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08-30-2009, 08:04 AM   #1
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Aperture problem with Pentax-A 50/1.7

Hey.

I just bought a Pentax-A 50mm F1.7 in a great shape! but the auto aperture doesn't work!
I adjusted the setting in the camera and put the lens on A mode but the camera still doesn't show me the aperture info.

What can I do?
Thanks!

08-30-2009, 08:33 AM   #2
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Check that when you move the aperture ring from the smallest aperture to the A setting one of the pins on the back of the lens should pop out so that it is just proud of the mount. Check that all of the contacts on the lens and the camera are clean. If both of these are OK then it should work.
08-30-2009, 08:39 AM   #3
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well the pin is out, and the connections are clear.
p.s the camera doesn't know the focal length also, I need to set it up when I start it.

Last edited by D0ri; 08-30-2009 at 09:42 AM.
08-30-2009, 10:14 AM   #4
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It is normal for the camera not to know the focal length as this lens won't have the data pin or the associated electronics. I presume the camera workes ok with other A lenses. Does the A pin on the lens feel free if you push it in (it should be spring loaded).

08-30-2009, 10:41 AM   #5
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If you have a multi-meter and know how to use it, you can measure the electrical resistance between the mount and the small pop-out pin on the mount. When the aperture is in the "A" position, the resistance should drop below 20 ohm. The camera monitors the resistance of this pin to determine whether the lens is in "A" setting or not.
When the lens is not in "A" setting, the pin is not pushed out by the spring behind it, thus the camera will see "infinite" resistance (open circuit) as it has no phycial contact with the pin on the lens.

If the resistance does not drop lets say below 100 ohm when the lens is in A setting, you will probably have to open the back of the lens to fix it. You will need to check the pin and the spring that's behind the pin and the metal bracket the spring rests on.

On my A50/1.7, I find that sometimes I have to apply some twisting pressure to the lens (i.e. wiggle it a little) to get the camera to recognize the lens is in "A" setting.
08-30-2009, 01:53 PM   #6
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Dori, have you used other SMC-A lenses on your camera before?
08-30-2009, 03:24 PM   #7
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I havn't used any other SMC-A lens on that camera before.
The resistance is jumping between 03 to 60, same when it's not on A mode.
I now see that the pin on the mount is not popping and it's stuck, here is some pictures:






the black pin next to the aperture numbers is spring loaded.

Last edited by D0ri; 08-30-2009 at 04:05 PM.
08-30-2009, 04:42 PM   #8
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If the pin do not pop out when the lens is in the A position, then that would explain the problem you are having. If you take a look at the pins on your camera mount, you will notice that the pin on the camera that reads this pin on the lens is recessed just below the camera mount (versus the other pins which are raised above the mount). Therefore unless the pin on the lens is popped up, the camera will think your lens is not in the A position (i.e. the camera sees an open circuit).

Here is how the pin looks like

Pentax 50mm lens fix

If you decide to take the lens apart to fix it, you should take pictures of the lens at each steps. This had helped me to fix my A50/1.7 and put it all back together. Be careful with all the small parts (bearing, spring, pin etc). They can be very hard to locate if you dropped them.

08-30-2009, 04:57 PM   #9
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Here is another thread on similar topic. I did almost lose the pin and spring when I tried to put it back together. The pin slipped out of the bracket and the spring launched the pin into the air and dropped onto the carpet. Fortunately, I found mine.

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/66792-problem-...m-f-1-7-a.html

Last edited by ma318; 09-01-2009 at 10:01 AM.
08-30-2009, 06:13 PM   #10
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I need to folow all the steps or just play a little with the pin and turn everything to it's place?
I don't know if it's worth it
08-30-2009, 07:10 PM   #11
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You can try to push the pin in a little to see whether that would free it.

There is also the possibility that someone before you may have tried to repair the lens and lost the spring in the repair process. I am afraid there is no way to know what is needed to repair it without opening it up.

If you are not comfortable with opening the lens to repair it, then you can sell it (and disclose the issue) to someone who is willing to repair it. The lens is still worth some money even in this condition.

Or you could just use it as a M lens (i.e. in manual stop down metering mode).

Last edited by ma318; 08-31-2009 at 08:12 PM.
08-30-2009, 07:22 PM   #12
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QuoteOriginally posted by D0ri Quote
I need to folow all the steps or just play a little with the pin and turn everything to it's place?
I don't know if it's worth it
At worst, you have a darn good SMC-M 50.

It actually isn't that difficult to fix that pin as originally designed. Disassemble it in a plastic bag to capture any unexpected spring-loaded parts launches and have a large magnet handy too. It may just be that there's crud or corrosion that needs to be scraped clean. If it's actually broken, you can probably jam the pin permanently in the extended position. But provide enough "give"
to ride over the bumps on the body plate.

Ultimately, if you'll only use the lens in the auto condition, just arrange to short the pin to the body by any feasible means. The auto/manual switch is only there for reverse compatibility with older bodies anyway. As a quick check, a very tiny piece/ball of aluminum foil carefully positioned to short the pins will confirm this "fix".

H2

Last edited by pacerr; 08-30-2009 at 09:14 PM.
08-31-2009, 05:55 AM   #13
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I'm so stupid!
I lost 2 parts (the pin, and the little metal bar that stands under it) and I have the little metal ball but I can't put it in it's place.

For now: the lens is working just like before, expect the aperture ring is very lousy (I haven't decided yet if it's a good thing, because before it was pretty annoying) and now I can finely decide what aperture I want to shoot with.

But the camera doesn't get the data of course.

My question is: if I'll put another metal peace instead of the pin, will it work?

Conclusion: never EVER open a lens you bought just yesterday in order to fix it, just take you money back and buy another one.

Conclusion2: I now have smc-m instead of smc-a
08-31-2009, 08:22 AM   #14
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Given what has happened, I would say "no", do not try to put any home-made metal piece into the lens to try to fix it. The home-made piece may end up scratching/damaging your camera mount or falling further inside your lens and jamming the aperture mechanism.

I recommend you look really hard for the lost pieces. (Use a magnet, a flash light, methodically go over every inches of the table and floor with your eyes very close to the surfaces) They did not disappear. They are still there around where you worked on your lens. It's just that you have not found them yet.

Was there a spring underneath the pin when you opened the lens?
If the spring was missing to begin with, then finding the pin and the metal piece alone would not resolve the problem.

Last edited by ma318; 09-01-2009 at 10:04 AM.
08-31-2009, 09:15 AM   #15
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there was no spring.
the pin was just flying through the wall from my hands and gone...
the metal bar is in the lens ><

Does the aperture ring supposed to be kind of stack/stop between every number or just go freely?

The only thing I'm pissing off about is that I paid 75$ for that piece of crap.
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