RF50mm f1.8 - focus shift on higher f-stops?

[maboleth]

I've tried to test this lens with and without an UV filter, to see if there would be any degradation in IQ. Turned out there was none. But, I've found a much bigger problem.

Apparently my lens shifts focus at higher f-stops.

Both images are taken at f/4, R7 on a tripod, using 2 sec counter and electronic shutter.

The first one is taken with Exposure + DOF preview, which makes the aperture blades turned on. It's very sharp, as it should be.This one is using just Exposure simulation, without DOF. Focus is done without aperture. There's a visible loss of sharpness of the subject. The background is now more sharp - focus shift.

I've tested it several times and it's always there. The focus is great at 1.8, but the focal plane shifts when the aperture ring is used.

I've tested it even on a manual focus. I pre-focus at a certain spot, but as f-stop progresses (f2, 2.8, f4), the focal plane shifts to the background.

[ebiggs1]

I doubt the RF 50mm f1.8 (I don't have one) has any significant focus shift. However, all lenses show some focus shift at very close distances.