Intrinsic Matrix using calibration app

I'm calibrating my camera using single camera calibration app.However,I get different values of f_x (or f_y) for every trial I run.I take different sets of images for each trial.I do understand that f_x and f_y are not the same because of non square pixel but why does the value of f_x=F*s_x change for the same camera. Shouldn't the focal length in pixel dimension be constant for a given camera?From what I understand both F and s_x are constant.The variation is sometimes of 200 pixels between f_x-trail 1 and f_x trial 2.

Answer 1

[Walter Roberson]

"Shouldn't the focal length in pixel dimension be constant for a given camera"

Not generally, no. That would require a fixed focal length camera, not an adjustable focal length or auto-focus camera.

I have been up all night so I do not recall at the moment if the aperture affects the equation.

[Jinang]

Thanks for your reply but the camera didn't have auto-focus.The mistake was that my checker board size was small and it didn't cover significant part of image.I calibrate it keeping it closer to camera and the variations are now negligible between each trials.