RE: filmscanners: Re: bit depth and dynamic range

[Jack Phipps <JPhipps@asf.com>]

Paul--

Scanner scan red, green and blue at a certain bit depth. Some Nikon scanners
scan at 12 bits per channel. However, a lot of software can't handle more
than 8 bits per channel (8x3=24 bits). When they can handle more, they
usually require 16 bits per channel (16x3=48 bits). So the Nikon scanner
scans at 12 bits (12x3=36) and if you want the extra bit depth it stores at
16 bits (48 bits). So the file size increases by 100% (if it is a tif for
example) while information increases by 50%. While this isn't very
efficient, it does allow for higher bit depth. Higher bit depth is important
if you are going to do a lot of changes in your image editor. Each time you
apply a curve or sharpen the image or resize the image you loose image
information due to round-off errors. If you did enough of these operations
your image would become poserized. If all you plan to do is to scan your
image and print it, you don't need the extra bit depth, 8 is enough.
Otherwise you want to start with a 16 bit scan, do as many operations as
possible in 16 bit mode (because you can't do all the operations you want to
in Photoshop 16 bit mode as you may need for example). Then convert and
proceed.

If you had a negative with an extremely high dynamic range (say a child
under a tree with a white house in the background in bright sun) and a
scanner with low dynamic range you would want to scan the image twice. Once
to expose for the child and once to expose for the house. Let's say you
could control the scan light, turn the light down for the house and turn it
up for the child (actually, the scanner does this for you when you adjust
the levels). Then you would combine both images into one. If you had a
scanner with an extremely high dynamic range, you could scan once and have
all the information you needed.

The scanners that use an infrared channel (fourth channel) for surface
defect detection typically don't let the user at this channel, so it doesn't
come into play in the scanner's specifications. The scanner uses this
information during the scan along with software to remove the effect of
dust, scratches, fingerprints, etc. from the output image.

As many readers on this list know, I'm biased, but I believe the most
important feature is the Digital ICE (the surface defect removal), Digital
ROC (Color Restoration) and Digital GEM (Grain Management). These tools
really make a difference in the quality of scans.

Good luck in picking a scanner!

Jack Phipps
Applied Science Fiction

>I've just been reading some Nikon literature about their new scanners, and
>I have some questions about the terminology.  The ad states that the
>coolscan IV ED yeilds 48 bit images (48 bits per pixel?).  Later, it
>states that the scanner has a color bit depth of 24 or 36 bits.  What does
>this refer to?  It also states that the scanner has a dynamic range of
>3.6.  What is a scanner's dynamic range?  Can anyone suggest an article
>somewhere on the web or elsewhere that explains all this?  Thanks.