Peter D said:
So, is it true that a 4800ppi scan produces unwanted noise/grain? And
if it produces grain/noise that may be wanted, what purpose woudl it be
wanted for? IOW, is there any _real_ value to scanning at 4800ppi? If
there is, tell me and I'll go out and buy a new scanner.
Of course it is true that scanning at higher resolution picks up more
rubbish, whether noise, grain or dirt and scratches is irrelevant, than
scanning at lower resolutions. However, that is not all that it picks
up - it does get more real information off of the slide or negative.
Back when the highest resolution from Nikon was a mere 2700ppi I was
using their then top of the range LS-2000 scanner but I also had my own
colour printing facility running as well. I was frequently disappointed
that when I created large prints from scans the grain was much more
visible than when I made equivalent sized prints from the same negative
or slide in the darkroom. And it wasn't just me - most people, when
shown side by side prints and asked which had more grain picked the
print from the scan.
The scanning process was clearly exaggerating the film grain, and it
didn't take long to work out what was probably happening: 2700ppi
*might* have been enough to resolve all of the image information but it
just wasn't enough to resolve the film grain or give a clear margin
between that and the image content. Consequently the grain was aliased
by the scanner and exaggerated in size to the same scale or larger than
the finest detail of the image.
If you are having problems understanding this, and even if you are not,
then there is a fairly good introduction to grain aliasing at:
http://www.photoscientia.co.uk/Grain.htm
Once grain aliasing happens, no amount of post processing can remove it
or clean the image up without losing some of that fine image detail.
There are plenty of packages around, such as GEM and NeatImage, that
claim to achieve this particular physical impossibility - and perhaps
millions of users of those packages who claim that they are successful -
but when tested rigorously under situations where the grain is about the
same size as or larger than the finest image details they *all* fail to
achieve their claims. Every one of them results in plasticised skin
tones and other textures, or significant residual grain which is still
visibly worse than that in a conventional photographic print.
The only solution to this particular problem is to prevent the aliasing
from occurring in the first place. Possible ways to do this are
defocusing, blurring or diffusing the image BEFORE it is scanned - but
all of these lose information and if your scanner has just enough
resolution to capture all of the image detail then all of these
techniques will also lose image content, producing prints which are
visibly softer than their chemical comparisons - even if the grain is
reduced.
That leaves scanning at a higher resolution as the only viable solution
and when Nikon introduced their 4000ppi LS-4000 scanner I waited a
respectable time for major design problems to be reported by others and
then, when content that they did not exist, upgraded to the higher
resolution device.
Several things were immediately clear from comparing printed output from
the LS-4000 with earlier chemical and LS-2000 scanned prints.
First, there was MUCH less grain in the LS-4000 scan than in the LS-2000
scan, when prints were compared side by side. Not surprising at all
since 2x the pixel density should significantly reduce grain aliasing -
and it certainly did.
Secondly, there was significantly more image detail in LS-4000 scan than
there was in the LS-2000 one - which completely debunked any arguments
that had been made, and which you appear to be repeating, that more than
2400ppi adds no more image information. Film certainly contains more
information than a 2400ppi scan can pull from it. Furthermore, others
have shown that the 5400ppi resolution of the Minolta SE can pull more
image detail off the film than a 4000ppi scan if it exists in the first
place. Perhaps a few of my images taken on a tripod with high shutter
speed and high resolution film justify this, but it comparison scans on
my typical images haven't shown this on the occasions I have tried the
Minolta, so it isn't something I am rushing to upgrade.
Finally though, it was clear that the prints made from the LS-4000 scan
actually had more image detail than the chemical prints. In short, they
were simply better, and I put this down to the limitations of my
darkroom equipment. The result was that I closed down the darkroom,
moved to the semi-digital film only process, got better results with
less mess and expense and freed up an extra room in my house! So
scanning at 4000ppi over 2700ppi was win, win and win again.