How to get the focal length from the field of view?
There is a not too complicated formula (all measures in mm)
(Distance to the Object * Width of Sensor)
Horizontal Focal Width = ---------------------------------------------------
(Width of Object + Width of Sensor)
(Distance to the Object *Height of Sensor)
Vertical Focal Width = ---------------------------------------------------
(Height of Object + Height of Sensor)
Example:
You have a pencil shaped object of 15cm length, and 1cm width.
Your camera has a standard 1/2" sensor and the object is in 0.2m distance.
The Formulas are (remember? mm !!!)
Horizontal focal width = (200mm*6.4mm) / (150mm+6.4mm) = 8.18mm
Vertical focal width = (200mm*4.8mm) / (10mm+4.8mm) = 64,68mm
To display the height of the object full screen, a f=64,60mm lens would be needed - which first of all doen't exist and the full length of the object wouldn't fit on the screen.
So we go for a f=8mm lens.
