The relative focal length calculation allows people to visualize 'regular' photograph image scale relative to, usually, thr 35 mm film camera 'everybody' was used to using. The same relative calculation was done way back when people would compare 35 mm film cameras to six by six or six by seven film cameras like the classic hasselblad or big Pentax ('normal' lens were 90mm instead of 50mm IIRC).
So if you use the identical lens on a full-frame camera and on an aps-c camera (as you can do in many digital camera systems), and then enlarge the resulting photograph to 8x10 the image scales will appear different despite the fact that the lens focal length did not change. The smaller sensor captures a smaller part of the image circle projected by the lens. The basic lens design has to be varied to give the appropriate image circle to cover the film plane for larger sensors or films, so 90mm Hasselblad lenses looked much different and are much larger than the same focal length lens specifically made for a 35 millimeter camera or, nowadays, specifically made for a small sensor digital camera.
Regular photography and astrophotography have totally different ways of looking at things but the physics don't change.
Edited by markb, 26 March 2020 - 02:25 PM.