Step Tablet 0-100% Values to 8-bit 0-255 Values

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Fragomeni

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Hi all,

I'm working on putting together a transfer function for Platinum & Palladium. I'm using a digitally printed step tablet containing steps representing values from 0% to 100%. I've printed the step tablet and taken values using a densitometer and I've created a curve using Ralph Lambrecht's technique explained in Way Beyond Monochrome 2. I'd spoken with Ralph a few years ago and we were able to get this to work for a different project but I'm running into some issues unique to the current version of Photoshop that I'm using. CS6 no longer has the option to input a transfer function within the print dialog box. This is the only space I was aware of that allowed 0-100% values to be input. When inputing the values into a typical curve, the 0-100% values don't match with the 255 values of an 8-bit image. I'm wondering if anyone can provide the equivalent 0 – 255 8-bit luminance code values for the 0-100% steps of the step tablet? I think this should allow me to convert the values into a proper curve that could be applied before printing which would have the same effect as the transfer function with values from 0-100%.

As an aside, I'm well aware that there are many other methods of producing a digital negative. My question is specific to the issue described above so offering countless alternatives and not touching on the original question is not entirely helpful so lets please try to stay on topic if possible.

Any help/insight here would be hugely appreciated!

P.S. I'm re-downloading CS4 in case it is the only option that I end up with (to get back the ability to enter transfer function directly).
 
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Fragomeni

Fragomeni

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Another approach here could be a calculator (if one exists) that will convert visual transmission density values as read on a densitometer to 8-bit values. If anyone knows of something like that or can explain how to convert transmission density values to 8-bit values, that could solve this for me and I may be able to use that to manually build a proper curve.

Thanks!
 
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Fragomeni

Fragomeni

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Talking to myself haha but I may have jumped the gun here forgetting that I can just read the 0-255 values using any image editing program and the use those values against the transmission values of the neg made using the step wedge compared to the reflection density values of the test print to make a curve. Going to see if that works.

Any insight still appreciated.
 

fnovau

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What was the color space your step tablet was generated in the beginning?. I think you should convert your densitometer readings D to reflection or transmission percentage through this simple equation: Reflection %=100*10^-D.
If for any reason your working color space during your step tablet generation was Lab then you must convert D to L* using this equation L*=116*((10^-D)^(1/3))-16 if your density D is less than 2.05
I hope this helps
-Francesc
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fnovau

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I believe the answer is very simple. If you know the color space your step tablet was generated and printed you have to transform density data D to lightness L* if you generated step tablet in Lab color space or K if in CMYK color space or grayscale For transform density D to lightness L* you can use this equation: L*=116*((10^-D)^(1/3))-16 if density D is <2.05 or use this that gives ink reflection or transmission % K=100*(10^-D)
I hope this helps
-Francesc
Fotografia estenopeica=el mon per un forat ...pero de cap per avall
http://www.novau.net[/URL
 

fnovau

Member
Joined
Sep 21, 2007
Messages
7
Location
Sitges, Spai
Format
Pinhole
I believe the answer is very simple. If you know the color space your step tablet was generated and printed you have to transform density data D to lightness L* if you generated step tablet in Lab color space or K if in CMYK color space or grayscale For transform density D to lightness L* you can use this equation: L*=116*((10^-D)^(1/3))-16 if density D is <2.05 or use this that gives ink reflection or transmission % K=100*(10^-D)
I hope this helps
-Francesc
Fotografia estenopeica=el mon per un forat ...pero de cap per avall
http://www.novau.net[/URL
 
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