Assertion: camera lens's length | /r/HasProperty | zoom setting
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Source sentences:
text: This means a) calibrate the camera to establish the exact focal length/principle distance for more accuracy (or at least use a reasonably close focal length estimate), and b) if using a zoom lens, take photos with the camera set at the same focal length (zoom setting) at which the camera was calibrated (for repeatability, use the widest angle or fully zoomed).