RE: How to translate log density readings to percent?

 
Hi Dave,
 
I use a 225lpi negative on a 3600dpi imagesetter (225 x 16 = 3600). The
source file resolution is 450dpi (2x factor). I agree that "logically",
an imagesetter negative should do fine for my purposes "without" severe
curve adjustments... BUT "pratically" this is not the case (at least for
me).
 
Let me describe my conditions:
 
I use textured paper (cold press) and when I contact print an
imagesetter negative emulsion to emulsion I get strange patterns on my
print. The patterns are caused by the fact that the portions of the
paper that are in close contact with the negative (lets name them "the
hills") clearly show halftone dots (with a loupe of course) but the
portions that are not in close contact (lets name them "the valleys")
don't show halftone dots - because of light
scattering/halation/diffusion??? whatever. Therefore, the tone of the
"hills" and "valleys" differ considerably given the same negative dot
percentage.
 
I overcame this problem by printing emulsion up (yes, I loose sharpness
but not much - and I can compensate for this by oversharpening the file
in Photoshop). That way, I use the thickness of the negative to turn
hills to valleys, and valleys to deeper valleys so there are no strange
patterns and tone difference (because there's no "true" contact)
anymore. When I print emulsion up, the density change - somehow - is not
"linear". That's why I opt to design a curve by careful measuring.
 
Will keep all what you said in my mind when the densitometer arrives.
Thanks again.
 
Regards,
Loris.
 
BTW, What is "undercutting"?
 
----- Original Message -----
From: Dave Soemarko
To: alt-photo-process-l@sask.usask.ca
Sent: Wednesday, September 08, 2004 12:21 AM
Subject: Re: How to translate log density readings to percent?

Loris,
 
Your method is not wrong but might be unnecessarily complicated. If you
are using an imagesetter negative, you almost don't need a densitometer.
Assuming your service bureau is doing a reasonable job of maintaining
the chemical and calibrating the system regularly, the negative itself
should be fine. When you print, you want to be able to have the 5% tone
and 95% tone printed as suggested before. If that is not possible, it
means that for your printing process, you need to use a negative of
lower resolution (or alternatively, you could print from 15% to 85%, for
example, and adjust the curve accordingly; but what that means is that
you lose some tonal values because you are not using the whole available
range).
 
What you are doing is that you are trying to match percent dots to
densities and compared that with the theoretical values and try to
adjust the curve to get to this theoretical value, but this is almost
like going through a detour. Your imagesetter negative can be considered
"ideal" negative (though not perfect), so if you are able to achieve the
5% and 10% tone, everything else should fall correctly theoretically,
but in reality there are some othe factors involved. Years ago I
mentioned about dot gain. Some people think there is no dot gain because
there is no bleeding as in mechanical printing. Actually while our
mechanism is different from traditional printing, the effect of dot gain
does occur because of undercutting, for example.
 
But what applies to us is that you could look at the 50% dot and see if
it printed too light or too dark, and when you need to adjust it, you
can look at the printed patches and determine the adjustment. You could
then adjust the curve.
 
I don't know if I can make myself clear in a short email, but you should
probably try to adjust the 5%, 50% and 95% tones to get a reasonable
print. Once you get the feel of it (and understand this calibration
process completely), you can fine tune and do the 25% and 75% tone.
 
 
Dave S
Received on Wed Sep 8 04:57:49 2004