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and the digital darkroom
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* What is gamma? Very simply, gamma sets how light or dark your images appear on screen. More exactly, gamma describes the relationship between the digital brightness levels in your image to the actual brightness on the screen. The eye (like the ear) responds logarithmically to increasing levels, so we need a logarithmic mapping of displayed brightness to digital signal level. The gamma describes what power law is used for this conversion, and so effects the apparent linearity of the steps in the stepwedge — and in your other images. For a much better explanation of gamma, follow the first two links given under More Info.

**Brightness and contrast controls are somewhat counter-intuitive in how they work. The Contrast control actually sets the gain or "volume" and is a non-critical control on your VDU - just set it for sufficient visibility.

Brightness despite its name actually sets the black point, and its setting is critical to whether you see what you expect to see. If the brightness is set too high you will see a background which is not truly black, if it is set too low, you will not see any shadow detail in your images. It is easy to mal-adjust a monitor so that you are missing a significant amount of dark-end information and not be aware of it.

How to set up your monitor

Note: The shorter simpler version of this setup procedure is here.



Click for step wedge in a new window with black background

This simple test pattern is evenly spaced from 0 to 255 brightness levels, with no profile embedded. If your monitor is correctly set up, you should be able to distinguish each step, and each step should be roughly visually distinct from its neighbours by the same amount. As well, the dark-end step differences should be about the same as the light-end step differences. Finally, the first step should be completely black, the same as the unscanned area of the screen at the sides or top and bottom of the screen.

Here is a simple method to adjust your monitor for black point and gamma*. These two adjustments should ensure that you see the correct brightness levels on your screen, and that you don't miss any information. This method uses the step wedge to set the black point (the brightness control on your monitor) and Adobe Gamma to set gamma*.
  1. Select the link above to get a new window with the step wedge on your screen. Set the room lighting to your normal viewing condition. Turn the monitor CONTRAST** up to near max.

  2. Adjust the monitor BRIGHTNESS** control until the black end of the step wedge shows a reasonable distinction between the completely black step and the next step. You should be able to distinguish between all the lower level steps roughly equally. Also make sure that the first step is completely black. If necessary, adjust the height of your VDU picture so that there is unscanned blank area at the top of the screen and compare the unscanned area with the black scanned area.

    When you are satisfied with the brightness setting, mark this setting on the brightness control and do not touch it again. You should adjust CONTRAST for viewing comfort from now on, not the brightness control.

What gamma to use?

  1. The gamma you should aim for is not as obvious as it might seem. Macs generally have an effective gamma of 1.8, while PCs are generally set to 2.2. I chose to set mine to 2.0 as an average, but this might not have been the best move. If you are not sure which gamma to use, stick to the defaults (2.2 for PC, 1.8 for Mac) or do some more reading at the links at More Info

Set gamma using Adobe Gamma

  1. If you don't have a recent version of Adobe Photoshop with the Adobe Gamma utility, go to step 5. If you do have Adobe Gamma utility, run it now. In Photoshop 5.5 you can do this from Help/Color Management/Open Adobe Gamma. Choose the gamma you want to set. If you don't know, read the para "What gamma to use?" above. Follow the Adobe instructions and adjust gamma only; do not adjust brightness or contrast.

    (Brightness adjustment as set up in the last step is a better adjustment IMO than the Adobe adjustment. The problem with the Adobe adjustment is that your final setting depends very much on how close you choose to set the "nearly black" image to "black". When I first tried Adobe Gamma, I set it as instructed so that the "nearly black" square was only just distinguishable from the true black — the result was that the bottom three steps of my step wedge were all merged. This is not a correct setting and would mean that the bottom 40 or 50 brightness levels (out of 256) in my 24 bit color range were wasted. In other words, anything darker than about one fifth of full brightness would be invisible on the screen).

    If you have followed this paragraph using the Adobe Gamma utility, you have basically finished, so go straight to step 7.

Set gamma without Adobe Gamma

  1. If you do not have a recent Photoshop with the Adobe Gamma utility, go to one of these links and choose a test pattern for your required gamma.

    Timo's famous site which contains some good gamma adjustment test patterns

    Excellent tool by Hans Brettel requires Java


  2. With the correct gamma test pattern on screen, adjust gamma using the display adjustment facilty that comes with your display adapter. For example with my Matrox G200, I access this adjustment (PC only — I don't know about Macs) by right clicking on an empty part of the screen, select Properties/Settings/Advanced/Color tab. Other display cards will have a different means of access, and cheaper cards don't provide gamma adjustment at all.

  3. If you made big changes to gamma you should check the step wedge again and if necessary go round the loop again.

Gamma is not linear - how to check your settings

It is important to realise that gamma is not a single figure, and may vary across the brightness range - there is a gamma associated with each brightness level on the screen. Ideally gamma is constant with brightness, but often it is not. How you adjust the 'linearity' of gamma is something I have not yet discovered, but you can check how well your system is adjusted by using the excellent tool by Hans Brettel. This is a java applet which allows you to measure your display gamma at any brightness level. In my case, I found that after using the above technique (Step 3) to set gamma to 2.0, my actual gamma was 1.6 at the dark end, 2.0 in the middle, and 2.4 at the bright end. This is confirmed by the fact that my step wedge has smaller apparent steps at the dark end and larger apparent steps at the bright end.

As I said, I dont know how to fix this, if anyone knows please tell me. My theory is that this occurs when the software gamma adjustment assumes a hardware gamma that is different from the actual hardware gamma.

When I recently mentioned the problem of monitor setup on a couple of forums, I was amazed at how many people checked their greyscale and wrote to me having realised that they were missing out on much of the darker end of their images. The moral is — especially if images are important to you — if in doubt, check it!

The issue of display/software set up is as complex as you want to make it. Everyone should do the basic adjustments described here, but to go further into color management is, as several people have said to me, to start out on a very steep and slippery slope!

If you want more info about gamma and the "black and white" side of monitor adjustment, try the sites listed below. They all cover gamma in general but I am not sure if I believe some of the basic theory described in any of them. In fact these sites do not agree on the theory and all seem to be pre-occupied with the transfer function (or "intrinsic gamma") of the cathode ray tube because it is the heart of most monitors. I don't think that the subject has anything to do with this except that the hardware and software have to know the tube characteristics to compensate. But 'display' gamma is just as relevant on an LCD screen, without a tube in sight!

I believe that gamma as discussed is really about the response of the eye, and only the eye, and defines only how we translate a digital value in our computers to final brightness on the screen. It seems to me that a gamma of 1.8 on a Mac will look the same as a gamma of 1.8 on a PC, despite much-discussed but irrelevant differences in internal hardware gamma corrections. The 1.8 refers to our attempted overall gamma, from digital image to brightness seen on the screen. So forget about tubes having gammas of 2.5 and Macs having different hardware gammas — totally irrelevant and unnecessarily confusing. If you disagree, please tell me why as I am always keen to learn.


More info...

Despite my doubts on some of their theory, these sites make good reading and are very informative...

Timo believes we should all use a gamma of 1, and has plenty of discussion as to why this is so. Inside this discussion is some good info, and his gamma test charts are quite good. Fortunately these charts cover gammas other than 1. But if you are struggling with the subject, this site may confuse you more. So be wary of his gamma=1 adjustments: if you follow this you will seeing things on your screen as no-one else sees them!

Copyright © 2003 Julian Robinson
This page updated: 27Jul2001/13Apr2005  

Please notify errors or comments by
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