> Is there a difference between optical resolution and "real resolution"?
> If you're saying "You don't need more that 2400," I'm with you. If
> you're saying, "Scan at 2400, 3200, or 6400 and it is still only 2400"
> with a scanner that has the spec of 6400 optical, then I don't
> understand.
This is where math doesn't always tell the whole story. Let's say we
are thinking about line-pairs. If you can get the bars and gaps to
line up perfectly with the pixels, you have 100 transfer rate at the
maximum resultion of the digitizing tool. When we talk about what film
is capable of in line-pairs, this translates to around 2400 ppi in
most real-world situations.
However, that assumes that the details are bars are lined up with the
pixels. The film might have an absolute resolution limit, but the
where of that limit is the variable part of the equation. This is easy
to illustrate angled lines in the photograph. On film, the line is
straight and smooth. On a digital image, the line is jagged. You need
to scan at a higher resolution to smooth out those lines.
Just because you've smoothed out the lines does not mean that the
resolution is any greater as the film and lens is still the limiting
factor. All it means is that you've evened up the transitions. Another
example of this is where two opposite colors come together. Where
green and red touch each other in the image, the film will have a nice
transition, whereas a digital image (either from camera or from
scanner) will end up with a different color (usually gray) line
between the two. The higher the pixel density during the scanning
process, the less of a line you'll end up with.
Likewise with grain aliasing. When you scan at a lower resolution, the
diameter of the optical sensor's pixel has not changed. It just is
moved by the stepper motor a little farther between pixels.
What the V700/750 is doing is the actual diameter of the pixel is
fixed. It's probably fixed in that 2400 region. At 2400, there is
little to no overlap in pixels. However, when you go to the higher
scanning resolutions, you aren't increasing resolution, what you are
doing is reducing the distance the stepper motor moves. This means
that the pixels in-between are pretty much an average of the imaging
area that the neighboring pixels cover. You will reduce the "aliasing"
effect because any one tiny blip in any one pixels is actually an
optical average of the neighboring pixels. It's effectively an AA
filter.
Drum scanners have multiple controls for the operator to tweek and
adapt to the film. One control is for the stepper resolution, another
is for the diameter. Other controls also exist, but those two are
pretty basic. The V700/750, with the higher resolution settings is
effectively giving you a wider diameter than the stepper resolution.
One trick of the trade is that when you have a grainy film, you
increase the diameter while increasing the resolution.
In a nutshell, I scan at the maximum ability of the scanner and reduce in post.
AG
--
Ken Norton
ken@xxxxxxxxxxx
http://www.zone-10.com
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