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04-29-2009, 08:23 AM   #1
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Question regarding using converters with lenses

Hi,

I have read that you can use certain Pentax AF converters with lenses to make manual lenses into semi-automatic lenses.

I have a Sigma f/8 600 mirror lens.

Which Pentax converter would be the correct one to use with this lens?

It's my understanding that you focus the lens in fairly close and then the converter finshes the job to pinpoint the shot.

Thank you for your help and / or corrections on any of my assumptions!

04-29-2009, 09:49 AM   #2
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This thread might give you some answers.

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/58372-get-auto...al-lenses.html
04-29-2009, 09:56 AM   #3
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Maybe I am wrong but I have read somewhere that manual lenses with f values slower than 5.6 can not become AF. And In case your lens is already an AF (slower than f5.6) you lose that function and you will need to focus manually.

Hope that helps.
04-29-2009, 11:55 AM   #4
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QuoteOriginally posted by cooldude14es Quote
And In case your lens is already an AF (slower than f5.6) you lose that function and you will need to focus manually.
Mirrors built with AF? Not this Sigma, anyway.

04-29-2009, 12:41 PM   #5
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QuoteOriginally posted by stl09 Quote
Hi,

I have read that you can use certain Pentax AF converters with lenses to make manual lenses into semi-automatic lenses.

I have a Sigma f/8 600 mirror lens.

Which Pentax converter would be the correct one to use with this lens?

It's my understanding that you focus the lens in fairly close and then the converter finshes the job to pinpoint the shot.

Thank you for your help and / or corrections on any of my assumptions!
There WAS exactly ONE AF-convertor, the "SMC Pentax-F 1.7x AF Adapter". This thing goes for ridiculous prices on ebay, today. It converts most any lens with a max. aperture of about f/4 to AF function.

Officially it only supports A-lenses with at least f/2.8, but in practice it is more flexible, working also with K/M-lenses or even M42 (with a K-mount adaptor of course). The necessary max. aperture is also not quite a rigidly fixed, as it also depends on the light. In bright sunshine it will easily support f/4-lenses or even f/5.6, but not reliably. In dimmer conditions it won't.

For your f/8 lens it is basically unuseable, not the least it becomes very, very long, with a focal length of 1020mm at f/13...

Your notion about focusing is very correct. The AF adapter cannot focus over the whole focusing range of all the lenses, that could possibly be used with it. Also, it achieves AF by internal focusing and that is limited, to preserve image quality. So, what you do is: you focus manually roughly and let the AF adapter do the fine focusing. That works quite well with suitable lenses.

Ben
04-29-2009, 01:05 PM   #6
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QuoteOriginally posted by Ben_Edict Quote
There WAS exactly ONE AF-convertor, the "SMC Pentax-F 1.7x AF Adapter". This thing goes for ridiculous prices on ebay, today. It converts most any lens with a max. aperture of about f/4 to AF function.

Officially it only supports A-lenses with at least f/2.8, but in practice it is more flexible, working also with K/M-lenses or even M42 (with a K-mount adaptor of course). The necessary max. aperture is also not quite a rigidly fixed, as it also depends on the light. In bright sunshine it will easily support f/4-lenses or even f/5.6, but not reliably. In dimmer conditions it won't.

For your f/8 lens it is basically unuseable, not the least it becomes very, very long, with a focal length of 1020mm at f/13...

Your notion about focusing is very correct. The AF adapter cannot focus over the whole focusing range of all the lenses, that could possibly be used with it. Also, it achieves AF by internal focusing and that is limited, to preserve image quality. So, what you do is: you focus manually roughly and let the AF adapter do the fine focusing. That works quite well with suitable lenses.

Ben
Ben,

It sounds like you just saved me a whole bunch of money and I thank you very much for that. I will save your information for future manual lenses that I might consider.
Thanks again for your help.
Kevin
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