andrew_certain Posted December 18, 2004 Share Posted December 18, 2004 I'm trying to understand the math behind calibration (I'm a geek), so I've been performing some experiments in Photoshop. I have an Apple Cinema Display calibrated with Eye-One to a gamma of 2.2, so I'm pretty confident in the fidelity of the display (I also had two of my images printed by a pro lab and they matched the screen). Anyway, I created a red/black checkerboard and surrounded it by a field of solid red. The red in the checkerboard was 255 and the solid red was 128. I then used levels to adjust the gamma until the tones matched. I'm not sure what I expected, but it wasn't what I got. They matched at a gamma of 1.6 (a resulting value of 165). Can anybody point me to a good (hopefully online) explanation of how this stuff works? Thanks. Andrew Link to comment Share on other sites More sharing options...
martian bachelor Posted December 19, 2004 Share Posted December 19, 2004 I was under the impression that Apple's monitors all operated at gamma=1.8, so you may have just more or less independently discovered/rediscovered that fact.<P> The needed gamma can vary with intensity level, and your experiment only tests three points. Try the following image, which you'll see is a more thorough test, and see if you get the result comes out a little differently: http://endlessrapture.info/pics/GAMMA2.gif . Link to comment Share on other sites More sharing options...
Craig_Cooper11664875449 Posted December 19, 2004 Share Posted December 19, 2004 HI Andrew, I have a calibrated Apple Cinema Display (20" plastic bezel) and I use it almost entirely for digital B&W. No matter how I play with the settings, I Ioose substantial shadow detail on the monitor (using a step wedge) if set at any gamma much over 1.8. So 1.8 it is! regards Craig Link to comment Share on other sites More sharing options...
ellis_vener_photography Posted December 19, 2004 Share Posted December 19, 2004 <I>I was under the impression that Apple's monitors all operated at gamma=1.8</I><P>Your impression is outdated.<P>What you are seeing Andrew has a lot to do with homo spaien's color perception. The painter Josef Albers wrote and illustated a fine book on th subject. Link to comment Share on other sites More sharing options...
andrew_certain Posted December 19, 2004 Author Share Posted December 19, 2004 Chris, Mac's traditionally used a gamma of 1.8, but (at least under OSX) you can set them anywhere you want. I'm still not sure how what you wrote would explain needing a gamma of 1.6. Thanks for the gif; what gamma is it supposed to represent? Craig, at least with mine, I get equivalent shadow detail (about every other level up to about 20) with 1.8 or 2.2. Ellis, thanks for the hint. I had no idea that the intensity perception in different color channels would be so different (though I guess had I studied the RGB -> HSV conversion equations more closely, it may have been obvious). Using a white/black checkerboard and 128 grey gives a required gamma of 2.3. Still not quite what I expect, but interesting. Andrew Link to comment Share on other sites More sharing options...
andrew_rodney1 Posted December 19, 2004 Share Posted December 19, 2004 There's no difference in a display for a Mac or a PC, the all have a native Tone Response Curve that's specified using a term known as Gamma. Most displays have a native gamma in the Neighborhood of 2.2. The Mac OS assumes a gamma of 1.8 which is silly but that's what it expects outside of non color managed applications. When you make a document to test all this fun stuff, the working space has a gamma (which doesn't have to match the gamma of the display). All kinds of things have their own gamma (images, displays, printers, etc). At the very least, you should assign the display profile to the image you're playing with so the numbers get sent to the screen and not though the rest of the CMS. I'm not sure what this test image is supposed to do for you... Andrew Rodney http://digitaldog.net/ Link to comment Share on other sites More sharing options...
gordonr Posted December 20, 2004 Share Posted December 20, 2004 Norman Koren has a very nice explanation with diagrams: <a href="http://www.normankoren.com/makingfineprints1A.html">http://www.normankoren.com/makingfineprints1A.html</a> Link to comment Share on other sites More sharing options...
andrew_certain Posted December 21, 2004 Author Share Posted December 21, 2004 Thanks. Andrew Link to comment Share on other sites More sharing options...
martian bachelor Posted December 23, 2004 Share Posted December 23, 2004 > ...but (at least under OSX) you can set them anywhere you want. Ok, I guess what I meant was that the *nominal* Apple value is 1.8. My PC's video card doesn't spit out an actual number - one just moves a slider (actually there are three, but you can lock them together) until you get it right. > I'm still not sure how what you wrote would explain needing a gamma> of 1.6. Thanks for the gif; what gamma is it supposed to represent? The tif doesn't represent any particular gamma. It just does your checkerboard type experiment using more than only three points in the 0-255 range. Investigate the pixel values in each of the nine boxes and you'll see what I mean. If you can perfectly null out all but one or two of the boxes then you've got a value which works well over the entire intensity range. If each color needs a substantially different setting to get a null then something's really flakey (and the grey squares are not likely to null very well). I'm not sure if I'd be too concerned over your value of 1.6 being that much different from the nominal value. Each system is bound to be a little bit different considering the number of components involved, and a tenth or two doesn't seem like much to me. If the part of the chain which you can control requires a little bit less gamma that just means the rest of the system has a bit more than usual. Link to comment Share on other sites More sharing options...
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