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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 "moir�"
a patterning and lucky for users of SilverFast the Descreen
filter is designed to reduce the effects of moir�. 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.
Step 1
The following steps are not cast in concrete. As with all SilverFast
tools it's wise to experiment. Experimentation is how you will learn.
Main SilverFast Control Panel
Step 2
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Ensure that "Preview Moir�"
is engaged.
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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.
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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
at work.
So why trade grain for softening of the image, surely we can do the
same in Photoshop? Not quite! The screenshot below demonstrates that there
is more to the SilverFast Descreen filter than simply removing the moir�
effect and softening. I usually engage the "use Unsharp Masking"
filter to help bring back some sharpness to the image. In fact I now find
that with some images very fine detail returns - notice the appearance of
a hair strand on the bottom right of the "Descreen" preview window.
Descreen and Unsharp Mask Filter
So what are the sliders doing? With SilverFast USM
takes place on-the-fly in Lab mode hence the % values and references to
grey in the user manual.
-
Intensity will determine the amount of
sharpening that takes place. Typically we should set low values between 15 and
40%. Occasionally values above 50% can be used, but these are rare.
-
Threshold determines where the sharpening will
take place ( i.e. contrast between two adjacent pixels). Values
between 2 and 6 are normal. If high Intensity values are used
it will be necessary to further increase the Threshold.
-
"Sharpen down to" is a bit more
complex to explain but basically determines how far into the shadows
the image will be sharpened. Go too far down and shadow noise will
become more pronounced than we would normally desire. It's probably
better to experiment with different values.
-
I tend to leave "Light and Dark Contour"
at the default values. Modifying either will tend to cause black or white specks to become less or more pronounced. Increase
"Light contour" value and
white specs will jump off the screen at you, likewise increasing the
"Dark contour" value will cause any dark specs to become even blacker.
Experimentation is the name of the game with these two sliders. If the
image has lots of speckles it is certainly worth adjusting one or both
sliders.
-
Engage the "Highlights soft" option
means that the sharpening effects will not be applied to highlights.
Each of the above adjustment sliders works in "real-time"
so you can easily see the effect of a change in the high-resolution scan
window. This window can be scaled up to 8x magnification, but I rarely find
is useful above 2x magnification. Hopefully, through some experimentation
you to will get less grainy scans than appears to be the case at present.
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