Levels

Create a layer.

Go to your gradient tool.

Make a black to white gradient.

Open up your levels window.

What you see is the histogram of this gradient.

This histogram shows the population of pixels in the image at any given value.

In a color image what we are looking at is 256 levels of grey in any one channel.

In this histogram since this is black and white there are 256 levels of grey to choose from. 128 is the medium level grey.

When you move the black slider back and forth, the historam is being remapped, pushing twords the white. What used to be dark grey is now black.

And the same with the white slider when moved twords the black. What was light grey is now white.

You can go to the out put slider on the bottom, to set your maximum values. Slide the white levels from 255 to 128, then in the histogram you will only have 50% grey, and nothing in the image will be greater than that. You can do the same thing with the black slider.

Now going from a grey scale image to a color photo, looking at its levels, using the same techniques in the grey scale sample. The image can look really contrasty, really blown, or at 50% grey can look underexposed.