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Teleconverter use with 70-210 f/4 (Beercan)

Started Mar 11, 2006 | Discussions thread
QuietOC
QuietOC Veteran Member • Posts: 6,838
Re: Teleconverter use with 70-210 f/4 (Beercan)

Dave Farmer wrote:

Autofocus needs a certain amount of light to work properly, and the cut-off point seems to be somewhere around f/8. If you try, for instance, a 2x on the long end of the 100-300/4.5-5.6 APO the effective aperture will be f/11, which will probably be insuffiicent for AF to work properly. f/8 might work, might not. It depends on the subject, contrast etc. but it's around the limit for AF to function properly.

At least, that's how it used to be! I guess it's possible that AF systems have improved and the limit has gone up... anyone know the answer to that one?

My Sony SLTs will focus my Minolta 100-400 F4.5-6.7 APO with a Kenko 2X teleconverter. The focus degrades to the point of just barely working. That lens doesn't even focus that well without a teleconverter at the long end.

The 70-210 F4 focuses alright on a 2X. The CA though is pretty awful. I'd recommend using the converter on a more corrected lens.

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