The problem discussed in the introduction is not actually grain
as we know it but "grain aliasing", and as indicated above has been around for quite
some time. Grain aliasing isn't real grain but an interference
pattern between film grain (dye clusters in colour film) and the CCD pixel
size. I don't propose to explain any further what the problem is or why
it occurs, others can do that much better than me (see http://www.photoscientia.co.uk/Grain.htm
for a very useful explanation). However I will explain a method that I
use to help reduce (not eliminate) the problem.
Not all scanner models appear to be effected to equally badly and
Nikon in particular with their GEM technology seem to have been reasonably
successful at minimising the problem. However, even with GEM soft
tradeoffs take place.
I tend not to use much colour negative type film but B&W is something
I do use. I also tend to be quite conservative in terms of film stock
and speed - read no faster than 200 ISO for colour and for B&W
Ilford FP4 Plus or Agfa APX100 and very occasionally Ilford HP5 Plus. I
have not used Kodak B&W films in many years and have little
intention of changing this 20 year habit. It's funny that the new
generation film stocks seem to be the ones causing the greatest problem.
Enough waffle - how do I overcome "grain aliasing"?
In truth, I don't, I simply try to work round the problem using a rarely
used filter provided within the SilverFast filter menu. "Grain
aliasing" produces a broadly similar effect to "moiré"
a patterning and lucky for users of SilverFast the Descreen
filter is designed to reduce the effects of moiré. The following
shows typical settings used when scanning Colour and B&W
negative stock with my Polaroid SprintScan 4000 and 120, both of which
have a maximum optical resolution of 4000ppi.
Ensure that "Preview Moiré"
Choose a "Screen" value
at or just below the actual scan resolution. I show 3900ppi in the
following example but quite often I simply choose 4000ppi.
Press the "Prescan"
(shown as "Update" on screenshot below) button and
select and area of the image that you wish to preview close-up. The
scanner will now perform a "full resolution" preview scan of
the selected image area and present it in the "Descreen" preview
panel (as shown below). Remember the area being previewed is full size
(100%) and so gives a true indication of how the grain aliasing is
really impacting upon the image.
Basic Descreen Configuration
It should be pretty obvious from the screenshot that the
grain effect has been reduced considerably, and whilst the fine detail of the
hair strand has been softened it has not been totally obliterated.
From this we can conclude an obvious drawback of using the "Descreen"
filter, i.e. - it will soften the image. The degree of softening varies
and sometimes it will be acceptable. However with other images the
softening effect of the "Descreen" filter will be unacceptable.
The screenshot above is good example of "Descreen" filter softening
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