One of the biggest confusions that you might notice as you venture into astrophotography is that photographers and astronomers measure their equipment differently. Photographers tend to refer to their lenses in terms of their focal length, while astronomers refer to their telescopes by the diameter of their opening. Thus, a 75mm f/6 telescope has a focal length of 450mm. Meanwhile, a 75mm camera lens at f/6 has an aperture of 12.5mm. If a photographer is told that someone is shooting a 155mm lens hand-held, it would not raise an eyebrow, but an astronomer told the same thing would be incredulous! I can only barely lift my 155mm telescope onto its mount!
(Originally posted by David Kodama)