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Incorrect result from camera calibrator app

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I used the camera calibrator app to calibrate lens distortion and I specified correct checkerboard size of 8mm. The calibration goes fine and i'm able to undistort images and points. However, the plot from showextrinsics() shows camera at 8mm distance from object but in reality it is 140mm. Why is this wrong?
My setup has a camera looking downwards and pattern is placed at the same working distance in all 30+ images where the lens is focused.
Calibration settings: All default settings - Standard lens, 2 coeffs, no tangential distortion, no skew, etc.
  4 Comments
Giridharan Kumaravelu
Giridharan Kumaravelu on 19 Mar 2024
Would you mind providing us with the calibration images to help troubleshoot this behavior?
cr
cr on 19 Mar 2024
Edited: cr on 19 Mar 2024
The folder is a little too big to be attached as a zipped file. Here is a link to google drive.
I do not recall if this is the exact same set of images yielding the results I shared above, but all 3 sets I have produced similar results in all aspects.
Thanks & Regards

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Accepted Answer

Giridharan Kumaravelu
Giridharan Kumaravelu on 22 Mar 2024
Edited: Giridharan Kumaravelu on 22 Mar 2024
Thanks for providing the calibration images. This is a limitation of the technique (Zhang's algorithm) that camera calibrator app uses, that it cannot calibrate a camera with calibration images that are coplanar. To accurately calibrate a camera with the app, you need some variance in the calibration images in the z-direction. Even a little wiggle in the z direction could avoid this degenerate configuration.
  6 Comments
cr
cr on 30 Apr 2024 at 10:23
Edited: cr on 30 Apr 2024 at 10:24
I think this is a great question for MW. In my applications I need calibration only for (un)distortion. Since the telecentrics have very less distortion I never considered calibration when using them. If your concern is shallow DoFs of telecentrics for changing distance to cam, for lenses with low magnification the DoF should be ok with changing the depth by a small fraction of a millimetre -I’m presuming Giridharan meant that will be fine, but remains to be seen.
Despite MW’s answer here I have a feeling distortion part is working fine. Perhaps we should look into the method deeper to understand what can be done to remove the limitation at least for distortion part of it.
Alexander Groeger
Alexander Groeger on 2 May 2024 at 8:19
I'm not an expert in this field but I think the telecentric (aka orthographic) cameras requiere a different camera model for calibration. Since in the case of orthographic projection the Z-dependancy of magnification is eliminated (constant magnification instead).
For my application I'm currently working with a model based on Chen et al's work:
But I still think it would make MW's camera calibrator app more complete if an orthographic model would be integrated as well.

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