Impact,on the calibration of a camera, of the square size

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Hi all,

I am at the 3rd year of my phd and I finally really need cameras for some tasks.

First of all I need to calibrate my camera. I need to find the intrinsic parameters. I don't like to have an approach like : "Ok it is working let's go on." and therefore I have a question.

I am using the following tutorial to calibrate my camera: http://wiki.ros.org/camera_calibratio...

And I was wondering what happens when I calibrate a camera and I give the wrong size of the square, after --square in the command:

$ rosrun camera_calibration cameracalibrator.py --size 8x6 --square 0.108 image:=/camera/image_raw camera:=/camera

I really would like to have some details about it. Explanation with formulas and so on because I am not able to understand. I did 3 calibrations and I generated 3 calibration files. To me they look almost the same but only 2 of them are with the right size of the square. For this reason a question came to my mind: why do I need to specify that number?

Thanks in advance for all your explanations!