I must not have explained myself well. I understand that the problem is
bowed film - I have a web page devoted to the issue. I am only saying that
while it seems intuitively that a diagonal offset from the centre should be
best, I think that in practice an offset along the "y " axis, not far from
the top or bottom edge is a better choice.
If you look at the measurementsof film "warped-ness" on said site
you will see that a focus location about where I suggested is a better
average in some cases and at least as good in other cases. In other words,
the suggested location would be a better general solution, IMO.
The problem that I am trying to avoid is when the film is not parallel to
the scanning plane in the longitudinal direction, as happens on my scanner
and at least one other person who contacted me. It also occurs when the
longitudinal distortion caused by the motorised feeder curling the film
into the back of the scanner causes an asymmetrical front-to-back slope in
the film. My suggestion was that the offset towards the diagonal has no
advantage over an offset in "y" only, and has a positive disadvantage in
some circumstances.
Julian R
(who notes regarding above page address that for the 2nd time in 6 months
my ISP has collapsed today (Austar this time) and so I'll have to change
again! I can really pick them).
At 15:13 04/12/01, you wrote:
>Julian wrote:
> >I can't see any actual advantage in putting the focus point towards a
> >corner rather than just near the top or bottom edge.
>
>Because the main headache with focussing has been bowed slides which form
>a kind of dome shape? A sensible average point would be halfway along a
>diagonal between the center and one corner.
>
>Using the motorised feeder with curled strips of film could be problematic
>depending on which end of the frame is curled. :( In this situation I'd
>think the best option would be to use the center focus point since about
>3/4 of the frame should be flat. The only real solution for curled film
>is the flim strip holder. If only the feeder held the film flat!
>
>Rob
>
>Rob Geraghty harper@wordweb.com
>
