I recently picked up an RM3di (used) and have noticed that the IQ180 and the camera disagree about pitch level.
1. The bubble level on the camera frame shows a bit more than a half bubble of tilt when the electronic level in the IQ shows no tilt.
2. The IQ level shows a bit more than 1 notch (not sure what those correspond to) of backwards tilt when the camera bubble shows level.
3. A carpenter's level agrees with the camera bubble. When the camera shows level, so does the carpenter's level.
4. I think the camera frame is fine. The rear extension "box" (held by the rear clamp) lies in the same plane as the lens axis (held in the front clamp). The misalignment is most likely in the Mamiya adapter or the back itself.
To verify, I made some shots of a flat target with the apo digitar 150 at f5.6, at a distance of about 12'. Electronic level on the IQ set to zero. At 0 lens tilt, both top and bottom of the target go a bit soft. At 0.5 down tilt, much worse. At 0.5 up tilt everything looks good across the frame. At 1' up tilt it is terrible.
From all this I conclude the sensor plane is tilted about 0.5' back from the lens axis. On my old Alpa I would have shimmed the adapter but that is not possible here. Tell the truth, I never much cared about shimming the Alpa for infinity, but pitch and yaw parallelism are important. Any suggestions? Anyone know what Arca considers a reasonable tolerance for this type of thing?