p.1 #1 · Strange Issue with GF 100-200? Sharp at f/5.6, but blurry at f/8!
I'd heard this was one of the least sharp native lenses for GF (which is a very high bar!) but I wanted to try it out anyway so bought it used from another user here on FM. It is reasonably small and light, and feels well made. The lens collar is really nice and easy to remove, though I do wish they'd cut the foot for Arca Swiss.
But this copy of the lens is performing very strangely.... it is reasonably sharp wide open at f/5.6, but it is very blurry stopped down to f/8! f/11 is slightly better than f/8 but it is never gets as sharp as it is at f/5.6.
I'll attach some test shots below. All taken on a sturdy tripod using the lens foot on a windless day with a 10s timer and stabilization turned off. I tried both autofocus and manual focus, and I updated the firmware for both the lens and the camera body (GFX100S). I took multiple shots over several days to make sure they were not user error and they all look like this.
I've never heard of a lens being sharper wide open than it is with the aperture closed down by one stop, so could any other GF100-200 owners please chime in?
p.1 #2 · Strange Issue with GF 100-200? Sharp at f/5.6, but blurry at f/8!
Two possible reasons:
#1: Shutter shock. Can't magnify the images so I have no idea of what shutter speeds you used.
#2: Focus shift. Some lenses move focus slightly when stopping down, because of spherical aberration.
Edit: It's also possible that such a slow lens actually is sharpest wide open, at least partly because of diffraction softening when stopping down. But then it should be worse at f/11.
p.1 #3 · Strange Issue with GF 100-200? Sharp at f/5.6, but blurry at f/8!
I found out the hard way to never use this lens with mechanical shutter. Once I switched away from mechanical shutter and used electronic shutter, sharpness issues disappeared.
p.1 #4 · Strange Issue with GF 100-200? Sharp at f/5.6, but blurry at f/8!
Hmm, something looks off to me. I don't remember seeing focus shift like that, and my copy performed similarly from f5.6 to f11. I think the first priority is to determine if the focus is moving versus nowhere being in sharp focus. You can try taking shots of a ruler or rug (anything where the focal plane is easily visible) to see if it's shifting stopped down. Or try manually focusing at f8, though it will be difficult. There's a button dance necessary to manually focus stopped down.
I agree with the others saying to use electronic shutter only for testing a telephoto.
