Stephen Henning <pighash.DeleteThis@aol.com> writes:
>If you have a DSLR, you already have the very best film scanner made.
>With a macro lens you can do slide copying just like all photographers
>did for many decades. The advantage is that you have full control of
>contrast and color temperature. So if you want the best film scanner
>made, then just get yourself a copy stand and enjoy quality. Would you
>rather spend you money on a good macro lens or a film scanner? I know
>where my money has already been spent.
The macro lens might be just as good as the lens in a film scanner, but
that doesn't make a DSLR plus slide copier the equal of a film scanner.
Most film scanners use a linear array sensor, not an area array. That
allows them to measure red, green, blue, and often infrared
transmittance individually at every pixel position, instead of using a
Bayer filter sensor. Measuring RGB independently increases colour
resolution and avoids colour/luma separation errors. Measuring infrared
is the basis of automatic dirt-removal methods. And getting linear
arrays with 5000-6000 pixels across the height of the frame is not
difficult, while DSLRs with comparable resolution are nonexistent.
Then, some scanners use an illuminated slit light source that moves
along with the sensor array (or the film moves and the light source and
array are stationary, which amounts to the same thing). With this
configuration, most of the flare light scattered by the lens from the
illuminated line on the film is scattered outside the sensor and
vanishes. Another way of looking at it: most of the flare that any one
pixel in an area-array sensor like a DSLR would see with an area light
source is not present with a line light source, because almost all of
the film area is dark at any given time.
Then there are drum scanners that illuminate only a small disc on the
film at a time, not a line. They get even more optical flare reduction
from this - essentially all flare light is rejected.
The slower pace of scanning and digitizing also may allow the scanner to
capture more bits per pixel. That, plus the better flare performance of
the slit-illuminated optics, means you can see further into the darker
areas of the transparency or negative.
And of course the scanner provides full control of contrast and colour
temperature. The colour temperature adjustment, in particular, is
generally done by adjusting illumination or exposure time, *not* scaling
the data after the A/D converter, so you get the full A/D converter
range (unlike a digital camera).
Finally, film scanners come with sofware that understands how to convert
negative density back into original-scene light. If you photograph a
negative with a DSLR, you have to do this yourself, and it's *not* just
a matter of inverting the channels to get this right.
All in all, a DSLR is not capable of matching a well-designed film
scanner.
Dave
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