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Rob's shot is really beautiful. Very nice. The golf ball all in focus look much more like a swing and not a tilt when you think about the planes involved. For perspective control in the case of the Leica, you would generally want to level so that the film (sensor) plane is parallel to the vertical plane of the camera body. In this case, even the second shot still has considerable keystoning. Close, but not quite. I usually use and angle finder in the studio to check the actual level and then match that to the camera. In the field, I just use the bubble levels in the Manfrotto 405 tripod head which are remarkably accurate for plumb and level. Here's an architectural example that shows and example of the completely vertical lines near the edge of the frame.