Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 187

3.5 Touchup Work 5—Using Perspective
Correction to Remove Converging
Verticals
3.5.1 Trying to Avoid Converging Verticals When
Taking Shots
Layers play an important role when it comes to removing converging verticals
in an image. Converging verticals occur mainly in architectural shots, when
the camera is pointed upward and focused on an object that is very close to
a vertical object. What usually happens is that the building's edges converge
vertically toward a third vanishing point.
The following tips can help you avoid or reduce such image flaws when
taking pictures:
• The greater the distance to the vertical object (e.g., a skyscraper), the
smaller the amount of distortion at the top of the image.
• Try not to use wide-angle lenses because a short focal length will cause
additional distortions (such as bulging). The longer the focal distance, the
fewer problems you'll have with additional distortions.
• “Shift” lenses are available for cameras with interchangeable lenses.
These cameras allow you to move the attached lens so that it's parallel
to the shooting level (camera's rear panel). This will suffice to somewhat
rectify the problem.
Because converging verticals may occur in spite of all your careful
preparations, most digital image editing programs provide a variety of
methods to remove such flaws from architectural images.