Digital BW, The Print

Hello All,

I was getting ready to make some prints this evening from an image I'd
previously worked up and considered finished.  After the first print I
felt it needed a tad more contrast, so I decided to use a transfer
curve until I had it worked out to my satisfaction, before I changed
the image.  After a few trials I ended up changing each end by 1% -
the 5% point became 4% and the 95% point became 96%.  This added just
the right amount of extra kick it needed.

I then decided to permanently alter the image by adding the same curve
in a layer, flattening and saving it.  The transfer curves are scaled
in % units and the adjustment curves are in RGB values, so I used a
calculator to make sure I entered an identical curve (each 1% is equal
to 2.56 RGB units).  

As I was doing this I was thinking about yesterday's thread where I
said I could see the difference in a print with as little as 2 RGB
units change.  That was a general statement from memory and an amount
I felt confident saying in public.  I had never actually done any
exact testing to see what degree of change could be detected.  I
realized that my 1% change (2.56 RGB units) had made a clear
difference in the print.

While entering the curve points I discovered I could not enter the
exact values because you can only enter integers, not fractional
values.  So instead of 2.56 RGB units I had to change the points by
either 2 or 3 units.  I chose 3 for the dark end (moved the 13 point
to 10) and 2 for the light end (the 243 point to 245).  The first
print came out looking a tiny bit too heavy in the blacks, so I
changed the dark end to a difference of 2 (13 to 11).  This time it
was a tiny bit too light  (compared to the transfer curve print which
was a difference of 2.56).  It was still darker than the original so I
decided to stick with that and flattened it and saved the file.

It suddenly dawned on me that a difference of one half an RGB unit was
detectable in the print.  Including the transfer curve print, the 95%
points in the three prints were increased by 2, 2.56, and 3,
respectively.  The difference in all three prints, while subtle, is
clearly visible.  That, to me, is amazing.  What an incredible degree
of control we have.  One half RGB unit is equivalent to about .2%

To have that degree of control with an enlarger we'd have to have the
light stopped down to a 500 second overall exposure for the print
(over 8 minutes).  This would allow us to dodge or burn for 1 second,
giving us the .2% adjustment.  The numbers work out ok, but in
thinking back to darkroom work, I doubt that burning for 1 second out
of an 8 minute exposure would yield a noticable difference.  Perhaps
the paper is not sensitive enough to show it.  Is my thinking off
here?  Anyway, it's interesting.

The 2nd amazing thing is when I thought about the fact that I'm
printing BO, and how the printer changes the spacing between the dots
to get the various shades.  It's one thing to do that at the upper end
where you can see individual dots getting closer or further apart, but
how does it do it down at the dark end?  How in the world does it make
a solid tone, so dark there are no distinguishable dots, a bit darker
when I change a value by a half an RGB unit?  Does it still "think" in
terms of individual dot spacing, all of them ever closer together and
overlapping, or does it know that at some point they are all
overlapping and it just changes the amount of ink it lays down? 
However it does it, it's incredible.

I'm a programmer, and I understand the principles by which all this is
done,  but I'm still in awe of it.


Regards,
Clayton


Info on black and white digital printing at    
http://www.cjcom.net/digiprnarts.htm