QTR/Changing the Embedded Profile of a Step Wedge

Ben Altman · Nov 20, 2010

Hi All,

I'm wondering if the group wisdom can give me some clarity...

Normally for my inkjet negatives I work with my files in Roy Harrington's QTR RGB-Lab profile. As I understand it, this is a very linear space which allows me to embed a profile without it doing much. Thus if I want to add gamma or other changes, I can build them into the QTR profile or apply a curve in Photoshop.

When I create and refine a QTR profile I embed RGB-Lab into the step wedge that I print for tests.

I'm currently working with someone else who has his files in Gray Gamma 2.2. For creating a QTR profile for him I believe I should embed that profile - GG 2.2 - in the step wedge for testing.

Here's the question. In Photoshop, should I use "Convert to Profile" or "Assign Profile" to change the step wedge?

My thinking is to use "Convert" as that leaves the intervals of the step wedge unchanged - for instance a mid-tone step is level 127 before and after the conversion. That way, given that QTR prints without color management, a level 127 in the file will print as a level 127 on the OHP, and everything will correspond.

But it's easy to get confused by this stuff, so I'd love it if someone can tell me if I have it right - and hopefully reduce the confusion, if any!

Ben

donbga · Nov 20, 2010

Ben Altman said:
Hi All,

I'm wondering if the group wisdom can give me some clarity...

Normally for my inkjet negatives I work with my files in Roy Harrington's QTR RGB-Lab profile. As I understand it, this is a very linear space which allows me to embed a profile without it doing much. Thus if I want to add gamma or other changes, I can build them into the QTR profile or apply a curve in Photoshop.

When I create and refine a QTR profile I embed RGB-Lab into the step wedge that I print for tests.

I'm currently working with someone else who has his files in Gray Gamma 2.2. For creating a QTR profile for him I believe I should embed that profile - GG 2.2 - in the step wedge for testing.

Here's the question. In Photoshop, should I use "Convert to Profile" or "Assign Profile" to change the step wedge?

My thinking is to use "Convert" as that leaves the intervals of the step wedge unchanged - for instance a mid-tone step is level 127 before and after the conversion. That way, given that QTR prints without color management, a level 127 in the file will print as a level 127 on the OHP, and everything will correspond.

But it's easy to get confused by this stuff, so I'd love it if someone can tell me if I have it right - and hopefully reduce the confusion, if any!

Ben

I believe you want to Assign Profile to tag a file when you do not want to change the internals.

'Convert To', does just that, converts from one color space to the other.

Perhaps this may help or confuse:

http://www.adobepress.com/articles/article.asp?p=1315593&seqNum=6

It all gets confusing.

Don Bryant

Ron-san · Nov 20, 2010

Ben Altman said:
Hi All,

I'm wondering if the group wisdom can give me some clarity...

Ben

Ben-- This may be more group grope than group wisdom, but here is what I do. For purely historical reasons I embed gamma 1.8 in all my image files and step tablets. But I often have to deal with image files that have other profiles embedded (ie, adobe RGB, generic gray, gamma 2.2, etc). My practice is to use the Convert to Profile function to embed gamma 1.8 in each of these files before I inverto to negative and print it. Then I proceed to print negatives from the file as usual. SO-- my suggestion is to use Convert to Profile to embed the same profile in the image file as you have in the step tablet file. This way you do not have to change any of your previous calibrations.
In my experience, if you use Convert to Profile to change the profile of your step tablet you may often find that the various steps of the tablet do not read correctly with the Photoshop eyedropper tool. In other words, on the occasions where I have converted the profile in the tablet I have then had to go back and jigger many of the step tablet steps to get them to read the same % gray as is written upon them.

I do not understand the underlying complexity behind converting profiles, but the above is a practical approach that works for me.

Cheers, Ron Reeder

pschwart · Nov 20, 2010

Don Bryant said:
I believe you want to Assign Profile to tag a file when you do not want to change the internals.

'Convert To', does just that, converts from one color space to the other.

Perhaps this may help or confuse:

http://www.adobepress.com/articles/article.asp?p=1315593&seqNum=6

It all gets confusing.

Don Bryant

CONVERT will change the color numbers in the file to reproduce the same color in the new color space; ASSIGN will reinterpret the existing numbers based on the new profile.

donbga · Nov 20, 2010

For purely historical reasons I embed gamma 1.8 in all my image files and step tablets.

Ron can you explain what you mean by '...purely historical reasons...'?

I do not understand the underlying complexity behind converting profiles, but the above is a practical approach that works for me.

If it ain't broke ...

Thanks Ron.

Don

Ben Altman · Nov 20, 2010

Thanks for responses, folks. Yes, I had it backwards, duh. Note to self - don't trust memory, check before posting. ASSIGN will leave internals unchanged, which is what I want.

Ron, I agree completely with your method for printing a file with a different profile on one's own system - Convert will keep the file looking the same so it will run ok on your set-up. I'm doing the opposite - setting up someone else's system so they can run prints in the profile of their choice without converting their files.

Assigning the GG 2.2 to the step wedge is what I need to do. That gives me a wedge with the right 0-255 Level number at each step and the correct embedded profile. However...

...There's a catch. Assign does not allow me to go to a Grayscale profile from an RGB profile. And if I change Mode to Grayscale, the values shift. A quick search on the net seems to indicate that there is no solution to this problem. I guess I'll just sit down and rebuild my step wedge in Grayscale. My head hurts!

Ben

Ben Altman · Nov 21, 2010

OK, I re-did the step wedge today. Didn't take too long using the magic wand and a little Photoshop Action.

Moral of the story for those who care about this arcane stuff: it's okay to change the Mode of a step wedge from Grayscale to RGB - the step values will not shift. But not the other way - going RGB to Grayscale screws things up.

Then, within either Mode, use Assign Profile to change the embedded profile without moving the values of the steps.

Edit - For image files, Ron's method for converting from RGB to Grayscale with Convert to Profile will work fine, because one wants the image to look the same after conversion, however the computer figures that out. It's just step-wedges that are the problem, because we want to keep the numbers the same.

Thanks everyone.

Ron-san · Nov 21, 2010

Don Bryant said:
Ron can you explain what you mean by '...purely historical reasons...'?

Don

Don-- Years ago I started using gamma 1.8 in my step tablet and in all my image files (I think because back then this was recommended for all Mac users). So like a good little boy I did it without really understanding the reason. And since it worked (for grayscale printing at least) I have never changed.
My understanding, though, is that I could just as well have put gamma 2.2 or generic gray or whatever in everything, just so long as I was always consistent.

Cheers, Ron Reeder

QTR/Changing the Embedded Profile of a Step Wedge

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