On 13 Apr 2004 11:26:04 -0700,
(E-Mail Removed) (Neil Baylis) wrote:
>Do film scanners contain low pass filters like digital cameras do, to
>prevent aliasing of high frequency image components?
>
>If they do, I'm assuming that it would only be effective at the
>highest resolution, which raises the question of what happens when you
>scan at a lower resolution?
>
>One way to do it would be to acquire the image at the highest
>resolution, and then blur it (digitally) before downsampling. Is this
>in fact what they do?
>
>Finally, I'm wondering if editing apps like Photoshop apply a low pass
>filter before they downsample.. seems like it would be necessary. The
>evidence of my own eyes tells me that Photoshop is not doing this, or
>is not doing it enough, because downsampling does seem to introduce
>aliasing artifacts.
Anti-aliasing needs to be implemented optically,
at the time of the scan. The AA filter characteristic
needs to be matched to the mechanical resolution
of the sensing element.
I suspect both the scanner driver and Photoshop
are using good intelligence when they downsample --
ie., not simply "throwing away" data but averaging
the original data. You can prove this easily by
looking at a featureless area of a "real" film scan,
both before and after the downsampling, with the
histogram tool. The downsampled version will
a smaller standard deviation.
rafe b.
http://www.terrapinphoto.com
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