User:Enochb/Sandbox

Let a = h/w = aspect ratio

Then the diagonal d = w √(1+a^2 )

For 35 mm film w = 36 mm, a = 2/3 giving d= 43.3mm

If the  effective focal length (EFL) is the focal length for 35 mm film needed to provide the same angle of view, then:

X 35 /EFL = x/f

For diagonal-based EFL: EFL = 43.3f/d = 43.3f/(w √(1+a^2 ))

For width based EFL:	EFL = 36f/w = 36f√(1+a^2)/d