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05-01-2010, 05:34 AM   #1
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Anyone tried to make their extension tubes P/KA?

I've got a set of P/K "Auto" extension tubes which work fine for stop-down metering if the lens has an aperture ring.

Lenses without an aperture ring require electrical contacts on the extension tubes. Otherwise the camera doesn't know it is dealing with an "A"-lens and one cannot set the aperture from the camera.

Has anyone tried to modify the mount of non-P/KA extension tubes so that it tells the camera that an "A"-lens is mounted? The main thing is to make sure that the camera's "*"-contact is shortened to ground by the extension tube. Since that camera contact is slightly recessed at the the body mount, simply scratching paint away at the corresponding place on the extension tube mount will probably not work. Or will it?

If one wants to communicate the correct aperture range to the camera, one would have to scratch away some more paint at strategic places. Doing nothing should result in the range "f/2.8-f/16" reported to the camera.

BTW, I've read that someone had trouble with their Jessops P/KA extension tubes. They didn't work on a *istD since they didn't shorten the "digital communication contact" of the camera. After removing paint at the corresponding position, the tubes worked as intended. I don't know if later Pentax DSLRs also require this.

P.S.: I know that a popular technique is to gut P/KA teleconverters. I may go that route eventually but it wouldn't buy me as much extension as the extension tubes I've already got.

05-01-2010, 05:52 AM   #2
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I've a modified PK lens to PKA, a 135mm f2.8 Porst, and everything works in automatic (P-TTl, Matrix Metering, Av-Tv-Sv-P-scene mode), but i removed the aperture lever so i can put the same F-stop on body and on the lens.
When you let the body handle the lever, since PKA lenses's lever behaves differently than older PK lens, you may end up with different f-stop between body and lens (EX. on body you have F8, but the lens is like f5.6 because the movment of the aperture lever is not enough to close it till f8) and by removing it you can match what you have dialed on the body with what you have dialed on the lens.

Now working with it may be a bit tricky, because i always meter using f2.8 on body and selecting the desidered aperture on the lens, then i lock the exposure (* simbol in viewfinder) and dial the correct F-stop on body then take the shoot.
You can also work by dialing the desidered f-stop on the body while the lens is wide open, then lock exposure and dial the right f number on the lens.
If i use the lens this way, it does work like a normal PKA lens.
You may also let the camera handle it, but you have to learn how much you have to compenstae exposure for the "X" f-stop

QuoteOriginally posted by Class A Quote
I've got a set of "Auto" extension tubes which work fine for stop-down metering if the lens has an aperture ring.
...cut
If it's "auto" you shouldn't need to use stop down metering, and should work even with lens without an actual aperture ring (EX. kit lens), i think that what's for the "Auto" in your extension tube.

I've a non Auto extension tube that do work well but only in stop-down metering and wont work with lens without an aperture ring, unless you want to use it completley closed.
05-01-2010, 06:33 AM   #3
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Thanks for your reply.

QuoteOriginally posted by matam Quote
I've a modified PK lens to PKA, a 135mm f2.8 Porst, ...
How did you add the "*"/"A"-contact to the lens? Just a matter of making it conduct at that position?

QuoteOriginally posted by matam Quote
If it's "auto" you shouldn't need to use stop down metering, and should work even with lens without an actual aperture ring...
No, the "Auto" just means that the tubes contain aperture levers. This way you can focus and meter wide open and the lens will only stop down (to the value set on the aperture ring) once you take the shot.
The "Auto" does not mean "P/KA", i.e., there are no electrical contacts.
05-01-2010, 06:56 AM   #4
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QuoteOriginally posted by Class A Quote
Thanks for your reply.
How did you add the "*"/"A"-contact to the lens? Just a matter of making it conduct at that position?
It wasn't done by me but yes, it's just a pin added with a light spring behind so it doesn't interfere when mounting the lens but at the same time it makes contact with the recessed "A" pin on the body, the other (the one's who tell the body what F-stop range the lense have) where made just by painting the existing mount ring with an insulating black painting and theyr use is only to match the f-stop between lens and camera (if you shorten only that pin ,whatever the lens you have mounted ,the body will think that the aperture goes from f1.2 to f22 ).



QuoteOriginally posted by Class A Quote
No, the "Auto" just means that the tubes contain aperture levers. This way you can focus and meter wide open and the lens will only stop down (to the value set on the aperture ring) once you take the shot.
The "Auto" does not mean "P/KA", i.e., there are no electrical contacts.
If your's tubes has aperture lever then i'm sure it will shorten also the "A" pin on body, and that's should be the difference between an "auto" and a normal extension tube.
An "A" lens doesn't have any electronic part in it, it just has a different lever movment and the mechanical contact to comunicate the F-stop range and when the apertuer is in "A" position.

05-01-2010, 10:38 AM   #5
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Homebrew extensions

I'm one of those who's built their own. I picked up a pair of quantaray "scratch and dent" full auto AF extension tubes at techforless.com for $20 each, unscrewed and tossed the glass elements, and now have 2 full auto with AF extension tubes, each about an inch long. They work great individually or stacked together.
05-01-2010, 10:15 PM   #6
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QuoteOriginally posted by matam Quote
It wasn't done by me but yes, it's just a pin added with a light spring behind
I see, thanks. I guess that goes beyond what I'm prepared to do as a DIY project.

QuoteOriginally posted by matam Quote
If your's tubes has aperture lever then i'm sure it will shorten also the "A" pin on body, and that's should be the difference between an "auto" and a normal extension tube.
No, the tubes don't shorten the "A" pin. Believe me "Auto" does not mean P/KA. Mine are not the only ones of this kind. When I bought them I thought they were as as "auto" as it gets, but they are not. It was a while ago when I didn't know that much about the K-mount yet.

QuoteOriginally posted by snathan Quote
I picked up a pair of quantaray "scratch and dent" full auto AF extension tubes at techforless.com for $20 each, unscrewed and tossed the glass elements, and now have 2 full auto with AF extension tubes, each about an inch long.
Very clever, thanks for sharing.
Unfortunately, shipping to New Zealand is $50.
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