GIMP For Photographers: Levels
Many people find using the GIMP or Photoshop a daunting prospect but in fact those packages are quite easy to use once you’ve practiced a few times. This will be the first in an occasional series to help photographers use the GIMP to post process their photos.
The Levels tool (right click on your image, select Colors, then Levels) is used to adjust the levels of the colours in your image by manipulating a histogram representing the image. In simple terms, you can make broad changes to the Red, Green, Blue and overall “Value” parts of your image.
The single most useful function of the Levels tool is the “Auto” button. Click that button and the histogram will be stretched out. Your image should look better. If the photo lacked contrast, it can suddenly become a lot “punchier”!
Here’s an example which will make things clearer.
This is a nice photo I took in Galway in 2005 with a Sony 717. Unfortunately, there’s a nasty yellowish sheen to the image. I probably shot this with the white balance set to cloudy. That can give a pleasing golden look to images but it’s not always welcome. The image also lacks contrast and looks under exposed. How do I fix that?
Fire up the GIMP Levels tool. Right click on the image, go to Colors, then Levels. This is the histogram for the image above. See how it’s all bunched into the middle? Now, click on the “Auto” button.
Wow! One click did that? The image looks so much better now! The swans actually look white and it’s brighter and shiny!
I opened the Levels tool again, just to see what effect “Auto” had on the histogram. Sure enough. It’s stretched from side to side.
Before and After Auto Levels
Wasn’t that easy?
You may have noticed the eyedropper buttons next to to the Auto button. Those are “Black”, “Grey” and “White” selectors. Click on one of those, your cursor will change to a eyedropper and then click on the corresponding colour in your image. They work pretty well, but can be confused. If it all goes wrong, just click the Reset button, or CTRL-z to undo if you’ve clicked OK.
You can also manipulate the histogram manually. Just drag the sliders left and right until your image looks ok. You can change the channel with the drop down at the top of the Levels dialog. Changing individual channels does interesting “cross processing” things to an image.
- The Levels tool on gimp.org docs
- Levels Tool – white, black and grey to the rescue! – a tutorial I wrote about the levels tool back in 2004!
Want to know more? Leave a comment. I’d love to hear what you’d like to know and learn about the GIMP.