Re: To David Soderman: A Little Test

["Ernst Dinkla" <E.Dinkla@xxxxxxxxx>]

----- Original Message -----
From: "Kennedy McEwen" <rkm@kennedym.demon.co.uk>
To: <scan@leben.com>
Sent: Saturday, April 13, 2002 12:14 AM
Subject: Re: To David Soderman: A Little Test


> In article <000401c1e25d$55712980$0100a8c0@KAIROS>, Ernst
Dinkla
> <E.Dinkla@chello.nl> writes
> >
> >Kennedy, what kind of glass could I use for a film carrier ?
> >Some companies that sell optical glass sell quality picked
float
> >glass, nothing special in fact. Up to 1.1 mm the thickness
> >variation is within 0.05 mm. On a piece of 300 x 115 mm it
will
> >be less than that. The Nikon carrier + scanner would allow  up
to
> >1.5 mm thick glass maybe 2 mm but nothing above that.
> >
> I would be more concerned with the surface quality (digs and
scratches)
> and the colour purity than the thickness variation itself.
>
> The main issue that you will have from the glass thickness is a
focus
> shift (which the autofocus should accommodate) due to the
optical path
> being around 0.67x the thickness of the glass on the CCD side
of the
> slide.  Secondary issues like spherical aberration and defocus
due to
> the thickness variation should be negligible.
>
> Most float glass should be adequate, but if you can get
selected glass
> for a sensible price step then go for it.  Can you get hold of
some
> etched anti-newton ring glass?

Thank you Kennedy,

The autofocus probably has a maximum range of approx. 1 mm I
guess. At least the thickness of the slides should be between 1
and 3,2 mm, That means a shift of 0,5 to 1,6 mm in the film
plane.  The surface of the slide carrier lies 1.5 mm below the
surface of the film carriers. This also suggests that there is
more tolerance for bulging downwards of film in the film
carriers. The auto focus is done per frame and then the scan is
done at that focus for that frame. It can't be that it focuses
continuously while scanning so there is an issue how plane the
glass is. Quality picked float glass above 1.1 mm has a thickness
tolerance of 0.2 mm. That must be too much. Maybe it would be
better to buy several float glass sheets here locally and measure
their thickness variation per spot with a gauge, check on surface
defects and measure the glass with a spectrometre that I happen
to have. I don't think that the optical glass wholesaler does
more than that.

I could get anti-newton glass but I do not like it. On the Agfa
Horizon Plus I can scan film in a negative carrier with
anti-newton glass or scan it on the glass bed itself, the last
mounted with Kami fluid and an extra sheet of transparent
polyester.  The quality of the last scan is a lot better. I even
wonder whether that anti-newton glass influences the sharpness,
it looks quite coarse.

The glass carrier FH etc G  of the Nikon 8000 has at least one
very fine anti-newton surface on the inside of the upper part. I
did not control the rest. The owner complained that he still gets
newton rings
in his scans. Nikon delivers a thin plastic frame filler to
create some distance between film and glass,
I wouldn't like to work with such a gadget and the photographer
didn't like it at all.

I have ordered a FH etc M carrier that is normally used for 6x9
6x6 slide scanning. That will be rebuild to a single-glass
carrier which allows mounting with fluid again and scanning of a
6 x 12 cm strip of a 4x5" film (in two scans + 1 stitch) as well
without destroying the 4x5" film. An 8.5 x 17 cm area could even
be scanned in 2 mountings + 4 scans + 3 stitches if the film is
cut to 8.5 cm wide. I doubt I will ever do the last, there are
friends with 4x5" scanners :-) The main reason to build another
carrier is that the glassless MF carrier needs very flat films in
order to work properly and the glass carriers Nikon delivers are
not free from newton rings. The FH etc GR is probably the best
but also expensive. At least that one covers a 63 x 88 mm scan
area.

Ernst



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