Re: How to translate log density readings to percent?

MessageLoris,

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

  ----- Original Message -----
  From: Loris Medici
  To: alt-photo-process-l@sask.usask.ca
  Sent: Tuesday, September 07, 2004 4:52 PM
  Subject: Re: How to translate log density readings to percent?

  Thanks for the second answer.

  Actually my primary intention is not to measure negative densities, but print densities (the 1.33 I was mentioning was the print's dmax, not negative's - for instance I remember S. Wang saying he gets around log 1.25 dmax with his double coated classic cyanotype prints, emulsion: 2A+1B, probably even gold toned vandykes or platinotypes don't go beyond 1.5 - 1.6 because of matte paper). I will use imagesetter negatives... I was planning to print a non adjusted 101 step tablet using a standard exposure time (which is the minimum exposure time to achive dmax of the emulsion/paper combination - determined by braketing exposure under clear transparency material - masking with fully exposed lith film; with a dmax figure not less than log 4 - probably around 5) and then reading densities of every patch so that I can "map screen/file densities to print densities". I thought this will give me the necessary values to design a "rough" curve (or "transfer function" if you like) and then I can repeat the cycle few more times (I guess this is the iteration you are mentioning) - from now on, applying the curve - for fine tuning (especially highlights). I think a densitometer will make all this struggle much more easier and precise (probably it will also serve to establish an exposure index/development time per process for large format in camera pinhole negatives - which is another story).

  I know people are calibrating their inkjet printing systems (such as Piezography) using this method (or a very close one). Don't know, maybe I'm establishing a strategy which is completely wrong (because I misunderstood conversations between inkjet printer users @ DigitalB&WThePrint Yahoo group!?) or unappropiate for alt. process printing?

  Regards,
  Loris.

    ----- Original Message -----
    From: Dave Soemarko
    To: alt-photo-process-l@sask.usask.ca
    Sent: Tuesday, September 07, 2004 10:44 PM
    Subject: Re: How to translate log density readings to percent?

    Loris,

    We are getting into some details. For using densitometry with digital negative, the basic assumption is that the transmission density of Dmax is at least 2.0. If the Dmax is less than that, it is hard to relate density to % dots because some light is transmitted through the clear area but some light are also transmitted through the "opague" area because the Dmax is not that high. It is calculatable, but it is not very practical because when you expose the negative, the same transmission of both areas will occur, so the percent dots alone doesn't give you all the needed information.

    If your Dmax is 1.33 only, you want to use a short-scale process, or mix your chemical in such a way so that it is contrasty. For curve adjustment, you can use a different approach, print a negative with 5%, 10 - 90%, 95%, for example, and adjust your process so that when you look at your print, at 5% you see a very slight but visible difference from Dmax and a 95% you see a very slight but visible difference from paper white. Then you can adjust 50%. Say if at 50% the print is too light. How much do you want it to go darker? You can look at the patches and say that the right value is somewhere between 20% to 30%. You adjust the curve to achieve that (you can try 25%). In most digital printing, one would correct for 25%, 50%, 75% tone instead of just 50%. With a few iteration, you can get the curve you want.

    Dave S
Received on Tue Sep 7 15:22:01 2004