>>In what units is it measured and what values should we expect for a
"cheap" scanner and a "good" scanner?
>>>>>>>

Well, the scale sorta runs along a 0-4 range....the high-end drum scanners
will have dynamic ranges (true) up near 4.0....the expensive desktop slide
scanners run around 3.5+....so, it used to be a few years ago, that anything
less than 3.0 would be considered "cheap"....I think you'd need something in
the 3.5 range to be safe...

I'm no expert on any of this, but I am a photographer and the thing to
remember is that these lower end flatbeds are good to okay for reflective
media like prints, but a negative or a transparency is a whole different
story....because light moves through them, and even in conventional
printing, the detail and the range on the neg or CT is often compressed &
lost in the final print....photo papers simply cannot hold the range of
detail from the highlights to the shadows...for this to happen, you usually
have to have a professionally shot image, one that was shot for
reproduction--taking into account this compression and subsequent loss of
detail in everything from photo papers to offset printing....my guess is
that a turn of the century glass plate is going to be anything but a CT shot
in a studio....

I know from just trying to make prints off old negs & plates, that they
often are not a good match for the papers made today...they were shot for a
different type of photo paper, and the range & density of the older negs is
just too much for modern materials...there's always some compromise or
loss...

We use an older Polaroid Sprintscan here for 35mm slides and some negs, and
an older Linotype Sapphir for prints and 4x5 CTs....these 2 work
_okay_...but compared to a drum scanner, they're really lousy...you'll see
it in the shadows, or in the highlights if you have to work with contrasty
images or densities that exceed what the scanner can hold...my limited
experience with scanning b&w negs, has shown me that these are even more of
a problem than slides...

So, what I would suggest would be to ask for help from a vendor who caters
to professional photographers and commercial graphics people, over an office
supply store...I'd look at the res numbers, higher bit depth, and as much
true dynamic range as you could afford...or else, maybe the answer lies in
making good contact prints and just scanning those....whatever the digital
thing is that you do, it will not be a long term storage solution
anyway....it's a good idea, I think...neat for the public, but it could turn
into a real nightmare for the scanner operator & photoshop person if the
originals are all over the place....

Here's a link to a good scanning article...:

http://www.rlg.org/visguides/

http://www.rit.edu/~661www1/sub_pages/frameset2.html

That last one is from the Image Permanence Institute and a pdf document they
have that's a guide to digital imaging for photo collections...

Well, happy holidays you all  & good luck with the project..

KT

Kent Thompson,  Photographer
North Carolina Museum of History

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