Angle of view

d is the dimension of the sensor in the same rotation as you want to know the angle of view (horizontally, vertically or diagonally but any rotation is possible). f is the effective focal length of the lens.

<math>\alpha = 2 \arctan \frac {d} {2 f}</math>

(Arctan (Inverse Tangent) can be found as atan or tan^-1 on some calculators)

As an example we calculate the field of view for a 30 mm lens on a Canon D20 that has a horizontal sensor dimension of 22.5 mm.

<math>\alpha = 2 \arctan \frac {22.5} {2 \times 30}</math>

That gives us a an angle of view of 41.1°