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I have a Sigma mini-wide II F2.8 PKA fit lens on a Samsung GX20. Using the lens on A setting will only display a maximum aperture size of F4.
The lens works perfectly well on a Pentax P50 Film SLR - opening the aperture and holding down the DOF preview button the iris opens nicely wide open and operating the aperture ring manualy and opening up to F2.8 shows a corresponding faster shutter speed as expected.
A Pentax 50mm F1.7 PKA lens has no problem on the GX20 displaying all apertures.
A mystery that I am very keen to try and resolve - if anyone can help?
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Sigma lenses are reverse-engineered, that is, they do not pay Pentax for a licence to use the K mount. This means that older lenses may well mis-report to the camera and not be properly recognised. You won't have the problem with Pentax lenses.
The Mini Wide 28mm is quite old now, so probably not worth having Sigma re-chip it, even if it were possible.
The only other thing you can try is to clean both camera and lens contacts with a soft dry cloth in case there is poor contact.
I have the 24mm version of this lens which works perfectly in A setting, and I would think if this works on your P50 it should work on the GX20. I can only suggest that the lens is not locking in place properly for some reason?
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