Re: More than 24 bits of color

[Russell Williams <williams@Adobe.COM>]

Sorry if I missed a post already covering this, but here goes.

The printer driver cannot accept a 16 bit per channel file and could not
put that level of subtlety onto paper anyway, but having >8 bits per
channel in your input can be very useful. This is especially true for
negative film, which contains a greater range of brightness from the
original scene than transparency film, albeit compressed into a *smaller*
range of brightness on the film itself. It's even more useful for high end
scanning back electronic cameras, which can capture even more dynamic range
than negative film. (Stephen Johnson, for instance, has been documenting
the national parks with digital cameras  that provide up to 6000x8000
pixels, 142MB files with more than 10 stops of exposure latitude.)

If you only wanted to linearly compress this huge range into the restricted
range that will end up on the print, there'd be no problem -- if you got a
good medium-tone scan, you'd always be fine. But often you have a very low
or high key image, or you have a high contrast image with critical detail
in both the highlights and shadows. 16 bits per channel images let you see
that detail; they allow more radical levels and curves adjustments without
posterization. (Radical levels/curves moves, especially on 8 bit data, can
also significantly emphasize the appearance of film grain and scanner
noise).

But what about scanner drivers that don't output 16 bit files? And what
about the larger file size and restricted editing capabilities in
Photoshop's 16 bit mode?

MY RECOMMENDATION:

1. If you have a >24 bit scanner that only outputs 24 bit files, use the
scanner's levels or curves adjustments to get the brightness range of the
image into the ballpark you want. For instance, if you're going to bring
out details in deep shadows, do that boost here. With a properly written
driver, this means that the data will be manipulated with 10 or 12 or
whatever bits of precision per channel and *after* the adjustments have
been applied, it will be scaled to fit into 8 bits. Do the finer
adjustments and all the editing that doesn't involve major levels/curves
adjustments in your image editer. You'll get better results than taking an
unadjusted scan into Photoshop or whatever and making those big
levels/curves moves there.

2. If your scanner will output 16 bit / channel files, *don't* use the
scanner's adjustments. Do a straight scan. If you have a profile for your
scanner, use it (by starting with a profile to profile conversion on the 16
bit/channel file if your scanner, like most, doesn't embed profiles). Do
your major levels/curves work at 16 bits/channel in Photoshop. Save this
file if you have the inclination and disk space, and then convert to 8
bits/channel for the rest of your editing.

This will generally allow you to print, then go back to the 8 bit/channel
file for tweaks, and re-print, without seeing any artifacts from 8 bit
editing and without paying the memory and computation time costs of always
working on a 16 bit file.

Russell Williams
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