Originally Posted by
Black_Dog
I have always thought of ultra wide-angle lenses threshold as the point where distortion becomes really noticeable, which is about 24mm on a 35mm film camera. I recently learned there is a technical reason for that.
An ultra-wide angle lens is one whose focal length exceeds the short dimension of the film or digital sensor.
A 35mm film negative or full frame sensor measures 36mm x 24mm, so anything wider than 24mm is considered ultra wide-angle for that format. Likewise, Canon's APS-C sensors measure 22.3mm x 14.9mm, so anything wider than about 15mm is ultra wide-angle.