On Angle of View, Focal Length & Crop Factor
Buy a lens today, and its barrel will almost certainly come marked with a focal length. To establish what the angle of view is, though, one would have to either whip out that old scientific calculator for a little trigonometry or, alternatively, locate the manufacturer's detailed specifications.
Crop Factor Basics
The first thing to know in the pursuit of crop factor enlightenment is that the entire system is based, rather arbitrarily, on the old 35 mm film standard. I say “arbitrarily” because a crop factor is simply a ratio, one that can just as easily be calculated using an 8" × 10" large-format camera or the tiny iPhone 7 camera as a reference.