What is a Histogram?
The Levels adjustment is in the form of something called a histogram. A histogram is a way of plotting the distribution of values within a dataset that is at least one hundred years old. Below is a histogram of the height distribution of fifty students:
Photoshop Levels histogram looks like this (below). Instead of height it maps light values from black (left) to white (right) and instead of amount of students it maps relative amount of pixels.
How to read Levels
Heres how we read it. The Levels below is showing us the even distribution of values of a simple black to white gradient.
The Levels below is showing us the clumped distribution of the values of a simple dark gray to light gray gradient.
Here is a more realistic example. The histogram is showing us that the the brightest values in the image are only light grey.
How to use Levels
As well as displaying data it Levels can also change the values. Here the white and black point sliders have been moved so that they meet the lightest and darkest points of the data. The effect of this move is to ensure that the lightest point of the image is white and its darkest is black.
Though it is possible to use levels for a range of color operations, it is best that you restrict its use to the normalization of lightness values. The lightness values of image are normalized when the lightest point is white and the darkest is black. On the whole all images need to be treated in this way if they are to be 'aesthetically viable' (I leave it up to you to discover exceptions). It is easy enough to, but a word of warning. This is important enough to put in bold:
|On no account must the black or white point sliders cut across the data. If they do then the data becomes 'clamped'. A clamped black means that all the dark grays have been converted into black and a clamped white is one where all the light grays have been converted into white. This is a bad thing. Bad. Got it?|