Digital BW, The Print

> -Dan's answer:
>
> Technically there is more of a case for 16-bit in B/W than in
> color images,
> however it's unlikely to make any real difference.

I find Dan's comments on this topic quite wrong.  Doing tonal moves in 8
bit, CAN make a REAL difference.  It's a fact.

Here's one of the rubs of this issue.  If you select 8 bit data from the
scanner, the scanner driver typically does the tonal corrections TO high bit
data, then converts the high bit data to 8 bit data and sends you the
corrected 8 bit data, so you don't have to do much tonal correcting in PS,
if any, unless you want to "mess-up" your image.

So, how do you get 8 bit data out of the scanner to do your tonal
corrections to it in the first place?  On my scanner, I would have to scan
in 16 bit raw, and once in PS, set the setpoints, and then convert it to 8
bit data...the do tonal corrections to it.

If you don't quite understand the implications of that, and why all that
matters, let me know and I can explain further...

> Bit depth works along the same lines, although it doesn't have nearly as
> pronounced an effect as, resolution.

That is not true, especially as a blanket statement.  It depends on the
output image size for resolution.  Resolution change from 1k to 4k SPI may
have no effect on a smaller print, but a huge effect on a larger print, as
well as it's certainly film size dependant.

Doing tonal changes in 8 bits can wipe out ranges of tones completely, or
can have little effect...it is very image dependant, and it is also image
size dependant.  I am assuming this discussion is limited to images of
sufficient size (say a minimum of 8 x 8, 8 x 10 or larger) such that the
image displays a large enough print area for you to see the full range of
tonality from a decent viewing distance.

> Similarly, less resolution provides a crisper, snappier look, if
> that's what
> you want, or a harsh and jagged look if it isn't.

And it also means your image is less accurate...since there is dropped
information in the reproduced image.  You lose detail, whether it's "crisper
and/or snappier" or not.  A line-art image is "crisper and/or snappier" but
contains no real detail.

> One can vary resolution quite a bit without hurting anything

Only on small images, that, again, is a very inaccurate blanket statement.
Scanning at anything but the optical resolution of the scanner (except in
the case of grain aliasing) WILL degrade the image.

> Varying bit depth has a similar effect, but much less of it. That is,
> pictures corrected in 8-bit will, if the correction was very aggressive,
> seem
> very slightly sharper than those done in 16-bit.

Hum, that's again, not necessarily true.  It wouldn't necessarily seem
sharper.  It depends on what, exactly, you do.  What happens when you do
tonal corrections in an 8 bit space, is you ALWAYS lose tones, that is if
the original image contained all 256 tones in the first place.  A lot of
pixels that used to have different values, will now have the same value...
An example is taking four pixels, values 100, 101, 102 and 103.  Performing
a move on them so every pixel with the values 100-103 now all have the value
103.  You just dropped three tones out of four...and if that happened over
the entire tonal range, you'd have 64 tones, instead of 256.

When you do tonal corrections, you are doing one of three things.  You are
compressing a region, which loses tonal values as shown above, changing the
white and/or black endpoints (set points, and it's really a compression
too), which also loses tonal values or you are expanding a region....which
doesn't necessarily lose pixel values, the expansion has to compress some
other region...

If the lost tones are in a region of tonality (or a region of the image) you
don't care about, OR, especially more the case, are tones that there aren't
many pixels of that tonality, it won't have as drastic a visual effect.  It
all depends on what quality of output you want, and where the dropped tones
lie.

> ...In
> B/W, this effect doesn't exist, so there would be a lot more
> cases where one
> might have a slight preference for a 16-bit correction.

"slight preference"...well, it's far more than a "slight preference".  I
think the issue is strongly masked by the fact that most people get their
tonality set in the scanner driver, which, as I said above, does the tonal
move on high bit data.  After getting the data in PS, they only do small
moves in PS on the 8 bit data.  That's fine to do, but doing full tonal
correction on 8 bit data is going to give you VERY VISIBLE posterization for
most images.

BTW, anyone is welcome to pass any of my remarks on as you see fit.

Also, I encourage anyone to do as I suggest above, and take a raw image from
your scanner, set the setpoints (NO tonal corrections) and convert it to 8
bits...then do your tonal corrections.  See how you like the results ;-)

Austin