teleconvertors

why is it that teleconvertors in auto focus do not work above a certain apeture and one has to use the manual mode to focus. the purpose of having an auto focus teleconvertor is totally lost. pentax needs to tackle this issue.

It may be due to the lack of light at smaller apertures. My 50-500 works well with a 1.7TC in bright sunlight yet soon stutters at lower light levels.

Best regards

I think the critical aperture may be f5.6 and if you're getting a result with a lens of smaller than that aperture, it's a bonus.

Actually it's worse than that. The 1.7x converter is designed for using with manual focus lenses of f2.8 aperture or wider.

The kenko i had recommended f4 or wider but did work at f5,6 and above with the sigma 150-500 on a good day.

Canon only work on f4 or faster unlees you pay thousands for one of their higher spec bodies so it's not just Pentax. I have had the Sigma 100-300mm f4 and 2x converter AF on my K-5.

My experience too, Mike. For the combination to work effectively, however, at least when using a Tamron-F 2X Pz-AF MC7, the light needs to be good.

Jon