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[OM] Re: autofocus in scanner

Subject: [OM] Re: autofocus in scanner
From: Moose <olymoose@xxxxxxxxx>
Date: Tue, 24 Jun 2008 02:02:02 -0700
Chuck Norcutt wrote:
> 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.
>   
It does vary with different brands and designs. The Nikons and the 
Minolta 5400 have had lots of reports of focus problems and some 
examples posted on the web. it seems that ther DOF is not always 
sufficient to cover come curved film without care in setting the focal 
point for AF or using manual focus. Some users have trouble and others 
don't.

The Canon film scanners have a different lens design with greater DOF 
and seem to have no focus troubles.

I seem to recall that at least some film scanners autofocus using 
triangulation, bouncing a light off the film and focusing to maximize he 
reflected light captured by a photocell. That way, scanner focus is 
independent of content or focus of the image itself and only controls 
lens to film distance.
> 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.
>   
True at low dpi settings with large objects lit by the platen light. Not 
necessarily true of high resolution settings lit by the film light in 
the lid. I odn't remember the details, but I know some scanners, maybe 
Epson V700 series, have adjuster blocks on film holders to make small 
changes in focal distance. Whichever one(s) it was, Vincent at photo-i 
found significant differences with height settings in his tests.

I've also seen claims in forum posts that slide mount thickness can make 
a focus sharpness difference on some flatbed scanners.

Moose

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