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06-23-2007, 07:05 AM   #1
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Help on a teleconverter for Sigma 135-400mm

What would be the best teleconverter for a Sigma 135-400mm on a K10D?

Would there be any restrictions?

Thanks to All!

06-23-2007, 07:58 AM   #2
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I would recommend a 1.4x converter if you are getting one, Tamron makes a good version as does Sigma (APO EX). Also the Pentax A 1.4x-S or the A 2x-S Both will cost more but produce a little better results. The 1.4's will multiply a 400mm to roughly a 560mm which has an effective focal length of 840mm on a DSLR and you only loose 1 stop (at f4 the lens becomes f5.6 etc). A 2x converter will cost you 2 stops.

You may loose the auto focus though, unless the lens has the drive slot. The Sigma APO and Tamron 1.4's have the drive slot for the auto focus feature. There may be others that have the drive slot as well. AF may slow down as well depending on the available light. You're adding more glass to the back of the lens and cutting down on the light getting to the sensors.

These converters do reduce the lenses image quality as well (some not too bad and others quite a lot). Any converter will show a loss in the final image.
06-24-2007, 07:52 AM   #3
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QuoteOriginally posted by rdrum76 Quote
What would be the best teleconverter for a Sigma 135-400mm on a K10D?

Would there be any restrictions?

Thanks to All!
there is no best TC for the 135-400, although AF might work at the shorter focal lengths, when you zoom out to 400 mm, F5.6 is just too slow for autofocus to work. You will be stuck in manual focus.
06-24-2007, 04:49 PM   #4
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I cant remember if that lens is an APO or not, but Sigma makes TCs for all of their lenses, including APO specific.

06-25-2007, 12:51 AM   #5
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QuoteOriginally posted by Peter Zack Quote does Sigma (APO EX)
Don't those only work with the Sigma EX lenses? (certainly doesn't fit my Pentax 80-320)

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