Gamma Vs. Brightness

(From The Yahoo Avid User's Group -- By Stephen Hullfish)

BRIGHTNESS does not usually exclusively refer to a control for black levels. What brightness usually does is raise the level of ALL of the pixels -‐ including black pixels.

So, brightness controls black, gamma and gain or highlights. But when you look at the image as you control brightness, one of the first things that you notice is that it is raising the black levels. By that definition, the contrast control on a monitor also will control the blacks, because increasing the contrast simultaneously lowers the blacks and raises the highlights and decreasing it does the reverse.

To explain GAMMA as a curve might be easier described by saying that you can think of a range of pixels going from black to white in a linear diagonal line and gamma is a curve that strays from that linear diagonal line either up or down (or sometime programs allow both, with a curve in one direction near black and a curve in another direction near white.)

If you have an AVID with color correction, this is like
THE CURVES TAB. The line starts out as a diagonal line with dark pixels at the bottom left, bright pixels at the top right, and an even distribution of brightness between them. By placing a point in the middle and moving it up or down, you are changing the gamma (the "trip") from black to white.

For a cinematographer, the "trip" from black to white is rarely straight. Each film stock has a unique curve for gamma.

For practical purposes in the AVID software, gamma can be thought of as the midtones of the picture. Of course, altering the gamma is a little like pinching the middle of a rubberband and moving it towards one end or the other, because levels of pixels at one end become stretched out and levels at the other end become compressed, so that the "trip" from black to mid-grey when you lower gamma is shorter (compressing all of the levels between black and mid-grey making mid-grey darker) and the "trip" from mid-grey to white is longer, stretching out all of the values between mid-grey and white.