Re: Sorry to ask again but...

[Sandy King <sanking@clemson.edu>]

Judy,

There are many different kinds of flatbed scanners and I don't know 
where the place of best focus in on all of them. However, all of the 
ones I have used appear to have the point of best focus not on the 
surface of the glass of the scanner but at somewhere around .5mm to 
1mm from the top. That is suggested by various film holders 
themselves. Therefore, placing a thin piece of glass on the top of 
the scanner, with the emulsion of the film placed in contact with 
that glass, appears to place the negative at a point where the focus 
is at least as correct as with the negative holder. Another issue is 
that lenses used in scanners appear to have really great depth of 
field and small differences in placement don't appear to make much 
difference. In any event, with the Epson 4870 I definitely see a 
small improvement in a scan made fluid mounted on glass as opposed to 
either placement over the glass in a holder or placement emulsion 
side down dry over the scanner glass.

As for the dry mounting fluid, some of the stuff being  used today is 
solvent and simply evaporates from the film so that little or no 
clean-up is necessary. Kami mounting fluid, for example, leaves 
little or no residue. However, if the mounting fluid drys out during 
the scan you have pretty much lost the advantage of dry mounting so 
for scanning large format negatives at high resolution I use  prefer 
Johnson's baby oil since it is more viscous and does not dry out as 
fast. After scanning I simply place the negative in a tray of warm 
water with a little liquid dishwater liquid and let it soak for a few 
minutes, then rinse it briefly and hang to dry.

I will look for that story about Clemson in the Times. Last time I 
remember Clemsion making much of a splash in the Times was when it 
was ranked the #1 or #1 party school in the USA. I see that Clemson 
did not make the top 30 in that category this year, though they 
finally got into the top 30 of public universities. I hope that 
neither of those rankings has anything to do with fact that both Sam 
and I retired this year!

Sandy







> Is there a limit to dumb questions -- or do we get an unlimited supply?
> I have absolutely NO intention of ever burying a neg in oil ever 
> again for any purpose whatsoever, but I do use my scanner in various 
> ways & Sandy's reply makes me wonder...
>
> If you add a piece of glass on top of the scan bed, doesn't that 
> spoil focus? I realize you can raise it a fraction of an inch to fit 
> negs into a multiple holder, but that's not as thick as a piece of 
> glass...  I also have an idea my scanner wouldn't scan anyway with 
> the added thickness of the frame Sandy suggests, because the cover 
> has to connect with something or other (UMax, large flatbed-- will 
> sometimes take an open magazine but VERY goosey about books). So 
> what, if any is the depth of field probably?
>
> I have, BTW, just finished a project requiring *many* scans, but 
> found that anything larger than say 4 by 6 inches had to be scanned 
> at 600, 800 or even 1000 dpi, or it simply wouldn't. (I'd get a 
> message saying scanner doesn't connect with the driver or words to 
> that effect.) It's goosey in other ways, too... possibly the 
> softwear needs a kick in the head, but once I figured out raising 
> the dpi it nearly always works -- and I get some guilt-free reading 
> while I wait... (The scans can come in at 100 or more megabytes but 
> I reduce them right away, so not a problem.)
>
> thanks in advance for enlightenment re depth of field, et al...
>
> PS. Sandy, Clemson was in the Times today -- something about a motor 
> vehicle, BMW as I recall... But I take it that's not your 
> department...?
>
> Judy
>
> On Mon, 28 Aug 2006, Sandy King wrote:
>
> > Matt,
> >
> > You don't want to mount directly to the scanner glass with fluid 
> > because there is the risk of it getting into the scanner.
> >
> > Better to have someone cut you a piece of glass that will fit over 
> > the scanner glass, then install a frame on this glass of the size 
> > of your negative, and glue the frame to the glass. This eliminates 
> > the risk of the fluid getting into the scanner, plus you can remove 
> > it to clean away from the scanner itself.