From here and here.

AFOV is typically specified as the full angle (in degrees) associated with the horizontal dimension (width) of the sensor that the lens is to be used with. Basically it means how wide(in degrees) a lens can see.

Here is the equation how to calculate AFOV:

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# 1. h: the size of sensor or film(normally in the horizontal direction).
# 2. f: the focal lenth of the lens.
AFOV(in radians) = 2 * arctan(h / 2f)

Here are some examples how to use the equation in Python:

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>>> import math
# Calculate the AFOV of a "50mm" lens + "36*24 sensor" camera:
>>> math.atan(36 / 100) * 2 / math.pi * 180
39.59775270904986
# Calculate the AFOV of a "75mm" lens + "60*60 film" camera:
>>> math.atan(60 / 150) * 2 / math.pi * 180
43.60281897270363

Note that the “ / math.pi * 180” part is converting the result from in radians to in degrees.

Another interesting thing we can see from the 2 examples is that, “76mm lens + 60*60” has even a little bit bigger AFOV than “50mm lens + 36*24”, which though are very similar.