>From: Winsor Crosby <wincros@xxxxxxxxxxxxx>
>
>It is amazing how pleased most people are
>with their 4 and 5 megapixel cameras and if you do the math it is
>impossible. I think it may have something to do with the fact that the
>scan of a piece of film is a second generation image adding the flaws
>of film to the flaws of scanning.
Sampling always produces artifacts. The two main ones are low pass and
aliasing, and they are at odds with each other. Selecting the proper sampling
aperture is always a compromise.
When you sample near high frequency components, it generates low frequency
components (heterodynes) that are arithmetically related to the high frequency
component and the sampling interval. These look ugly! Do a search on "grain
aliasing" for more info on this nasty affect.
If you sample far enough below the grain frequency to avoid alias artifacts,
you end up clipping authentic high frequency information in the image, making
it look soft. This is why you always need to do a bit of sharpening after a
scan, adding back in synthetic high-frequency detail that was lost during
scanning.
Electronic sampling systems (such as that used in CD players) are able to use
extremely sharp filters to both capture authentic high frequency information,
yet avoid aliasing. Unfortunately, optical sampling systems cannot do such a
thing easily, and typically have a Gaussian response, which often results in
BOTH softening and grain aliasing at the same time!
Perhaps as Moore's law progresses, we'll see sensors so cheap that one can scan
with a staggered array, with multiple sensors at precise fractions of a
wavelength of light apart from each other. Then you could use synthetic
aperture imaging to low-pass the spectral information and get good high
frequency content without aliasing. But I suspect both film, and the need for
such super-quality scans, will be long gone by then!
To make a long story short, a good 5MP digicam will produce a better image than
a 5MP film scan.
--
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