I always assumed that there was a fixed relation between focal length and angle of view. But compare the technical data for the following lenses, all from the same manufacturer:
Why do two lenses by the same manufacturer have the same angle of view at different focal lengths?
Answer
They're measuring the first on a crop-sensor camera and the second on a full-frame; specifically for a Canon APS-C sensor, 15mm x 1.6 = 24mm.
Canon does this in general when referring to EF-S vs. EF lenses; see for example How can a 24-70mm and a 10-22mm both be "wide angle" lenses?, which has the same effective answer.
The relationship between focal length and angle of view is fixed, for a given sensor size. More on this at What is “angle of view” in photography?, if it's not already clear.
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