You know how approx 50mm is 1:1 as far as what the eye sees.. at 18mm, things look further out, and 300mm brings you close in (obviously). I'm wondering if there's a mathematical formula for that. Something with trig probably, involving triangles.. and would also let you factor in crop factor for certain DSLRs. To be more clear, a formula that will let me take the size of an object and figure out how large (or small) it would be at a particular focal length. Anyone know?