Scanning glass mounted slides in Nikon ED IV

  • Thread starter Thread starter Cedders
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Cedders

I have a large number of glass slides to scan. I am wondering whether the
auto focus on the Nikon accounts for the thickness of the glass, or does it
focus on the surface of the glass? I've tried using the manual focus
adjustments but can't seem to get a really sharp image. Any ideas on this
one?

Thanks
Cedric
 
Cedders said:
I have a large number of glass slides to scan. I am wondering whether the
auto focus on the Nikon accounts for the thickness of the glass, or does it
focus on the surface of the glass? I've tried using the manual focus
adjustments but can't seem to get a really sharp image. Any ideas on this
one?
Click to expand...
Autofocus requires a texture to focus on, which is usually the film
grain itself. Consequently it *should* focus on the film emulsion,
taking account of the thickness of the glass slide mount. However, if
you are having problems focussing manually then it may be that the
thickness of the glass itself is taking the image out of the range of
focus adjustment, and autofocus will not help if there is inadequate
focus adjustment range.
 
Kennedy McEwen said:
Autofocus requires a texture to focus on, which is usually the film
grain itself. Consequently it *should* focus on the film emulsion,
taking account of the thickness of the glass slide mount. However, if
you are having problems focussing manually then it may be that the
thickness of the glass itself is taking the image out of the range of
focus adjustment, and autofocus will not help if there is inadequate
focus adjustment range.
Click to expand...

I know that projectors don't autofocus in the way that cameras do, so
perhaps it's the same with scanners.

Projectors use a light-beam from the side, and measure where it falls
after reflection off the surface of the slide. If scanners work in the
same way, then grain is irrelevant - it will reflect from the first
smooth surface it sees, which, in the case of glass-mounted slides, is
the glass itself.
 
Alan F Cross said:
I know that projectors don't autofocus in the way that cameras do, so
perhaps it's the same with scanners.

Projectors use a light-beam from the side, and measure where it falls
after reflection off the surface of the slide.
Click to expand...

True, but projectors can't autofocus on a user definable portion of the
image. It would be fairly difficult to implement that type of autofocus
with the same flexibility in focus point that the scanner has. It would
also require additional hardware in every unit, thus being much less
cost effective than a local sharpness measure as the focus position is
adjusted.
 
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