Teleplus MC4 2X Teleconverter for Pentax Mount

tom_kkh · 03-12-2010, 11:13 PM

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?

SOldBear · 03-13-2010, 12:31 AM

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.

tom_kkh · 03-13-2010, 01:48 AM

Originally posted by SOldBear

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

SOldBear · 03-13-2010, 11:08 AM

Originally posted by tom_kkh

But how come it works with the sigma 70-300mm?

When the Sigma is at 300mm? I have no clue.

Originally posted by tom_kkh

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.

tom_kkh · 03-13-2010, 08:47 PM

Originally posted by SOldBear

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.

Damn Brit · 03-13-2010, 08:59 PM

Moved to lens forum.

03-12-2010, 11:13 PM	#1
tom_kkh New Member Join Date: Jan 2010 Posts: 24	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, 01:48 AM	#3
tom_kkh New Member Join Date: Jan 2010 Posts: 24 Original Poster	Originally posted by SOldBear 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.

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03-13-2010, 12:31 AM	#2
SOldBear Veteran Member Join Date: Feb 2009 Location: Alameda, CA Posts: 3,206	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, 11:08 AM	#4
SOldBear Veteran Member Join Date: Feb 2009 Location: Alameda, CA Posts: 3,206	Originally posted by tom_kkh But how come it works with the sigma 70-300mm? When the Sigma is at 300mm? I have no clue. Originally posted by tom_kkh 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
tom_kkh New Member Join Date: Jan 2010 Posts: 24 Original Poster	Originally posted by SOldBear 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
Damn Brit Guest	Moved to lens forum.