[nuke12]

I'm not big on word play. Call it effective focal length, interpreted focal length, apparent focal length, whatever? It comes from this formula;



D = sensor size horizontal width in millimetres (36mm used)

f = effective focal length in millimetres

a = horizontal angle of view



f = D/(2*(TAN(a/2)))



That is this formula transposed.



a = 2*ARCTAN(D/(2*f))



The first formula on the wiki page for angle of view.

http://en.wikipedia.org/wiki/Angle_of_view



EDIT: Typo. Changed 35mm to 36mm for the 36x24 full frame format.