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01-05-2009, 05:23 PM   #1
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Autofocus teleconverters

I've never used a teleconverter before, and would like to have one to use occasionally with the 55-300 (which I just got and really like). What's available that actually works with autofocus? I know there's a new Pentax one coming out soon. How about the Sigma 2x? Does it soften the images much?

Thanks!

01-05-2009, 06:16 PM   #2
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QuoteOriginally posted by mabelsound Quote
I've never used a teleconverter before, and would like to have one to use occasionally with the 55-300 (which I just got and really like). What's available that actually works with autofocus? I know there's a new Pentax one coming out soon. How about the Sigma 2x? Does it soften the images much?

Thanks!
I just received a Kenko 1.5x AF Teleconverter; it works with auto focus lenses and automatically focuses them. It will also mount non-autofocus lenses but you still have to focus manually.

I have yet to evaluate its effect on image quality.
01-06-2009, 04:00 PM   #3
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there are a couple of things to look out for with a TC and your 50-300mm lens.

First of all, what maximum apature is your lens.

You may find the camera won't autofocus (except in bright sunlight) because auto focus usuallly stops at an absolute of F6.7, and your zoom is probably F5.6 at 300mm, adding even a 1.4 X TC will make this F8. a 2x makes it F11.

Second, if you have a K10D or K20D, most teleconverters give you an exposure problem (over exposure) because these cameras have special compensation in the metering that needs to know maximum apature. Since the TC usually passes the apature info streight through, the camera reads the lens only and does not account for the impact of the TC, This can be corrected in auto modes with exposure compensation, but on the K10D exposure compensation does not work in manual mode.

third, the TC does not correct for focal length, and as a result, the shake reduction is not optimum, because it is adjusting for the lens only not the lens plus TC

finally, relating specifically to sigma TCs, they may interfere mechanically with your lens, they are only made for a select few fast long sigma and pentax lenses
01-06-2009, 08:17 PM   #4
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Thanks for all that information. Yes, I believe the 55-300 is indeed an f/4-5.6 lens, and I'd be most likely to take advantage of the teleconverter at 300mm. I wonder if there's anyone out there who has used the Kenko 1.5x converter with the DA 55-300, who can tell me if the thing autofocuses fully extended?

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