Problems in focusing when calibrating a DSLR--using OpenCV for Calibration

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Hi,

I am using 2 CANON EOS60ds as a stereo setup to do photogrammetry. I am using OpenCV to calibrate both the cameras using a high precision Circlegrid calibration pattern. I am using a signal generator to trigger both cameras at the same time, earlier i tried to use Gphoto2 to trigger them but there was bit of time delay, so i relied on signal generator to trigger them.

The problem in the calibration is that, the cameras lose focus even if i move the pattern a little bit, so i am getting incorrect results, since i m not moving the pattern too much between frames(We have to cover entire field of view of cameras in calibration and all the euler angles should change). To prevent it i have to increase aperture to increase the DOF(depth of field) for both cameras, and this further causes me to decrease shutter speed. The problem in decreasing shutter speed is that it will cause shaky pictures if the calibration pattern moves.

If anybody has any experience calibrating DSLRs, and can give me some ideas, i would be grateful.