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09-29-2010, 09:17 AM   #1
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K-x with older lenses - quick question

I have had my K-x with the two lens kit (18-55 & 55-300) for a couple weeks and love it. Now I want to try using the older Pentax manual lenses. I ordered a lens online that was described as "Pentax-A 50mm 1:2 SMC Lens" which I would believe is the lens referred to in the forum's lens database as "SMC Pentax-A 50mm F2". My understanding is that this lens will mount on the K-x with no adapter plate required. I believe all I have to do is set the lens aperture ring to the A position and the camera will adjust the aperture and I will need to focus manually. Do I have this right?

Also, I remember reading somewhere that some lenses may short out the electrical contacts on the K-x body and - presumably - damage it. Is that true? If so, when do I have to worry about that?

Thanks for any help!

09-29-2010, 09:22 AM   #2
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QuoteOriginally posted by ptx101 Quote
I ordered a lens online that was described as "Pentax-A 50mm 1:2 SMC Lens" which I would believe is the lens referred to in the forum's lens database as "SMC Pentax-A 50mm F2". My understanding is that this lens will mount on the K-x with no adapter plate required. I believe all I have to do is set the lens aperture ring to the A position and the camera will adjust the aperture and I will need to focus manually. Do I have this right?!
That is correct, mounts just like your DA lenses, and maintains auto exposure with manual focus.
09-29-2010, 09:27 AM   #3
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QuoteOriginally posted by ptx101 Quote
I have had my K-x with the two lens kit (18-55 & 55-300) for a couple weeks and love it. Now I want to try using the older Pentax manual lenses. I ordered a lens online that was described as "Pentax-A 50mm 1:2 SMC Lens" which I would believe is the lens referred to in the forum's lens database as "SMC Pentax-A 50mm F2". My understanding is that this lens will mount on the K-x with no adapter plate required. I believe all I have to do is set the lens aperture ring to the A position and the camera will adjust the aperture and I will need to focus manually. Do I have this right?

Also, I remember reading somewhere that some lenses may short out the electrical contacts on the K-x body and - presumably - damage it. Is that true? If so, when do I have to worry about that?

Thanks for any help!
Short answer Yes the lens will work as you described

Longer answer

The aperture coding of the pentax lenses is achieved by 6 contacts, starting from the lens locking pin. aLL of these pins they are either open or shorted to tell the camera whether a lens is in the A position (third pin from the locking pin, and recessed on the camera body so a non A lens does not contact it) , AND WHAT THE MAXIMUM AND MINIMUM APERTURE ARE

The 7th pin and last one is for data feedback from the auto focus lenses, and if shorted to the lens mount tells the camera that a non AF lens is attached. this , on some bodies needs to be shorted for non AF lenses to achieve focus confirmation.

Regardless all 7 pins on the lens mount are OK to short because they are designed to be shorted to the lens mount in certain conditions including a lens being put on or off. There is no requirement to turn power off when dismounting lenses is there? even if there was, no one would accept a camera damaged by such an innocent action.

What I can't comment on is the two power pins for the KAF2/KAF3 mount that are recessed in the mirror box. One of these supplies power for the SDM, the other , I am only guessing is for control. I don't know how they would behave if shorted, but there is NO issue at all in shorting any or all of the pins on the face of the mount.
09-29-2010, 05:24 PM   #4
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When you've read about lens contacts being shorted, most likely, you didn't read that this was a bad thing - it's something people often do deliberately to get certain lenses to function in certain ways that they otherwise wouldn't. But we're so accustomed to associating shorting out with something bad, that you probably just assumed it was something bad.

09-29-2010, 06:24 PM   #5
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Works great!

audiobomber and Lowell Goudge - thanks for your replies. The lens came in today's mail and I have taken a few pictures and it seems quite fine. The only imperfection I can see is a small speck of dust inside the lens about 2/3 of the distance from the center of the lens to the outside of the lens. I took some shots of just a white background and can't see it showing up anywhere so I guess that is OK. The lens works as advertised in terms of aperture adjustment via the camera and the manual focusing is a breeze. I hope to take it on a hike tomorrow to get some more pictures with it.

Marc - you are correct, I never would have guessed that the contacts were shorted out to function correctly.

Thanks for all your help!
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