Levels in GIMPGIMP gives you the power to view and edit the image histogram for a digital pictures. This ability helps you accomplish a number of tasks. You can check the exposure quality of your image - something which can be very valuable as the histogram may tell you something your eyes missed. You can also control the relative value scale for brightness of the picture overall and also for the three color channels - Red, Green and Blue.
Automatically Adjust LevelsThe easiest way to adjust the color levels in GIMP is with some of the Automatic color adjustment tools. These tools intelligently go through your image - literally pixel by pixel - and adjust the colors and brightness levels based on a programmed understanding of color, human optics and photographic technology. These tools can give excellent results and are worth experimenting with.Spend a little bit of time playing with:
Colors >> Auto >> Enhance
Undo and RedoAs you play with the picture, if you don't like a result or want to try a different effect on your picture you can press the Ctrl key plus Z (Ctrl+Z) to undo the last action you took. To redo an action is Ctrl+Y. When I am editing a picture I will often hold Ctrl and alternate between pressing Z and Y to undo and redo an action over and over. This lets me compare different parts of the image before and after and I find it a useful trick when doing this type of image adjusting.If you aren't satisfied with using automatic adjustment tools on your photos and would rather take more control over how the color levels of your photo are adjusted read on!
Access the Color Levels ToolAccess the Levels tool in the Colors menu.Colors >> LevelsÂ…
This will bring up a levels box which looks like this.
There are a number of different settings but the most important ones for this article are highlighted. You will see an image histogram for the value channel (black/white or dark/light). Below the histogram chart is a gradient scale going from black on the left to white on the right.
 Original Image and Histogram Underneath this gradient are three sliders which looks like little triangles of black, grey and white. These can be clicked with the mouse and moved left or right. Below each slider is a box with a value in it. The box on the right is the value for the black slider and controls what the lowest values (shadows) in the image. This is actually the brightness range of the pixels which can be anywhere from 0(black)-255(white). This box will be set at 0 by default. Likewise, the white slider controls the upper end of the image range (highlights) and is controlled by moving or by adjusting the number in the right box (by default 255).
 White Slider to the Left
 Grey Slider to the Right
Check out this video demonstration. Full screen is recommended to see what is actually happening!
As long as you have the Value channel selected you will only be adjusting the overall brightness and contrast of the image because these changes affect all color channels. If you want to manipulate the individual channels one at a time select one from the drop down menu. You will notice that the histogram changes slightly (or not so slightly) for each color. That is because the picture will have different amounts of each color. You can adjust the sliders in the same way as the value channel but now the effect is only on one color channel at a time. Here is a video of the color channels being tweaked:
The best way to get comfortable with this tool is to use it. There is no magic answer for correcting every picture, you just have to develop an understanding as to what changes will have what affect on the picture. Good luck and have fun with this powerful tool!
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