Adjusting the black and white endpoints

This page has big images, doesn't work well in some screen resolutions, sorry. You can get your money back. :)

There's more than one way to do this of course, but when I hear people talk about "lightening" or "darkening" an image or adjusting the "contrast" it makes me cringe. I've seen similar reactions from people who teach digital imaging as part of photography courses. You might get it to work, sort of, but you'll probably make a mess.

One alternative is to use some statistics, and no, you don't need a calculator. That's already done in Photoshop, or The Gimp, or Paint Shop Pro, you just have to use it. This is The Gimp but the same concept applies.

I'm working here with a photograph taken from over 2 miles away which has a lot of atmospheric haze. Notice in the histogram that the amplitude of blacks are way down, which you can see because the left-most side of the peak is about half-scale. To get the darks more normal we want that peak to come all the way to the left. The whites aren't so bad, they just need a little tweak. This isn't typical, usually both ends need work. The highlights account for the whites in this case. Original

blacks done Dragging the black pointer to the left-most part of the peak sets that portion of the histogram to black. Notice that the darkest parts of the picture now approach black.
The white end doesn't have to be moved very far, but the same idea applies. Not much change with this picture but with most overrdoing this will lose details in the white areas. whites done

Usually that's all that's needed to increase the contrast but in this case it doesn't look right. Fixing that is less of an exact method, statistics can't help, you have to go by eye and individual preferences will vary.

There's a number called simply gamma which is roughly the ratio of input value to output. Here I adjusted it the wrong way on purpose to show the effect. It's the middle slider under the histogram and starts out at a value of 1. Moving it the wrong way here causes too much contrast but sometimes moving it this way is what's needed. gamma wrong
final gamma This is the gamma setting I arrived at. Usually you can drag the slider and see the effect immediately.
This looks a little too much like a set of Christmas lights. Usually, somewhere in the program there's a slider for satuation or the amount of color. Here we need to back this down a little. desaturation done

The result still isn't beautiful, looking maybe a bit too much like an old faded postcard, but it's better than what we started with:


Not all images will be the same, but the same principles apply. Don't adjust the contrast!

AB1JX / calcs