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[Definition] Gamma

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  • [Definition] Gamma

    Gamma is a display of the contrast of an image represented by a graph. Traditionally, the vertical axis represents the output values, with the lowest point being the shadows. The horizontal axis represents the input values, with the leftmost point representing the shadows.

    In a perfect theorhetical image, every input value would map to exactly the same output value. If you drew a line from each input and output value and marked where they intersected, then connected all these intersection marks, you'd have a perfectly straight line at a 45-degree angle. The gamma (or "slope") of this image would then be "1", since every input value devided by the matching output value would be 1 (any number divided by that same number is 1).

    If you were to change the input or output values in such a way that the slope of the gamma curve (actually a line in this theorhetical example) were to be more than 45 degrees you'd be increasing the contrast uniformly across the entire brightness range of the image. Likewise, if you were to change the values so the slope was less than 45 degrees you'd be decreasing the overall contrast.

    In the real world, most values are in the mid-tone areas, so we seek to give them good contrast for differentiation. Highlight and shadow areas are flattened to compensate, since we're less sensitive to those areas.

    A graphic representation of an image with a higher midtone contrast and lower shadow and highlight contrast would resemble the letter "S", so the most common gamma curve is known as an "S curve".

    When you use your image editing program to adjust "curves", it is the gamma curve you are adjusting. In other words, you're adjusting (actually remapping) the output values against the input values.
    Last edited by Doug Nelson; 02-02-2004, 09:58 AM.
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  • #2
    Thanks, I've always wondered exactly what "gamma" is. But isn't gamma also used as a term for the middle slider in the levels dialogue box (PS)? Does moving that slider create the equivalent of an S-curve? If not, what does it do - and is it really a "gamma" adjustment?



    • #3
      I've always associated gamma with the midtones, but I never saw a technical explanation for it. Excellent choice for a term to discuss.



      • #4
        Moving the mid-tone values without changing the shadow or highlights would be the equivalent of increasing the slope of the straight portion in the middle of the S-curve. This middle portion is usually the longest section, and therefore is usually used to define the gamma as a single number for simplicity's sake.
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        • #5
          In wet style photography, the Horz. and Vertical values are reversed for negatives, with the H being the amount of light getting to the film, and the V being the density of the film AFTER IT IS DEVEOPLED. We refer to it as a DlogE curve (Density / log of the Exposure). I think I remember this coming from a couple of guys called Hurter and Drifield (watch that spelling). They were from England I believe.
          We used to use a thing called a "reproduction cycle" to set up for film copy work that projected exactly how to set up the equipment so we could make copies based on the gamma of the orginal photo, the gamma of the copy negative, the gamma of the printer optics and the gamma of the printing paper. It would work about 95% of the time.
          Remember that scenes have gamma too. A brite sunny day with deep shadows and very brite highlites is a day with a high gamma, while a cloudy overcast day with hardly no highlites and not really any dark shadows is a low gamma day. (those low gamma days are quite common were I live!!!). We would change the film developing time to add to or delete some of the contrast produced by the gamma of the scene.
          Anyway, just thought I would add this for whatever background info you wanted. Thanks for reading and I do find this new feature quite helpfull.


          • #6
            Is this related to levels? I always use levels to tweek digital photos, but I've never really understood how to use curves.



            • #7
              Moving the midtone slider in levels would be a rough analogy to increasing/descreasing gamma, though with very little fine control.
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              • #8
                If the internet has an 'official' (majority peer review accepted) FAQ on gamma - then this is it:






                Stephen Marsh.


                • #9
                  Timo's views on gamma-correction are very controversial, and even contradicted by Adobe, but I'm sure he knows plenty about gamma in general.

                  This is not the place to get into a pro or con of his views, since megabytes of discussion on this already exist on usenet (use google to search on his name to read for yourself).

                  Just be aware he is considered by many to be a guru, but many more (including just about all the other gurus) discount his theories as "eccentric" (putting it nicely).
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                  • #10
                    Originally posted by Doug Nelson
                    Gamma is a display of the contrast of an image represented by a graph. Traditionally, the vertical axis represents the input values, with the lowest point being the shadows. The horizontal axis represents the output values, with the leftmost point representing the shadows.
                    I just found this section! Believe it or not. (This site is big) I don't want to step on anyone's toes here, but ... that definition at least doesn't correspond to the RGB Curves in Photoshop.

                    In Curves in Photoshop the axes are opposite of what you write here. The X axis are the input values and the Y axis represents the output values. You can see this for yourself in the Input/Output boxes, and you can experience it visually: if you "lift" the curve, darker midtones become lighter.

                    So by lifting the curve, you get "more for less". If you're adjusting RGB, you get more light. If you're adjusting CMYK (reversing the axes) for output, you get more ink. Isn't this correct?


                    • #11
                      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
                        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.
                        Attached Files


                        • #13
                          Mmmmm, Gamma and Gamut are two different concepts

                          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!


                          • #14
                            We were talking about this over at Adobe's U2U and Chris Cox chimed in.


                            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:

                            I've been meaning to do more research into this.
                            In particular, the hardware side of gamma.
                            Last edited by Stroker; 02-14-2005, 07:13 AM.


                            • #15

                              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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