampling at too high a frequency such that sampled frequencies above
the Nyquist limit (1/2 the sampling frequency) are included in the sampling.
They then cause heterodyning artifacts, similar to well-known Moire effects.
It's easier to comprehend in 1-dimensional space than 2D. Consider a very bad
CD recorder/player that samples audio frequencies at a 10,000 Hz rate. If you
feed the recorder an audio frequency of 6,200 Hz, the player will produce equal
amplitude frequencies of 3,800 and 1,200 Hz -- not at all what was intended,
and pretty awful sounding.
This is what so-called "grain aliasing" is all about: scanning at a frequency
that is near the RMS grain frequency of the image. Whereas if one scanned at a
much lower frequency, it would accurately reproduce whatever it picked up,
without ugly non-harmonic components.
Audio (and other 1D sampling systems) include a low-pass filter, to keep
frequencies near the Nyquist limit from producing sampling artifacts. CCD
scanners have no such capability, although drum scanners perform low-pass
filtering by varying the illuminated aperture that is sampled.
So yes, a lens CAN be "too sharp" for digital, if it allows spatial frequencies
at or above the Nyquist limit to be sampled. If your lens resolves (for
example) 150 lpmm, but your sensing elements are placed very much farther apart
than 300 lpmm, you'll have aliasing artifacts.
--
: Jan Steinman -- nature Transography(TM): <http://www.Bytesmiths.com>
: Bytesmiths -- artists' services: <http://www.Bytesmiths.com/Services>
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