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maboleth · ‎12-02-2024

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.

maboleth · ‎12-02-2024

Ok, so I tested this lens again and there's a certain pattern of the focus-shift.

From 1.8 - 2.2 - focus is the same or getting better.
From 2.5 - 2.8 there's a slight shift and degradation in sharpness of the subject.
From 3.2 - 4 there's a visible degradation in sharpness and focus shifts towards background. This is only evident when letters and small details are focused.
From 5.6 onward, focus is restored and back to the subject.

So, if someone needs critical, pixel-peeping sharpness of the subject from f3.2 to f4, it's best to use Exp. Simulation + DOF, if the camera permits full-time Aperture ring usage, as R7 does.

View solution in original post

maboleth · ‎12-04-2024

Well, R7 is 32mpx and is using just the central part of the lens, hence cropped sensor.
On the other hand, R6mk2 is FF, 24mpx and is using full diameter.

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