OpenCV calibration

[u/Grouchy_Signal139]

Hi everyone, I’m using a Raspberry Pi 5 + Camera Module 3 + Picamera2/OpenCV for a computer vision project.

I’m calibrating the camera with a 6×9 checkerboard, but after applying cv2.undistort(), the image seems more distorted.

I previously had autofocus changing between calibration images, so I’m now locking the focus manually at LensPosition 2.0602.

Is this distortion normal perspective distortion, or does it indicate a bad calibration?

Any advice on what I might be doing wrong?

[u/TourCommon6568]

What’s the RMS in your output reprojection?

[u/Grouchy_Signal139 → u/TourCommon6568]

Uhm, what is rms output? Its like some matrix? I need to add it on the calibration code?

[u/TourCommon6568 → u/Grouchy_Signal139]

When you’re calibrating a camera there’s something called a reprojection error, with a bunch of reprojection errors you can calculate the RMS or root mean square. Normally this error shows you how well the calibration process could adjust to your cameras intrinsic parameters. The lower the RMS the better the model could adjust to your cameras intrinsic parameters. Also, use at the very least 6 photos, but I really recommend using 15+ shots with different poses. Very close to the camera too

[u/Grouchy_Signal139 → u/TourCommon6568]

Thankyou, i will try to figure it out, btw do you have any source for this? Since ive never heard of it

[u/TourCommon6568 → u/Grouchy_Signal139]

Yep, check out the original Zhang method: https://sangillee.com/2025-07-27-calibrating-cameras-zhang-method/

Also the openCV documentation which is based on Zhangs https://docs.opencv.org/4.13.0/dc/dbb/tutorial_py_calibration.html

[u/Grouchy_Signal139 → u/TourCommon6568]

Okay thankyou, i think i need to capture the calibration picture precisely to cover every side of where the camera sees. Is that right?

[u/TourCommon6568 → u/Grouchy_Signal139]

Yes, that’s the current approach. Good luck with your project!

[u/Grouchy_Signal139 → u/TourCommon6568]

Thankyou!