Originally posted by Mike.P® Eh?
What's that all about?
All fairly self-explanatory:
Quote: This lens is equipped with a Petal-type hood to block out extraneous light. The filter size of this lens is 95mm and the lens is supplied with a step down ring enabling digital cameras with an APS-C size image sensor to use an 86mm filter.
Filter size for full-frame is huge (95mm), requiring expensive filters. Since APS-C cameras don't reach the full extent of the frame, a step-down ring is provided to use a somewhat smaller and more economical filter size.
Quote: *It is not possible to use a Polarizing filter with the step down ring. If you attach a Polarizing filter, you may not be able to remove it from the lens again.
Using the step-down ring only provides enough coverage to attach one filter. Polarizing filters often stand out further than most regular filters to allow for rotation of the filter when mounted. Using a polarizing filter on the step-down ring would hence result in vignetting at the corners of the frame, even on an APS-C camera.
Quote: *For Pentax and Sony mounts, it is not possible to use the AF and the built-in OS function of this lens when attaching it to film SLR cameras as well as Pentax *ist series and K100D.
The built-in autofocus motor and stabilizer function aren't compatible with older Pentax cameras. The autofocus system used is HSM (Sigma's version of SDM / Supersonic Drive Motor), with no support for screw drive, hence older screw-drive only bodies can't use AF with the lens. Pentax doesn't have in-lens stabilization, and FWICR, the stabilizer function in Sigma's lenses is piggybacking off power from contacts used for SDM or power zoom (if anybody's confirmed which, I've not read of it). So again, older bodies without the appropriate contacts can't power the in-lens stabilization.
Also not mentioned but worth noting is that in-body stabilization should be disabled if in-lens stabilization is used (and at the focal lengths of the Bigma, that's basically all the time I'd say).