What is Gamma besides a Greek letter?

[Grandpa Ron]

I was playing with a photo editor and most of the commands were straight forward. However, I could not figure out the specific use for Gamma.

When, why and how do you adjust it?

 

[TCampbell]

Gamma describe the difference between actual variations in brightness vs. what your eye perceives as variations in brightness.

In a simple linear brightness scale where 0 represents the darkest black, 100 represents the brightest white, and 50 represents the middle... your eyes wont perceive things that way. You will perceive the 50 value as being just a little dimmer than the brightest white. Also, what your eyes perceive as being about 50% bright is, in reality, a much dimmer value. Human eyes are much better at noticing slight variations in brightness in darker areas than we are at noticing the same amount of variations in brightness in lighter areas.

If you've ever wondered why Ansel Adams says to use "18% gray" as "middle" gray... and wondered why 18% is "middle gray" and not 50% gray... that's gamma!

Instead of brightness from black to white being a simple linear scale... the gamma version causes a more aggressive ramp up in brightness at the dark end and only a very slight ramp up toward the bright end.