Originally Posted by GlenC
The correct gamma setting is not as simple as picking a number. The reason many are using a "bump" at the lower IRE is due to the fact their gamma is too high for their system.
For those who don't fully understand the effect of gamma, think of it like a staircase, with each step getting a little taller. With higher gamma settings, the first steps are really shallow, then they get larger, faster, as they approach 100 IRE. The goal is to get black as black as possible, while still being able to see the detail as you rise out of black, like 2, 3, 5 IRE, etc.
Okay, now you set gamma to 2.4, set black and white levels, then with a 50% APL pattern, you cannot see the 2, 3, 5 IRE detail. To fix the problem, some try to reduce the gamma at low IRE by bumping the luminance in the low IREs. Now with this "fix", you take a few larger first steps, then a few flatter steps till you are back on line with the gamma. When improperly done, one can crush some low APL detail, by making the differences in IRE increases, after the "bump" equal. For instance, you bump at 5 IRE but the effect may cause luminance changes somewhere between 6 to 10 IRE to be so close that you cannot see the difference, thus loosing detail.
Properly setting gamma for the specific theater system will allow the best black level while maintaining sub 10 IRE detail to be visible in medium to low APL scenes.
Another factor in proper gamma is the brightness and contrast ratio of a projector and the eye's ability to see the dark detail with bright content in the same scene. What good is having a 2.4 or 2.5 gamma if you cannot see the low IRE detail because your eye is crushing the blacks?
I don't remember the exact numbers, however, IIRC, it starts with 5, 10......
Some shows can vary, but I prefer to have gamma set somewhere between 2.1 and 2.2. With this, things like strands of dark hair are visible rather than a black mass with no detail. Lapels, wrinkles, etc. are visible on black suits and sweaters. Now the Factory gamma settings are not absolutely accurate. I have made corrections with the custom gamma controls, and further corrected the gamma with the Lumagen. A good, linear measuring device is necessary, to accurately adjust gamma.