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Adjusting the gamma value


show more Adjusting the gamma value provides you with in-depth training on Design. Taught by Deke McClelland as part of the Photoshop CS4 One-on-One: Advanced show less
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Adjusting the gamma value

Just to bring you up to speed, I'm working inside the Semi-corrected.psd image and I created a new levels adjustment layer right here. Turned off the old one, and I went ahead and assigned these values right here, a black point value of 12 and a white point value of 192, which means that we are clipping all the colors that are darker than 12 to black, and we are clipping all of the colors that are lighter than 192 to white, and we are spreading the remaining colors in between from 12 to 192, those luminance levels.

We are spreading them across the entire histogram, so 12 becomes zero, 192 become 255, and everybody else gets spread out in between. Now right there in the center is the most important input levels options of them all, which is the gamma value, and it really is super important, because unlike the black point and the white point both of which clip colors, the gamma value refuses to clip. It never sends colors into the luminance oblivion, the way the others do. So for example, what I mean is if I were to drag this black point way up to here, for example to 84, then I'm saying, send any color that has a luminance level of 84 or darker across all of the different channels, send those colors to black. That's a lot of good data inside of this image. A lot of very good details would go to black and they would get clipped away and they would be sent, as I say, to this luminance oblivion, which is a bad thing.

Now gamma can't do that, gamma always respects what's there, and sometimes you don't want to respect it, you want to change it, another times you do want to respect it. So when you are in a respecting mood, and you want to just effect those middle colors inside of the image, well, gamma is your way to go. Let me show you how it works. If you go ahead and drag the gamma value over here to the left, you are going to brighten the midtones inside of the image without clipping anything to white. So even though we are brightening the heck out of some of the lighter colors, we are not clipping them. If you were to move this gray slider over to the right, then you would darken the midtones. Again, you are darkening up some of those dark colors there, but you are not clipping them the way were just a moment ago, and you will just have to accept that as being the case, because they do look like they are altered dark probably on your monitor or inside your video here, but it is true, we are not clipping.

So anyway, let me tell you a couple of things, first of all don't go too far with brightening the gamma value, especially if your image has a fair amount of noise in it. Notice here inside of this image, I shot this image with an ISO of 1600, so what that means is that I was inviting a lot of noise into this image in the first place, and you are really going to notice the noise inside of the shadow details. So when you start brightening up your midtones here, you are going to bring out a lot of that noise as you can see me doing here around Max's collar. So even before I changed the midtone value, I'll change it back to one here, that gamma value there. We had a lot of noise to start with. You are just exaggerating that noise as you start to brighten it up.

Now when you are darkening colors you don't tend to exaggerate noise, because you are not going to see as much noise inside of the highlight detail of the image. So noise is really going to hide out inside of the shadows. But anyway, just something to bear in mind, my experiences is that, images tend to require midtone brightening, more than midtone darkening, especially if you are going to prepress where things tend to darken up, thanks to dot game. Anyway, I'm going to take this gamma value up to 1.15 for this image, so just a little bit of brightening going on there. Now I'll tell you why in the world this value is measured as 1.0, versus for example, 127 or 128 or something along those lines? But before I do that, I want you to know that we are going to get into some math. So those of you who hate math and couldn't care less about why this value is what it is.

You just need to know that if you move the gray slider over to the left, it's going to brighten things, and you move it to the right, it's going to darken things, and if you make this value right here, this gamma value higher than 1.0, you are going to brighten the image, and if you make it lower than 1.0, you are going to darken the image. And if that's all you need to know, then bye, and I'm just trying to spare you from the mathematics that's coming. Those people who like a little bit of math in their copy here, I'm going to share some math with you. Here's what's going on. Just so you know why in the world this is 1.0? Because you might think, if zero is black. And 255 is white.

You might think. well, right there smacked up in the center, 255 divided 2 would get you to 127 or 128, something in that range, really 127.5, but you can't do that. So why isn't this value is 127 or 128, why is it have to be 1.0? Well, it's measured as an exponent. See I told you that if don't like math you wouldn't want to listen to this. Exponents are like to the power of. So in other words, if I change this value to 2, let's say, not 2.0, to just 2, then I'm squaring the luminance levels inside of the image. I'm taking them to the second power.

Which is brightening the heck out of them. And if take this value, this gamma value to 0.5, it's to the half power, essentially which is going to darken the heck out of the luminance levels. What that does, it totally respects black and white. It doesn't hurt black and white at all. And this also explains by the way, why 1.0 is no change whatsoever, because anything to the first power is not getting modified. So the gamma value 1.0, which is the default, is a neutral gamma value. Now those of you who know thing the truth about math and have struck with the discussion so far, and those of you who want to skip, definitely skip.

But you might say, this isn't really worked out, Deke, because if white is 255, and you square it, you take it to the second power right there. Let's go ahead and do that, then that's going to brighten the heck out of white. I mean with 255 or anything resembling that, 255 squared is going to be like 100 million. I mean. It's a really high number. It's something in the tens of thousands I think. That means that we are clipping colors like crazy, so how can you say ascending these luminance levels to the second power is not going to clip white for example. And the reason is these are normalized value. So in other words, zero is mapped to zero, that's fine. 255 is mapped to one, so one to any power is going to remain one, so white is not going to get clipped. And so just the colors between black and white are going to be effected, then you would say to me, if we are normalizing the colors.

And medium gray, which is 127-128, becomes 0.5 in this environment. 0.5 to the second value is 0.25, so it actually darkens the colors. What Adobe has done is it actually takes one divided by two. So that's how the math is done, so it takes the middle gray for example, and if you square it, you are really setting it to the half power, and if you take it to the half power, you are really taking it to one divided by the half power, which is two, so you are really squaring it. So isn't that wonderfully confusing, but really sufficed to say that, of course what I said in the first place, which is, you drag it over to the left to make things lighter, you drag it over to the right, it makes things darker, but I know some of you really get into this stuff. And by that I mean, I do, and probably nobody else does, but I just wanted to explain this.

Yes, I think by now I have safely lost every single viewer of this movie successfully awesome, now then having done that, for those of you who are just kicking back and listening, in the next exercise I'm going to show you something really great, how to preview your clip shadows and your clipped highlights right here inside of the image window. It's a really great technique. All of you will want to know about this. Of course, nobody is listening to me anymore, so it's not going to matter, but definitely stay tuned.

Adjusting the gamma value
Video duration: 7m 35s 20h 57m Intermediate

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Adjusting the gamma value provides you with in-depth training on Design. Taught by Deke McClelland as part of the Photoshop CS4 One-on-One: Advanced

Subjects:
Design Photography
Software:
Photoshop
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