What is Gamma?

[PlacervilleGuy]

Can someone explain what Gamma is and what adjusting the control does to the picture? (Or, point me to a link)

[Deciazulado]

Gamma is the overall slope, or contrast, of the grey tones between the maximum white and the 0 black. For example if you say you have gamma with a slope of 2.2, it means tones ranging from 0-255 in 8 bit color file increase in in value on screen raised to the power of 2.2 (so for example a grey tone of level 80 on a calibrated monitor will be displayed 4.59 times as bright as a grey tone of level 40 ) (2 raised to the 2.2 power = 4.59)

If you plot this level changes in a logarithmic scale graph, you would find that the line graphed has a slope of 2.2, (each change of horizontal value of 1 in the chart becomes a change of 2.2 values in the vertical) a much steeper (contrastier) slope than the standard 45 degree diagonal slope of gamma 1, which has a linear 1 to 1 relationship of levels vs ouput.

What this means in practical terms is, if you have your display/source combination properly calibrated so the maximum white tone of the video reaches the maximum output of your display and the minimum black tone of the video reaches (or approximates) 0 light output on your display, when you change the gamma, you change the middle tones, increasing image contrast (difference between the light grey tones and dark grey tones), making the dark tones darker still when you move (increase) gamma in the direction of 2.2->2.4->2.6; and decreasing image contrast, making darker tones lighter, when you move (decrease) the gamma setting in the direction of 2.2->2.0->1.8

For example if you watch with very bright lights on, you might find decreasing the gamma a bit might make things look a little better by lightening the dark tones sightly, or vice-versa, if you turn off all the lights and have a small monitor/are far away from it and it's surrounded by darkness, increasing the gamma might make for a subjectively better picture by darkening the tones slightly.

[Blacklac]

I find it a very convenient feature on my Pioneer 51 for quick adjustments.