You can invoke this command by choosing Image|Adjust|Levels from Photoshop’s menu or by pressing Ctrl-L.

What interests you in this window is the graph with three triangular sliders below. This is called the histogram, and it shows how many pixels of each of 256 possible brightness levels there are in the image’s raster. Black (0) is on the right, white (255) is on the left; you can see a steep peak on the right which means there are a lot of white and near-white pixels in the picture — I used a clean pencil sketch, so most of the scan is clean paper. The scan is rather well-balanced — the paper is white, and the histogram shows it.

Now let’s look at our scan’s histogram. It looks different.



There is a great peak on the right, but it is not white: it occupies a wide range of light gray tones. If you look at the scan you’ll see what it is. It’s still the paper, but since in our scan it is gray and full of wrinkles making for a lot of “noise” we see what we see. The rest of the histogram is still the dark tones of the graphite lines. (There frequently is a gap on the left as well when there are no pure black pixels in the scan, and even the darkest “black” is in fact gray. But this is not the case with our scan because it is too dark overall.)

It is clear that we do not need the space beyond our actual “black”; it is no less clear that we don’t need the noise and garbage that makes up the background of our scan. We’d make the graphite black and the paper white, and that would satisfy us. And we can do this with the help of little triangular sliders below the histogram.

Continue...