Dave Krueger
Member
Just some math. Using the simple lens equation; 1/p + 1/q = 1/f
If subject is at 3600mm (12ft) and focal point is 3400mm (8 inches in front) and focal length is 80mm
focal plane should be 81.82mm behind the lens if correctly focused
focal plane is actually 81.92mm behind the lens as focused
Difference is 0.1mm
The target distance on the first set of tests with the error of 8 inches, was about 8 feet, but I understand your point. The mispositioning of the lens or film plane will be very small. But, the 8 inches was the focusing error with the offset set to +30, so the mispositioning of the film relative to the lens, is greater than the 8 inch error would imply.
The bottom line is that the film is too far away from the lens and that could be caused by the film back. That doesn't explain why the viewing screen seems to be off in the same direction as the film plane (but maybe not by the exact same amount). Of course, it's not impossible for the viewing screen and film plane to both be off, but I've been treating that as very unlikely.
If we assume that the lens to film distance was correct when the camera was calibrated at the factory, something must have changed between now and then. The two things that would have changed, of course, are the lens and film back, which spits in the face of my theory that the problem is in the camera body (although I was kind of counting the film back as part of the body). Good thing I'm sending all three pieces back.