Updated: Sep 26, 2020
Levels and Curves are the first, and in many cases the final, adjustments that I use to improve both black and white and full colour images. They’re an absolutely indispensable pair of tools.
Here’s a miserable picture of a lovely view: a balloon flight through the Napa Valley in California. In colour it’s pretty good, but a straight conversion of a colour image into greyscale usually leaves a lot to be desired: it’s flat, dark and muddy.
Time for the Levels window.
Either use ‘cmnd / cntrl + L’, or 'image > adjustments > levels'. On the left is the slider for setting the black point, on the right is one for the white point, and in the middle there’s one for the mid tones. Grab one and move it, and you’ll soon see what it does.
The initial adjustment is to fix the a gap at both ends of the black pile of data, which is representation the actual pixels in the image. Right now, the tone doesn’t go all the way out to black, or white. However, if you pull those sliders in towards the pile, the tone range is much improved.
Incidentally, try not to pull the sliders in too far so they go under the pixel pile. If you do, you just burned what used to be shades into being black, or white. And if you press OK, you probably lost those shades for good.
The appearance here is improved even more by dragging the middle slider to the left. I never move this one until after I’ve got the black and white points set. All the pixels to the the left of it are
darker than 50%, and those to the right of it are lighter. So moving it left really brightens the mid-tones.
The last adjustment is to fix contrast. But don’t use the ‘brightness and contrast’ adjustment to do it. In fact, you should probably forget about that adjustment completely. It’s a lot better than it used to be, but it’s still really easy to utterly destroy data at both ends of the spectrum. Instead, use the Curves window. Again, either just open it from 'image > adjustments' or use ‘cmnd / cntrl + M’.
Click in the middle of the diagonal line, and you’ll put a ‘stop’ there. You can drag that around and turn the line into a curve which accentuates the tone without burning anything out.
Sometimes all an image needs is a slight ‘S’ curve to add a bit of contrast, but in this case I went a little further – as you can see. If you decide one of those dots is a mistake, just grab it and drag it down and out of the window.