Look at HistogramTutPic5 for this part of the tut.
HistogramTutPic5.jpg
Again, it's important to understand that you're using a Response Curve to control an image. You've actually set two breakpoints (places on the curve where it's direction changes. In this case, we've adjusted the luminance so that the dark levels are even darker and the light levels even lighter.
While here, let's take note of the display adjustments in the bottom center. The up & down buttons here are to control the local display. The vertical scale of the spectrum was automatically set by the program when we first enter the operation. However, the vertical bars may increase as we make corrections, and there is no correction after the first one. We can use the buttons to set the display, without actually changing anything in the image.
Now... That Gray triangle thumb between the black & white controls is going to be used to control the straightness of the line between the black & white breakpoints. Yes, I know that was a real mouthful! In order to give it a simple nickname that is easily said, everyone has agreed to call it 'Gamma'! In fact that's the name you see just above the local amplitude controls.
Up to this point, we've left it at a setting of 1.00, which is the proper math method of stating that a line is straight and has no curve. Nor am I going to get into the mathematics of what the numerical numbers represent, since I don't believe it'll really add anything to our use of the tool. All we need is to know that we can change the value by either changing the numerical number up and down or by dragging the Gamma thumb left or right.
Look at HistogramTutPic6.
HistogramTutPic6.jpg
I've dragged the gray thumb to the left, until I obtained a gamma of 0.5. Look at what it did to the response curve. Looks like a ski ramp, doesn't it?
But notice that the at the extreme light and dark luminance points we controlled with the white & black thumbs doesn't change. We only changed the response of the mid range by changing the gamma.
As a side note, if you're a fan of the TV show: CSI, have you noticed that to give it a "Dark" atmosphere, they usually set the gamma to around .3?
Now, let's examine what happened to the image for a gamma of .5. As a rule of thumb (talk about a bad pun!) When you drag the gamma thumb to the left, you're making the mid-range luminance darker. The white and black points stay where they were set.
Look at HistogramTutPic7
HistogramTutPic7.jpg
Here, I've just moved the gamma thumb until I obtained a gamma of 1.5. I'm putting the numerical values in, so you can follow along, but in my actual usage, I rarely bother with them. I just study the output image and move the sliders looking for the effect I want. Al;so, sometimes it's just fun to Easter egg for an unusual effect.
Look at the response curve. It's now bulging upward from the breakpoints. How about the preview picture? Notice the vent openings still look black but all of the dark mid-ranges are lighter?
Side note!! Being able to adjust the Gamma of an image is so important, it rates a position as an independent tool! Adjust>Brightness and Contrast>Gamma Adjustment You cannot set the start and end breakpoints, but it works just the same as the Histogram!
To review: Gamma is a means of adjusting the mid range of the response curve. Without changing the defined white and black points, We can now make the mid-range lighter or darker.
For the image that we've been working with, a gamma of less than one doesn't really enhance the image, and a gamma of 1.5 seems too much. My personal taste is to set it around 1.2
As you adjust your images, make them show the effect you're wanting to show.
When you are ready, Part 5 is waiting for you.