Page "Camera" Paragraph 29

When photographing rapidly moving objects, the use of a focal-plane shutter can produce some unexpected effects, since the film closest to the start position of the curtains is exposed earlier than the film closest to the end position.

Typically this can result in a moving object leaving a slanting image.

The direction of the slant depends on the direction the shutter curtains run in ( noting also that as in all cameras the image is inverted and reversed by the lens, i. e. " top-left " is at the bottom right of the sensor as seen by a photographer behind the camera ).