Even OEM lenses don't maintain 100% forward compatibility. For instance, M
series lenses won't work in shutter priority or full auto on A series or
newer bodies. And I have seen some comments about worse compatibility
problems with the new *ist because you don't have an "A" lock on older
lenses. I doubt a newer Sigmas, especially the EX series, will have any
more problems than an FA series lens on new Pentax bodies.
Now having said that, if the rumoured new lenses offer image stabilization,
that's a horse of a different colour. But there are also rumours that Sigma
will offer their 80-400 IS lens in a Pentax mount (currently only available
in Nikon, Canon and Sigma mounts)
The capability issue that I mentioned had nothing to do with K/M lenses type limitation. It is the fact that Sigma didn't pay for the royality for the proper chip design. As a result, problems could range from metering to AF. Most of the problems I have read are related to Canon EOS & Nikon AF systems. I do not know if Pentax is on the list, but should be aware of. I know in some countries, Sigma would provide "one" chip upgrade, but some don't and they would charge the customers for that.
regards, Alan Chan
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