Keystoning

Misho Baranovic, on the pesky issue of keystoning in pictures:

This form of distortion is very common across architectural, street and travel photography. It’s most often seen when tall buildings ‘fall’ or ‘lean’ within a picture. This distortion has become so common that most people have stopped noticing it within their pictures or just think it has something to do with the focal length of their lens.

It’s a great overview of what the keystone problem is and the variety of ways it can be fixed.