Why is Zone V 50% gray?

[Helen B]

Here, as an example, is a plot of density vs Photoshop K value for a print with a DMax of 2.42. 18% grey occurs at 52% K, or an RGB value of 122. Oddly enough, that would produce about the right correspondence between original and print. I chose the data I had immediately available - a printer profile I am linearising right now.

Best,
Helen

[bjorke]

Prints are part of our world.Looking to my right out the workroom window I see probably a 1200::1 luminance ratio. An excellent print hanging to my left probably has less than 100::1. The idea that luminance values reflected from the real world should MATCH the luminance values of the print are a bit absurd. Dmax may be "black" and DMin "white" but there's no requirement that they map to exact values from nature or that values between do so, either.

[RalphLambrecht]

I fully agree. God has no problem with about 1000:1, Sony is between 400 and 700:1, and the darkroom is limited to 100:1. The problem is, what is the 'best' representation of this unachievable contrast?

[RalphLambrecht]

Helen

We are getting close. I got to 53% for 18% gray for a Dmax of 2.1 and Dmin of 0.05. Now, I want to develop an equation for it.

[L Gebhardt]

L

I looked through these files again and these caught my eye again. Where did you get these from? What are they for? How did you use them and how did they work for you?

I answered some of this on page 4. They came from the software that drives a LightJet film recorder/photo paper printer. I use them to print on RA4 paper with the lightjet. I also used these files to create my own custom calibration curve to create digital negatives for contact printing on Azo. By ploting a paper curve I was able to work back to what values I needed on film to get which ever density I needed on Azo. I then produced a calibration target for the lightjet and linearized against this. The net result is image files with a gamma of 1.8 (the lightjet doesn't understand ICC profiles) contact print with a very good match to the monitor. As always this is subject to the limitations of the display and the paper and viewing conditions.


So my use of these files was to avoid doing the work I think you are doing now.

[RalphLambrecht]

Well, since everyone is sharing, I'll show the current efforts and what I'm after in a graph. Attached are the tone reproduction cycle for a digital negative on Ilford MG-IV FB, and the transfer function to make it work in Photoshop.

The curve in Q4 is what I'm after. Once that is done, all else is easy.

[Chan Tran]

Helen

We are getting close. I got to 53% for 18% gray for a Dmax of 2.1 and Dmin of 0.05. Now, I want to develop an equation for it.

If you can supply me with the plot points I think I can come up with an equation.

[RalphLambrecht]

Chan Tran

I didn't explain myself correctly. I'm not looking for an equation through existing points (I can handle that). I'm looking for an equation, which indepentdently gives me a curve matching but not using my empiric data closely. It must be something close but different to the theoretical gamma function (rel log density = LOG(100/K%)*gamma).

[Helen B]

Do you need to go far from the theoretical function? The following is a fairly close description of the print profiles that I have looked at:

OD = DR*(K/100)^gamma + DMin

Where OD is the log density (ie for 18% grey the OD is 0.74); DR is the density range of the print; K is the Photoshop value in the range 0-100 for 'blackness' given for greyscale images (not CMYK); K is (255-Val)*100/255 where Val is the 8-bit pixel value or equivalent.

For the DMax = 2.42 case I mentioned above, the gamma works out at 1.9 The formula can be used to determine the relationship between gamma and DR to achieve OD = 0.74 when K = 50 (for example) for any given DMin.

Best,
Helen

[RalphLambrecht]

Helen

I couldn't make that work. This formula returns 0.496 for K=50% and it should return 0.71 [0.66 (theoretical gamma) + 0.05].

[Kirk Keyes]

Ralph - did you work up your Reproduction Cycle graph by hand, or do you have some software that you did it in? I'd love to get ahold of some software that can do that sort of thing...

[Helen B]

Helen

I couldn't make that work. This formula returns 0.496 for K=50% and it should return 0.71 [0.66 (theoretical gamma) + 0.05].

What numbers are you using for DR and gamma to give 0.496?

Best,
Helen

[RalphLambrecht]

Helen

I used DR = Dmax - Dmin = 2.10 - 0.05 = 2.05, and gamma = 2.2.

[RalphLambrecht]

I do it 'manually' in Adobe Illustrator. It's more flexible than any customized software would be. Also, the curve smoothing abilities are unbeatable. It takes about 1/2 hour to do modify my template for a new film, paper, scanner, imagesetter etc.

[Helen B]

Ralph,

How did you arrive at that gamma? Something around 1.5 would fit that DR and mid-grey better.

Best,
Helen

[RalphLambrecht]

Helen

This is my monitor gamma.