Bad triangulation in stereo camera setup on FOV borders

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Hello,

I have a stereo camera setup and want to triangulate objects. I use cv::stereoCalibrate to get all the intrinsic and extrinsic camera matrices and I use cv::triangulatePoints for the triangulation.

Now to my question. When I position the object around the middle of both camera frames, I get decent results. When I move the object to the borders of the FOV of the cameras, the triangulation is very bad. What is the reason for this? Are my cameras the problem (bad lens distortion? or something else?) and if so, is their a software solution to get better results or should I change the camera?

Any help is very much appreciated. I am also happy to provide more information if needed.