Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 192

If you want to tag along again, open the image convergingverticals.png
in the SampleImages folder on the DVD. We'll begin by using the filter. By
handling the filter correctly you can omit practically all distortions in one step.
When you select the Lens Distortion filter ( Filters > Distorts> Lens
Distortion ), a window opens with six slide controls to choose from.
It is hard to tell the difference between Main and Edge . Both distort the
image convexly (outward, similar to a fisheye lens) by increasing the value
and concavely (inwards) by reducing the value. Whereas Main affects the
whole image equally, Edge only bends the edges, leaving the center of the
image unchanged. Depending on the direction you select, you can straighten
a cambered image. Essentially, these are the settings that correct barrel- and
pillow-shaped distortions. For our image, the setting Main is recommended
to do the correction.
Figure 3.16
The Lens Distortion preview window depicting the suggested settings.