On Sat, 07 Apr 2001 15:07:11 +0930 Mark T. (markthom@camtech.net.au)
wrote:
> Eeek. I thought grain-aliasing and film resolution was covered in
> either lesson 1 or 2 when you do Filmscanning 101..! :)
When I first came across this, and began to suspect it was an aliasing
phenomenon, I was unable to find any references anywhere. Not one. It
didn't exist, and nobody had questioned why images which produce
near-grainless prints should suddenly produce easily-visible grain in
scans. Nevertheless, it seemed to be that it was completely intelligible
as an aliasing artifact, so I wrote it up as a tentative explanation.
About a year later, Pete at Photoscientia noticed the same phenomenon and
did some research. Like me, he found no reference material, except the
material I had posted about it at my site. He contacted me and we
discussed what we were both seeing and that we were not hallucinating but
it appeared that scanners were, which was reassuring for both of us. We
agreed that the fundamental mechanism was aliasing arising from grain
pattern interference with the matrix of pixel geometry. His investigations
resulted in the feature at which
remains the most thorough attempt at an explanation - you still won't
find it in any text books AFAIK.
Apart from Pete's Acer review at www.photoscientia.co.uk, I have still not
seen *any* other review of a scanner which mentions it, though many
wrongly assert that 2700ppi is enough to image film grain even from ISO100
materials. I have even been contacted by a manufacturer rep and asked if I
could suggest any reason why a user was reporting massively exaggerated
grain with ISO400 film, so I don't think this problem is widely correctly
perceived, let alone understood - probably because many reviewers and
others within digital have minimal experience of film photography.
It may be that the engineers who design scanners have a huge file on the
problem, and I would be astonished if they do not as aliasing is very well
understood and many techniques are being developed to deal with it,
especially within digicams. But mfr's. mouthpieces, the marketeers, are
hardly going to tell us about it, as it devalues the sellable notion of
scanning as a near-perfect process.
Regards
Tony Sleep
- Online portfolio & exhibit; + film scanner
info & comparisons
