Opencv: Find focal lenth in mm in an analog camera

Question

I have sucessfully calibrated an analog camera using opencv. The ouput focal length and principal points are in pixels.

I know in digital cameras you can easily multiply the size of the pixel in the sensor by the focal length in pixels and get the focal length in mm (or whatever).

How can I do with this analog camera to get the focal length in mm?

Answer 1

The lens manufacturers usually write focal length on the lens. Even the name of the lens contains it, eg "canon lens 1.8 50mm".

If not, you can try to measure it manually. Get lens apart from the camera. Take a small well illuminated object, place it in 1-3 meters in from of lens and sheet of paper back from it. Get sharp and focused image of the object on the paper. Now measure following:

a - distance from lens to the object; y - object size; y' - object image size on the paper;

f = a/(1+y/y') - focus distance.

Answer 2

If your output is in pixels, you must be digitizing the analog input at some point. You just need to figure out the size of the pixel that you are creating.

For example, if you are scanning film in, then you use the pixel size of the scanner.