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Calibration of a high resolution camera


I'm trying to calibrate a high resolution camera of roughly 4000x3000 pixels hence 12 Mega pixels. (It's a canon EOS 1100D). I'm wondering what's the maximum calibration accuracy I can hope to reach with that kind of resolution. Here are some numbers I get:

-both with checker board pattern and asymmetric circle grid, I get a error around 1 pixel. I'm using between 46 and 60 valid shots of the board, with the camera mounted on a tripod, each shot carefully taken to make sure I have the best possible focus on the pattern. I cover most of the picture surface when combining all the shots. I used several grids from 6x9 points to 10x17 points. I varied all angles to make sure the algorithm converges. As 'good' calibration values should be between 0 and 1, I'm a bit concerned about a value around 1.

-on the other hand, if I reduce my high res shots to say 640x480, my error drops to 1/6.25=0.16 which seems to be more in line with some values mentioned on the web.

What's your experience? Somebody got to calibrate high res cameras with better results? Or is it really the best I can hope, given the noise/measures uncertainty and the algorithm used by Open CV?

Thank you for your help,