Originally Posted by spacediver
interesting, I wonder why they made that statement then. I also wonder why they didn't have in mind the target luminances at those two points so they could preserve the proper gamma while making grayscale adjustments (and I do understand that a 10 or 20 point adjustment is better than a 2 point adjustment, and that gain and offset refer to 80% and 30% respectively, and that with a 10 point adjustment those terms lose their meaning - at least I think
You can't really establish a target luminance for black... most meters will not measure the black of a really good flat panel or of most projection setups. If you get a measurement, it's likely to not be accurate anyway. And the room has to be darker than the TV or projection system for the measurement to be accurate. I've had sensitive light meters in my room (not calibration meters, light meters) I was experimenting with and the meter would pickup the presence or absence of a single LED on a piece of equipment even when the equipment was behind an opaque screen (screen free-standing in the room on "legs", in front of the equipment racks). So unless you are going to make the room pitch black including covering every LED with opaque tape, you will never get a accurate black level measurement. For 100% white, yes, you can decide your target luminance level is going to be 33 fL and measure 100% white repeatedly so that when you adjust gains, you maintain the luminance level you want. But it can be tricky because what we perceive as white or gray is 72% green light, 21% red light, and 7% blue light. So if you want to make a +3 adjustment to the green gain control, you'll make 100% white a fair bit brighter while a +3 blue gain adjustment will barely influence how bright 100% white is after the adjustment. So your +3 green gain adjustment might be better done as +1 on green and -2 red and -3 blue or some variation of that where you increase some controls and decrease others to maintain your desired 100% white level... but since you can't measure the black accurately, you have to use your powers of observation -- which is also necessary because at, say, 5% white, leep in mind that there will be so little blue light that almost no colorimeter or spectroradiometer within the price range of pro calibrators (let alone home calibrators who won't spend that much) will be accurate measuring the blue light at 5% white and measuring blue at 0% white is even harder... remember, you could easily be measuring .001 fL or less down in those dark ranges and only 7% of those tiny amounts of light will be blue light. So you always have to rely on your eyes to tell you what's going on at black or near-black. I've seen strong magenta, and blue tints at very los light levels even though the meter was saying the measurement was neutral gray. You HAVE to believe your eyes at that point because the meter just gets increasingly unreliable as the light levels decrease. So your meter can tell you that your darkest step is good, but you could have a red tint to star-fields and errors like that are VERY obvious and very difficult to live with.
So, again, being observant about how the TV behaves when you move offset or gain controls is critical. That's not something you can probably learn from a video because there are so many different scenarios you can run into with different models, different years, and different brands of TV or projection setup.
Gain and offset do NOT refer to 80% and 30% respectively. Your goal with the gain and offset controls is to get the ENTIRE grayscale as accurate as it can be with just those 2 controls (3 settings for each, red, green, and blue). It is my experience that you almost NEVER get the best result using 80% and 30% with gain and offset controls. I look at the ENTIRE grayscale after making an adjustment because the offset controls typically affect everything from 0% to 70% or so while the gain controls usually control the entire range from 30% to 100%. Meaning there's a fair bit of overlap in the middle between 30% and 70% and you'd never see the effects of that overlap if you were only looking at 80 and 30.
"They" (essentially everybody) makes statements without thinking through the logic or perhaps from not understanding the underlying concepts completely or simply by not thinking far enough through what they are saying and what they are intending to say. They "meant well" but simply didn't use an appropriate comparable situation when you get right down to it.
The danger of a 10 minute or 1 hour video is that it can't tell you all this stuff.