Originally posted by 1martin2 I'm new to the forum so excuse me if this question has been answered before.
I have owned a Pentax K100D since 2007 and have had good results with its standard 18-55 lens and with the Tamron 70-300 zoom I bought at the same time. However, I would now prefer not to carry both lenses and have to keep swapping them (as I don't have a second body) and so am looking for a compatible 18-270 lens which will work with the K100D and allow auto-focus operation, which I have read may be an issue with the current Pentax 18-270 zoom.
Can anyone advise my best options, please?
Generally speaking, Pentax's compatibility policy is such that all older lenses will work on a newer body, but lenses newer than the camera may or may not.
The full compatibility is broken down here. A checkbox means you can use the lens without limitations in metering, and the AF/MF icon indicates the type of focusing available:
The Pentax Camera Lens Compatibility Chart
Here is a list of all Pentax autofocus lenses that deliver autofocus on the K100D:
Lenses with Screwdrive Autofocus | PentaxForums.com
What won't work:
Lenses with the KAF3 mount (look for SDM or DC in the lens name) will only work in manual focus mode. Unfortunately, the DA 18-270mm is part of this group. I'd recommend the older DA 18-250mm instead. To verify which mount a lens has, look under the "Mount Variant" heading in the specs tables on the page above, or in the
lens database. Most new lenses are KAF3 because this designation means "silent AF only".
Retractable RE lenses (DA 55-300mm RE PLM, DA 18-50mm RE) will not be able to focus at all, and thus can't be used. KAF4 lenses won't work either because the camera won't be able to control the aperture.
TLDR: any lens with the K, KA, KAF, or KAF2 mount will work fine. KAF3 and KAF4 lenses are either not compatible with the K100D, or only manual focus.
By the way, have you considered upgrading the body at all? Something like the K-50 works with all Pentax lenses, and it will blow your K100D out of the park as far as image quality and performance goes
Adam
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