Digital BW, The Print

David Sinai writes:

> I was curious to see if the histogram of this file would be
> different if the file was converted to 8 bit. After converting the 
> file to 8 bit, I reviewed the histogram and so no changes.

The histogram only shows data with 256 levels, so you can't see any
difference between 8-bit and 16-bit modes with the histogram.  To show
the 16-bit data, you'd need a histogram 65,536 pixels wide, which
obviously is not practical.  In 16-bit mode, every pixel you see
horizontally in the histogram is actually the sum of 256 different
levels (256 x 256 = 65,536 levels).

> Shouldn't there be a different histogram once the file is converted
> to 8 bit?

It would be nice, but as explained above, it wouldn't fit on the screen.

> Going from 16 bit to 8 bit means that the tonal range has
> been reduced, correct?

Yes.

> More importantly, I could not detect any visible differences on the
> screen when the image was in 8 bit vs. 16 bit.

In black and white, very few monitors can show any difference.  And you
must be in at least a 32-bit color mode to even be able to hold any
difference in display memory (and that's still smaller than the 48-bit
mode you'd need to show full 16-bit images).

> I decided to work with the 8 bit file as there are more PS
> features available this way.

But there is also less data.

The advantage to 16-bit mode is that you have far more headroom for
modifications.  For example, if you need to drastically change the
curves in the image, you can do so in 16-bit mode with almost no loss of
quality, whereas you'll get serious posterization in 8-bit mode,
particularly after multiple operations, since the effect is cumulative.

> Should there be any discernable tonality differences on a print
> created with a 16 bit file vs. 8 bit when printing with an Epson 
> 2200 using the BO method?

No.  The advantage to 16-bit is in the extra data you have for
manipulation.  You don't need to stay in 16-bit mode once you are
finished retouching and adjusting the image, and you can store it in
8-bit mode if you want (just remember, once you move it to 8-bit mode,
you won't have as much data if you ever decide to modify it again).

> I printed the 8 bit and the 16 bit file and saw no differences. Is
> this what you would expect?

Yes.

> Everything I've read, says to stick with 16 bit.

As long as you are adjusting or retouching the image and/or you expect
to do so in the future, 16-bit is better than 8-bit.  For actually
printing the image and archiving it, though, it doesn't matter, since no
display or print device can really resolve more than 256 levels, anyway.

> Does the rule "Stay in 16 bit" apply when printing BO on Epson 2200?

No.  It doesn't apply for any kind of printer or display.  None of them
can come close to resolving 65,536 different gray levels.