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RE: filmscanners:Focusing & film flatness

At 12:49 PM 4/04/01 EDT, Rich wrote:
>>...they ended up relenting and giving me a much better lens with
>>sufficient depth of field....
>This is an interesting statement.  The only things that affects depth of
>in a lens is its apeture or focal length.  A 'much better lens' doesn't
>necessarily imply either. ...
>But ... a lens with a somewhat concave field is actually an advantage
since it would give
>you a sharp image on a curved surface.   In the projector business, it is
>that a 'better lens' has a slightly curved field to match the assumed
>of a slide.

Yep, they told me that the 'average' projector lens is just designed for a
flat plane, as you suggest.  But the new one they gave me, a Leitz
Colorplan I think it is, was designed for a slightly curved plane, in the
direction that most slides bow.  I presume they designed it for a middle
ground - it certainly does cope very well with flat slides as well as the
typical cardboard job, and of course those slides that 'pop' as they heat
up from the light.  (Was the bowing deliberately done by Kodak to avoid
that, I wonder??  Back in those days, I would imagine the vast majority of
slides were destined for projection, rather than the enlarger or a
film-scanner, where flatness is much more of a virtue!)  

>Increasing a projector lens' depth of field by reducing it's
>apeture is impractical since it would result in a much dimmer image on the

I didn't take notice of any aperture restriction.  (It's stored at the
moment, and I'm too lazy to drag it out and look!)  But I wonder just how
much restriction you would need to gain the required result?   This is
heading off-topic, so no answer required!

>So the question is, are the lenses in film scanners flat field, or are they
>slightly dished to accomodate film curvature?  Or are some small apeture,
>depth of field lenses working with more sensitive ccds. 
>Tony praises a fixed focus Minolta scanner which would have to fit the later
>category.   How about some others?

I know my Acer copes well in this area, so I just decided to push it and
find out.  I put 2 bowed slides in, one reversed, got it to focus on the
first (which I presume it does towards the centre of frame) and scanned
both at the same focus plane.  Sure enough, first one was sharp, inc.
corners, second one was blurred in centre, although the edges were OK..
That tells me that the depth of field is just about right, maybe 0.4mm (?)
as a wild guess..

Regards, Mark T.

Mark Thomas   markthom@camtech.net.au


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