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08-31-2010, 05:39 AM   #1
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2X Converters

On a pentax K10d, can I use a 2X converter? Will it still allow auto focus? I'm sure some are better than others, I'm useing a Sigma 1.4-6.3, 500mm. Sure would appreciate some input. thanks

08-31-2010, 06:15 AM   #2
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Yes, you can use the converter, and yes, the AF will be slower on a lens with slow aperture such as the sigma 50-500mm.

See here for reviews of the Sigma 2x teleconverter for Pentax: SIGMA APO 2x EX DG Lens Reviews - Pentax Third-Party Lens Review Database

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08-31-2010, 08:24 AM   #3
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A couple of qualifications with using a TC on the sigma 50-500mm (AKA BIGMA)

first of all, sigmas own TCs do not support HSM at this time so if the lens is an HSM version autofocus is lost.

Second, the lens is slow enough that a 2x converter will not focus reliably and even a 1.4x converter at the long end will not AF reliabily due to the slow final aperture.

third, the sigma TC protrudes into the back of the lens, and there is a limit to going all the way down to 50mm on the lens, when using a sigma TC

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