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RE: filmscanners:Focusing & film flatness
--- You wrote:
When I bought an expensive slide-projector about 10 years ago, I took it
straight back when it gave out of focus edges on curved slides. After some
argument, they ended up relenting and giving me a much better lens with
sufficient depth of field. It copes easily with flat and curved slides,
and so does my current scanner, a low end 2720 model.
--- end of quote ---
This is an interesting statement. The only things that affects depth of field
in a lens is its apeture or focal length. A 'much better lens' doesn't
necessarily imply either. What would affect edge sharpness is the flatness of
the lens' focal field (not necessarily the correct technical term.) Not all
camera lenses focus correctly on a flat surface and we are likely to spend extra
bucks on a flat field macro lens for really accurate copying work, for example.
We assume our expensive enlarging lenses are flat field.
But under some practical conditions, where film planes aren't actually flat, 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 likely
that a 'better lens' has a slightly curved field to match the assumed curvature
of a slide. 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
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, high
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?