Help - baffling depth of field test results!
I've been doing tests on the relative effects of diffraction and depth of field with various focal lengths in 35mm. The latest was with a 21mm Zeiss ZF. I photographed a long hallway at apertures ranging from f11-f22, focusing at ~infinity, and then repeating the exposure series with focus set to hyperfocal distance. The hallway had detail from near to essentially infinity which I could use to evaluate definition in the resulting prints.
As expected, once you get to f22 diffraction effects on both near and far detail, regardless of point of focus, are quite noticeable. What I did not expect, and do not understand, is the following. The effect was observed at all tested apertures but was most visible at f11 due to the lower diffraction:
At all apertures, the shots focused at ~infinity showed visibly better focus not only on far objects as expected, but also on very near detail, when compared to the shots focused at the hyperfocal distance (ie a point of focus closer to the film plane)!
I am at a loss to explain this. The only issue in the test is since the subject was an empty hallway, there is nothing close to the film plane in the center of the field of view. The near details used for evaluation were anywhere from about mid way off center, to the edges of the frame.
What is going on here? Is this due to field curvature? Or is it that lenses are optimized for infinity focus and will show better definition at that setting for subjects at all distanced even if depth of field is reduced?
I don't get it.