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03-12-2010, 11:13 PM   #1
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Teleplus MC4 2X Teleconverter for Pentax Mount

I have a Teleplus MC4 2X Teleconverter for Pentax Mount but it doesn't seems to work on Pentax dal 55-300mm and tamron 18-250mm lens.

It is unable to auto focus on both above lens but it works fine with a sigma 70-300mm.

I have tried with a different KX body with the same result. Weird

anyone knows the reason?

03-13-2010, 12:31 AM   #2
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The reason is the effective max. aperture.

Autofocus can work reliably only down to about F/5.6 or F/6.7 at best. In best lighting condition and if there is something contrasty to lock on, it can work down to F/8, maybe with some hunting.

With a 2X TC, the effective max. aperture is 2X of the max. aperture of the lens. For example, the Tamron 18-250mm F/3.5-6.3 when combined with the TC becomes a 36-500mm F/7-12.6, too slow for autofocus.
03-13-2010, 01:48 AM   #3
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QuoteOriginally posted by SOldBear Quote
The reason is the effective max. aperture.

Autofocus can work reliably only down to about F/5.6 or F/6.7 at best. In best lighting condition and if there is something contrasty to lock on, it can work down to F/8, maybe with some hunting.

With a 2X TC, the effective max. aperture is 2X of the max. aperture of the lens. For example, the Tamron 18-250mm F/3.5-6.3 when combined with the TC becomes a 36-500mm F/7-12.6, too slow for autofocus.
thanks for the clarification. But how come it works with the sigma 70-300mm?

any idea if there are any teleconvertor that can works with Tamron 18-250mm lens?

TIA

Last edited by tom_kkh; 03-13-2010 at 02:14 AM.
03-13-2010, 11:08 AM   #4
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QuoteOriginally posted by tom_kkh Quote
But how come it works with the sigma 70-300mm?
When the Sigma is at 300mm? I have no clue.

QuoteOriginally posted by tom_kkh Quote
any idea if there are any teleconvertor that can works with Tamron 18-250mm lens?
It works with my Tamron 1.4X TC. But at 250mm, it needs something contrasty to lock on for autofocus. And the IQ is disappointing. I wouldn't use a TC with that lens.

03-13-2010, 08:47 PM   #5
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QuoteOriginally posted by SOldBear Quote
When the Sigma is at 300mm? I have no clue.



It works with my Tamron 1.4X TC. But at 250mm, it needs something contrasty to lock on for autofocus. And the IQ is disappointing. I wouldn't use a TC with that lens.
Thanks for the kind advice. Appreciated.
03-13-2010, 08:59 PM   #6
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Moved to lens forum.
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