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[OM] Re: autofocus in scanner

Subject: [OM] Re: autofocus in scanner
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Wed, 18 Jun 2008 05:52:17 -0400
It's necessary for the scanner to focus on the film and, more 
specifically, focus on the emulsion side of the film.  The scanner could 
have a fixed focus position if all film was perfectly flat or all slide 
mounts were exactly the same thickness but that's not true.  The scanner 
lens does have some depth of field but it is very shallow.  If the film 
is curved the center and edges will be at different distances and the 
scanner may have to focus to find the best compromise.

Most flatbed scanners typically have much greater depth of field, 
perhaps 10mm or more.  You can see this if you scan a three-dimensional 
object.  I don't think focusing is necessary on a flatbed.

Chuck Norcutt

Komtanoo Pinpimai wrote:
> I'm always curious what autofocus in scanner is actually mean because when
> we take a photo,
> we already have made focus on the camera lens, why do we need to do it
> another time when scanning.
> 
> I bought Minolta Dual IV film scanner years ago. On the spec, it's supposed
> to support autofocus,
> but when I enable it and scan with its software, it usually stopped scanning
> and freezes the process, the same happened with vuescan.
> I guess it's probably hardware or driver issue, I didn't bother returning it
> but turned the autoscan off, since then.
> The output of scanning is pretty okay, no weird blurring, or sign of
> out-of-focus which makes me more curious of how
> autofocus in scanning is supposed to improve my images.
> 
> It also sounds like autofocus in film scanner makes more sense than flatbed
> because we can place a film any where on flatbed scanner. It shouldn't know
> how to autofocus.
> So, another of my guess is scanning film on flatbed scanner cannot do
> autofocus, am I right ?
> 
> 
> 
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