RE: filmscanners: Scanwit: Seeing through mount?

["Mark T." <markthomasz@ozemail.com.au>]

At 09:36 PM 13/07/01 -0600, Frank wrote:
> >..you are just seeing a shadow effect where the slide
> > mount is blocking only some of the light path along the edge.
>
>Yup, I took a couple and looked at them under a microscope at low power and
>what you describe is definitely what is happening - one side has a slightly
>smaller aperture. However, this still says the Scanwit is able to "see
>though" that one smaller side's edge.

I would rephrase that - I think you are still only seeing through the film. 
My guess is the wider aperture is on the side of the ccd?  So at the edge 
the ccd sees the film very darkly illuminated from the light leaking around 
the inner edge of the mount.  Ie (forgive ASCII art - done in arial, 
probably won't line up very well in other fonts..  :)

| | | |              light source
| | | |
| | | | =====   slide mount (smaller aperture)
| | | \ \ \          leaking light!
=========  film

            ===   slide mount (wider aperture)
++++++++    ccd sensor
aa  bbb ccc

(I've left out lenses and other unnecessary items!)

So at aa you see your normal image, at bbb you see a weakly illuminated bit 
of image,  and at ccc it should be black (probably cropped out of your 
sample).  To check this theory, just turn the slide around - I reckon the 
edges will be black.

>And what the result seems to be is that area has higher contrast with no 
>more noise than the film area next to
>it.

But do you *want* higher contrast?  I played with your image, and by 
selectively fiddling with the different areas, couldn't see any real 
advantage to the darker area - in fact if anything, the reverse.  Is this a 
Provia slide by the way?  If it is, then it looks like your scanner is like 
mine - I find the noise is just far enough below the max density of most 
slides to not be a problem, but some Provia's, Velvias and K-Chromes start 
to push the envelope..

>I think the bottom line is that my next scanner will definitely have a
>manual exposure mode!

Agreed.  But for the market the Scanwit is aimed at, I can understand their 
thinking...

MarkT.