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Photo Retouching "Improving" photos, post-production, correction, etc.

[Definition] Gamma

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  #11  
Old 02-02-2004, 09:57 AM
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Doug Nelson Doug Nelson is offline
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It does sound like I reversed them. But I was much younger, then

I fixed them in the original, so I won't lead anyone else astray.
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  #12  
Old 02-12-2005, 01:52 PM
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Gama and the Print world

The term Gamma or "Gamut" in the Print world refers to colors that may or may not be printable with CMYK inks. You can see this easily in a very colorful and saturated photograph. Bright reds, oranges, emerald greens, turquoise blues are notorious for being "out of gamma" and not reproducible in CMYK. In PhotoShop, if you do a "Select ->Color Range" and the pop up menu --> Out of Gamut you will get a selection of the RGB colors that are going to "shift" or become duller when printed in process (CMYK) color work.

The attached file is a good example. Most all of the red/pink is out of gamma for print purposes.

When working on photos to be printed at high resolution in CMYK (brochures, magazines etc.) you should convert your RGB files to CMYK before saving as TIFF. (Retain your origianl PSD RGB file in case you need to go back and make changes). Also, when you need to select colors for Text or fills, be sure to set PhotoShop's VIEW dropdown menu to "View Gamut Warning". This reduces your color palette, but will assure that the colors you select will be printed correctly.
A Pantone Process color book is a great tool.

If you have to include vector art (such as corporate logos) in your PS work, keep a Pantone "Spot to Process" color guide on hand and that your customers understand that their corporate PMS colors may shift and make appropriate adjustments to the Illustrator colors if necessory. My local community college still has a problem with their PMS 166 orange. All their offset printed materials are perfect, (geeee.. the local printer just pulls a can of PMS 166 ink off the shelf and prints the plate!!), but they just don't understand that in the CMYK world there is no PMS 166 unless they want to pay for an additional sport color with its additional plate charge.
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File Type: jpg CALADIUMS20.jpg (74.1 KB, 14 views)
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  #13  
Old 02-13-2005, 10:16 PM
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Mmmmm, Gamma and Gamut are two different concepts

Quote:
Originally Posted by Swampy
The term Gamma or "Gamut" in the Print world...
The description of Gamut is very good, but it has nothing to do with Gamma!
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Old 02-14-2005, 07:01 AM
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We were talking about this over at Adobe's U2U and Chris Cox chimed in.

Gamma:
output=input(1/gamma)

That's what the mid-tone slider in Levels uses.
That's also why the slider has a funny range compared to the other sliders.
Something like 0.1 to 1.0 and 1.0 to 10.

If you graph it in the range (0,0)-(1,1), you should be able to see it.
Java(?) grapher: http://people.hofstra.edu/staff/stev...Graf/Graf.html
Examples:
x^(1/0.5)
x^(1/2.2)

I've been meaning to do more research into this.
In particular, the hardware side of gamma.

Last edited by Stroker; 02-14-2005 at 07:13 AM.
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  #15  
Old 02-14-2005, 09:07 AM
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Smile

Rex

Quote:
Originally Posted by Doug Nelson
Moving the midtone slider in levels would be a rough analogy to increasing/descreasing gamma, though with very little fine control.
Doug is correct and using the midtone slider in levels is a crude way to correct gamma/gamut problems in a print job.

Open my attached file below, turn on View Gamut Warning in PhotoShop's View menu and all the out of gamut areas are masked. In a Level Adjustment window you can reduce the masked areas by sliding the midtone slider. You can also reduce the out of gamut areas by sliding the input/output levels below. Another way to make corrections is to reduce the saturation for those areas.

When speaking of gamma correction the dictionary defines it as:

NOUN: A correction to the contrast of images and displays, performed by either software or hardware, and designed to correct for the fact that the intensity displayed on a cathode-ray tube is not linearly related to the input voltage.

So when I think gamma, I think in terms of hardware, (monitors, scanners etc) and how they produce the image on the screen. When I think of gamut, I think in terms of the range of colors that CMYK inks can produce from that image onto paper.

For those of us in the print world, setting white points and gamma and calibrating our monitors is a primary step to translating accurate colors to the monitor. Gamut corrections help us make the translations from screen representation into CMYK print ranges.

A couple years ago I had the task of creating a magazine cover from a piece of art that was acryllic painted on glass. Scanning alone was a bitch, but I finally purchased some grayboard to include on the side of the painting to have a perfect 50% gray to key on. After a quick levels adjustment using the grayboard, I had a spot on representation of the painting on the screen including transluscent areas where the paint was "thin". The subject was caladium leaves (my home town is the caladium capital of the world so it's a subject I deal with often) with lots of reds and greens.

A quick check of the gamuts showed just about everything out of range!! I guess you would say my gamma was great, my gamut sucked. I spent three days working on color corrections and proofing on my Epson 900N (which is fairly close to my print house's when I use their color profile). I did 9 different versions of the graphic, ganged them on one document and sent out for a Fuji match print.

The best match to the original painting and the original screen representation was so out of wack it seemed almost impossible. In order to get the subtile greens I had to set the gamuts to turquoises and to get the subtile reds I had to use purples! It looked absolutly UGLY on the screen, but printed great.

In the world of four color process prepress gamma is the starting point, but gamut pays the bills.
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