Bill wrote
>>
"Even if digital is close to or surpasses film you would think there would
be some concern about the glass. When did that go away? Or do I just not get
it? :-)"
An interesting point is that, several years ago, about the same time we were
being "educated" about the way light exits a lens, and what is needed for
digital, someone (camera manufacturer, chip manufacturer, or something like
that) was presenting the theory that if a lens is "too sharp" it makes for
bad digital photos. I remember this theory was received with a surprisingly
little raising of eyebrows.
<<
Bill the lens "too sharp" is very well understood issue. Unless you sample the
picture at a resolution (pixels per inch etc) higher than that that the lens
is capable of, you get "aliasing" on the finest detail in the picture. This can
be subtle as in increased noise, or gross, as in moire patterns wherever fine
repetitive patterns exsisted in the original scene. Color Moire is
particularly objectionable. To combat this in the high end cameras using older
35mm lenses they usually just add a plate right in front of the "film plane"
that effectively smears the light over adjacent pixels. In signal
processing/engineering terms this acts as a "low pass filter" and removes the
high frequency signals that are being undersampled by the too small number of
pixels per inch. In photographic terms it cuts the resolution that the lens is
capable of. If you look on some of the digital camera discussion groups there
are long discussions of how to remove these plates so you can get your m!
oney's worth!
This is also why you should always scan using a very high resolution and then
use photoshop etc to decimate the image to the actual lower resolution you
need. This reduces aliasing of high frequency fine detail and aliased grain
noise etc. Not all photo editing software will decimate by averaging. If
decimation is strictly via ignoring the pixels discarded ,this does not help
aliasing and it then just takes longer. The after the fact "anti-aliasing"
filtering plug-in software in general, is not able to eliminate aliasing
without causing other problems.
Regards,
Tim Hughes
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