Logic suggests its best to stick to fixed lens or min/max zoom, though I wonder: does splitting calibration settings for photos shot at different focal lengths result in noticeably worse reconstruction?
Real world example: when capturing a monument, I can shoot it from most directions from the same distance @18mm, but there's an obstacle from one side, preventing me from shooting up close - am I better off shooting still @18mm with monument being rather small in frame, or can I zoom in and not worry about different calibration params for that photo?
-Andrew