How to get the camera scale factor

I want to obtain the scale factor by calibration, so as to obtain the relationship between the length of the object in the image and the actual length.

However, when I complete the calibration, I can get the intrinsic and extrinsic parameters, but it seems that I still lack some information (distance between the sensor plane and lens center) to get the scale factor.

Please also provide suggestions from professionals in this area, thank you.

Camera Calibration Toolbox for Matlab: http://www.vision.caltech.edu/bouguetj/calib_doc/htmls/example.html

Answer 1

[Matt J]

To get that, you need to image an object of known length at some known reference depth from the camera. You would also need to know the depth of any object you later wished to apply it to.

[chihyu]

Dear Matt, what do you mean is that if my subject is a 15 cm ruler (I know the exact length), then how many pixels does the length of the subject occupies in the captured image.

Then calculate the pixel/ unit length, but how do I get the reference depth from the camera