GIMP offers a range of tools and features that can help you improve your digital photos and the Levels adjustment feature can offer an easy way to tweak a picture.
If you've ever opened the Levels dialog in GIMP, it can look a little overwhelming, but it is a very simple tool to use. It doesn't offer the same flexibility as Curves, but for many images, it is just as effective a tool for making the most of a photo.
The accompanying image shows a photo split in two with the right hand side showing the result of a Levels adjustment.
Open the Levels Dialog
When you've opened a photo that you think could be improved, go to Colors > Levels to open the Levels dialog. It may look a bit confusing, but making adjustments using Levels is quite straightforward and simply a case of dragging sliders.
The Input Levels slider is where most of the magic is carried out, but the Output Levels slider can also help to improve the appearance of some photos.
The Input Levels shows the histogram of the image and below it are three sliders. The one on the left is for black values, the right hand one for white values and the one slider inbetween them is to set the mid point between black and white.
If you move the black slider across until it is level with the left edge of the histogram, you can ensure that your image contains pure black tones. Sliding the white slider left until it touches the right edge of the histogram will ensure that the image contains pure white pixels also.
In practice though, keeping the two sliders just outside of the histogram retains detail throughout your photo, while maximizing contrast.
Moving the middle slider left lightens the image and moving right darkens it.
The Input Levels slider is responsible for most of the work when improving your photos, however the Output Levels slider can also play a part. Moving the white slider to the left reduces the value of white pixels so that in reality, the final image will include no white pixels. Moving the black slider to the right will make black pixels become dark gray.
This effectively reduces the contrast of the image, but this can be used to counter blown out highlight areas in a photo, for example small areas of bright white clouds in a darker sky.
Dragging the white slider left will make pure whites pale gray and this can help to recover small areas of a photo, though if the blown areas are too large, the image will start to look flat.