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Hi Mr Bump
Seems the problem is random. There is no firmware update at Pentax, and Sigma claim this is the first time they've heard of it. I have changed lenses and bodies (istDL), and had the same happening on the istDL, but with the Sigma 105DG Macro. Which seems to pretty much eliminate dirty contact points.
What happens when attempting to reduce the aperture to 2.8 is that it either refuses to go below 4 on the K5, or 3.5 on the istDL.
I suppose it is one of those problems one has to live with: switch off, fit a different lens, switch on, off, then revert to the Sigma, problem solved....Thanks for the offered help.
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