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QTR-Quadtone RIP

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I don't understand gamma, I guess.

I don't understand gamma, I guess.

2015-08-02 by jp432r@...

I think the problem I'm having has to do with gamma, but I don't have a solid grasp of what gamma does in each part of the environment I can't work out a solution. In fact I'm not really sure if it's the problem.

I'm making digital negatives for contact printing

I have carefully calibrated QTR using a gamma of 1.8. Step tablets print great. Scans with an embedded gamma of 1.8 print well, but the stuff I shot with my digital camera, post processed in Photoshop, inverted to negative, and printed using the calibrated QTR settings print very poorly. Very dark and muddy.

I don't really know the gamma of the files my camera creates, but I assume it is 2.1 or 2.2.
I've searched the web for info on converting a file's embedded profile from 2.x to 1.8 without luck.

I could start over and create a step tablet with gamma 2.1 and then recalibrate, but I've been working on this a week and I'm hoping to avoid that.

Does anyone have any suggestions on how to approach the problem?

Jeff


Re: [QuadtoneRIP] I don't understand gamma, I guess.

2015-08-02 by Alan Vlach

Don’t know if gamma is your problem, but if you want to make sure your images are in gamma 1.8 go into Photoshop/Edit/ Color Settings and make sure the workspace you are using is set to gamma 1.8 and that you have checked all the boxes about profile mismatches. That way if you open an image and it is not gamma 1.8 you will be notified and asked if you want to convert it.


On Aug 2, 2015, at 2:19 AM, jp432r@... [QuadtoneRIP] <QuadtoneRIP@yahoogroups.com> wrote:

I think the problem I'm having has to do with gamma, but I don't have a solid grasp of what gamma does in each part of the environment I can't work out a solution. In fact I'm not really sure if it's the problem.

I'm making digital negatives for contact printing

I have carefully calibrated QTR using a gamma of 1.8. Step tablets print great. Scans with an embedded gamma of 1.8 print well, but the stuff I shot with my digital camera, post processed in Photoshop, inverted to negative, and printed using the calibrated QTR settings print very poorly. Very dark and muddy.

I don't really know the gamma of the files my camera creates, but I assume it is 2.1 or 2.2.
I've searched the web for info on converting a file's embedded profile from 2.x to 1.8 without luck.

I could start over and create a step tablet with gamma 2.1 and then recalibrate, but I've been working on this a week and I'm hoping to avoid that.

Does anyone have any suggestions on how to approach the problem?

Jeff




Re: I don't understand gamma, I guess.

2015-08-03 by richard@...

There are lots of places the word gamma is used along the way, so it isn't exactly clear where to look first.

When you say you calibrated QTR with a gamma 1.8 do you mean that the step wedge you used had an embedded gray gamma 1.8? or that some other setting of 1.8 in a photoshop levels adjustment (Mike Ware approach), or in the print dialog box? Or, was this in the Gray_Gamma input in the QTR profile creation tools (or text file).

The gamma from your digital camera is based on the color settings in the camera, or in the editing/raw conversion process (assuming you coming from raw capture). You can choose to use ProPhoto RGB as an export color space, which has a gamma of 1.8, which would translate directly to your current QTR calibration. I am working more with capture 1 and different process recipes for different output types depending on where things are ending up. The same can be done in Adobe Camera Raw.

If your digital camera is just spitting out jpegs then they are probably either Adobe RGB 1998 or sRGB, both of which have a gamma of 2.2. If your gray working space in photoshop is 1.8, then if you just convert the color files to grayscale it should convert it directly to the working space and it should look correct (although depending on how they are inverted prior to sending them to QTR it might end up wonky)

The nice thing about having a linear space in QTR, especially when working with digital negatives, is that the gray curve correction and linearization step for each process and setup will make the the whole range print straight so there is no worry about the shadows at gg 1.8 or 2.2 being flipped to the highlights and causing that to go weird in the print. Of course all that is dependent on everything being the same throughout the process... which seems to be what is monkeying things up here.

I just posted about a spreadsheet tool for curve creation for inkjet prints, but I did create one for working with a densitometer and converts density to LAB so that it can work easily with QTR and digital negatives.


Richard Boutwell

Re: I don't understand gamma, I guess.

2015-08-03 by jp432r@...

Paragraph 2 - Yes. I mean that the step wedge I used had an imbedded gray gamma of 1.8.

I didn't know of the ability to convert from RAW into a gamma 1.8 space. I will give that a try, although I'm a bit miffed at having to do all the post processing again, but it seems that's the only way I can ever really learn something.

Thank you for your kind assistance.

Jeff

Re: I don't understand gamma, I guess.

2015-08-03 by richard@...

yiu shouldn't need to reprocess everything. Just set your color settings in the raw concmverter for future photographs. 

For the ones you already have processed, you might want to try testing the difference between just assigning a gamma 1.8 to those pictures and converting the current color space to gray gamma 1.8. You might want to heck levels afterward to make sure you will get what you expect when printing the negative/making the print. 

Richard Boutwell

Move to quarantaine

This moves the raw source file on disk only. The archive index is not changed automatically, so you still need to run a manual refresh afterward.