The Curves Tool

The curves tool is a very basic tool that can be used to improve photos with a few clicks of the mouse. It is used to change the brightness and contrast of an image. It can also modify the separate Red, Green and Blue channels of an image too. The Curves Tool has a histogram to represent the shadow, midtone and highlight detail in the image. In the GIMP, you access it by right clicking on an image and go to Colors->Curves.

This is the second article in my GIMP for Photographers series, but as usual, all of this applies to Photoshop, or any other image application with a Curves Tool. The first tutorial was on The Levels Tool, and worth a read if you missed it!

Here’s an image I shot at the Lord Mayor’s Picnic in Fitzgerald’s Park a few months ago, and the Curve Tool below it. Notice the histogram? The photo is fairly well exposed, but some highlights are “clipped”, as the histogram hits right hand side without sloping off.

Brightening Curve

It’s easy to brighten an image. Just drag points on the line up.

Darkening Curve

Now, let’s darken the image by dragging points down.

Contrast Curve

A classic use of the Curves Tool is to increase contrast in an image. You do this by darkening the shadows, and brightening the highlights. The curve looks sort of like an “S” when you do this. Don’t go overboard on this though, because it’s easy to lose detail in either direction.

If for some reason your image has too much contrast, a quick inverted S curve will solve that problem,

Wacky Colours

You can select any of the Red, Green and Blue channels and do strange things to your photos. Here’s what happens when you play with the Red Channel.

And here’s what happens when you change multiple colour channels in different ways.

Colour Picker

After you have opened the Curves Tool, click anywhere in the image. Notice how a vertical line goes up and down the histogram/line? That vertical line is the colour of the pixel where you clicked. That can be useful if you’re trying to modify a particular part of a photo. This is what you get when you click on the black coat on the left of the image above.

Aperture ƒ/6.3
Camera Canon EOS 20D
Focal length 18mm
ISO 200
Shutter speed 1/250s




  1. Nice tutorial Donn. I’ve been using curves quite a bit since you told me about it months ago, very useful for gig photos. Increasing the green curve has the same effect as reducing red without losing detail and increasing the blue light is great for getting more realistic skin tones under odd colour lights. All trial and error of course which means it takes quite a bit of time!

  2. Hi there,
    I am new at this so pardon my ignorance. I did not see any difference between the two first histograms shown. Is that due to the lack of severity of change in pixels? When you add a dot to the diagonal line and adjust it, does that affect the pixels on the histogram that are right beneath the dot?

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