Re: How to translate log density readings to percent?

From: Dave Soemarko ^lt;[email protected]>
Date: 09/07/04-01:44:11 PM Z
Message-id: <02e501c49513$0cb3e550$0a808080@wds>


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

  ----- Original Message -----
  From: Loris Medici
  Sent: Tuesday, September 07, 2004 1:55 PM
  Subject: Re: How to translate log density readings to percent?

  Dave, thanks for the contribution.

  But how shall I see that I'm dealing with dmax if I don't map the hypothetical max density value of 95.32% to 100%?

  I'm asking all this because I'm about to puchase a densitometer (which reads in log values - not percent) and want to figure out how I can use it to make the most correct adjustment curves for my digital negatives. AFAIK, I need to convert the log readings to percentages so that I can design curves in Photoshop. My approach was: the densitometers that read percent values can both set dmin (zeroing) and dmax boundaries in measurement, but the "normal" densitometers have only zeroing (no dmax preset). Therefore, I thought I should map dmax myself by means of calculation.

  Thanks again,

    ----- Original Message -----
    From: Dave Soemarko
    Sent: Tuesday, September 07, 2004 7:35 PM
    Subject: Re: How to translate log density readings to percent?


    You do not need to map (Dmin, Dmax) to (0%, 100%). The values you have for R and 1-R are already in percentage.

    That is, if you subtract the real Dmin (0.12) and consider Dmin to be 0, then at density of 1.33 the reflectance is 4.68%; the opacity is 95.32%. You do not map that to 100%. That would be incorrect.

    Dave S
Received on Tue Sep 7 13:44:41 2004

This archive was generated by hypermail 2.1.8 : 10/01/04-09:17:55 AM Z CST