The difference of FOV at the same focal length (f) is due to different detector sizes.
The three first digits of the serial number tell the detector size of the camera.
Here is a table of detector sizes and corresponding FOV of lenses used in the A20 and A40 cameras: (Bold 36mm is built-in)
| Type | Ser no | Hor size | ||||||
| A40 | 9 mm | 18 mm | 36 mm | 72 mm | 124 mm | |||
| 236... | 14.8 mm | 80° | 45° | 24° | 12° | 7° | ||
| 250... | 14.4 mm | 77.8° | 43,8° | 23.4° | 11.7° | 6.8° | ||
| A20 | 4.5 mm | 9,2 mm | 17 mm | 36 mm | 54 mm | 87 mm | ||
| 227... | 7.4 mm | 80° | 45° | 25° | 11.7° | 7.8° | 4.9° | |
| 249... | 5.6 mm | 63,8° | 33,8° | 18.7° | 8.9° | 5.9° | 3.7° |
(Horizontal FOV = 2*atan(Hsize/(2*f)))
For corresponding data for the A320, A325 and T/B-series, Click Here.