Is there a set point when wide becomes ultra wide?

[neuroanatomist]

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.

That's an interesting idea, but I believe it is more likely coincidence and lens design. For example, the EF 28mm f/1.8 has worse barrel distortion than the EF 14mm f/2.8L. Likewise, ultrawide zooms (16-35, 17-40) set to 20-24mm have less barrel distortion than the 24-105L at 24-28mm.