[filmscanners] RE: Scanning with too much resolution? (was: PS sharpening...)

["Paul D. DeRocco" <pderocco@ix.netcom.com>]

What's happening is this. If you have an image that has lots of
high-frequency (spatial frequency, that is) irregular detail, such as grass
or leaves, and you downsample "correctly", the detail is averaged out. But
if you shoot or scan the image originally at a lower resolution, your camera
or scanner may not produce the reduced resolution correctly, but may merely
take point samples that are further apart. (The correct method would be to
blur the image to roughly the size of the pixels.) This results in the
high-frequency energy being aliased down to lower frequencies, so you end up
with the same general sort of irregular detail that you would have had in
the higher-res image. It isn't a true image at all, but in some cases, it
may be more pleasing to the eye, because people expect grass or trees to
have some sort of random detail in it.

That said, it certainly isn't true that lower resolution images are
_usually_ better than higher resolution ones--it's the exception, not the
rule.

--

Ciao,               Paul D. DeRocco
Paul                mailto:pderocco@ix.netcom.com

> Julian wrote:
>
> Margulis makes a claim in this chapter that struck me as totally
> counter-intuitive: sometimes a lower resolution scan will
> produce a better looking output than one made at higher resolution.
>
> According to Margulis, the higher res scan produces an image
> with smoother
> color variations, due to the statistical averaging in the downsampling
> process.

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