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08-11-2008, 11:22 AM   #1
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M42 screw mount lens diaphrams!

Hi everyone, I have just registered for this forum so forgive me if I mess up. I am having a problem (maybe) with a vintage Takumar M42 screw mount, 50mm f/4 macro lens I have purchased recently. The lens is the first screw mount I have ever owned and don't have a screw mount body to try it on. When I press the diaphram stop down pin on the rear of the lens the diaphram readily stops down to the selected opening on the aperture ring. When I release the stop down pin it returns to it's normal position but the diaphram remains in the stoped down position until I return the aperture ring to the maximum aperture which happens to be f/4 at which time the diaphram opens up with the aperture ring. The lens is spotless and looks to be in mint condition with it's original case. I am wondering if this is normal behavior sense the lens is not mounted on a body or if there is something wrong. The Asahi part # is 43912. Any advise or help in this matter will be greatly appreciated.

08-11-2008, 11:58 AM   #2
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I think it is the manual/auto switch on the lens if it is on Auto the lens will stay wide open you have to have the switch turned to manual for the lens to stay at the aperture you select on the ring. could be wrong but I think thats may be the issue
08-11-2008, 12:20 PM   #3
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That reads like an oily, slow or stuck aperture. The only cure i know of is a professional cleaning though...
[edit]
You can try to exercise the aperture mechanism a bit. Sometimes it returns to normal speed, but this is not a long term solution.
08-11-2008, 12:26 PM   #4
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If it's in Manual mode, changing the aperture will immediately stop the lens down. Pushing the pin should not have any effect. In Auto mode, the aperture remains open until you push the pin... and should return to open when you release it.

Therefore I suspect there is something going on with the A/M switch linkage... sticky apertures would be slow closing which you do not describe. I'd eliminate the first and easiest option, like gokenin suggests: make sure you stay on Auto mode while you test it out.

However, as you don't have a screwmount body, you'll be using Manual mode exclusively. So as long as the lens stops down OK you're good to shoot.

08-11-2008, 12:35 PM   #5
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I think I'll go with UliBär's diagnostic. I have a M42 that behaves like that.
08-11-2008, 01:06 PM   #6
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QuoteOriginally posted by UliBär Quote
That reads like an oily, slow or stuck aperture. The only cure i know of is a professional cleaning though...
[edit]
You can try to exercise the aperture mechanism a bit. Sometimes it returns to normal speed, but this is not a long term solution.
cleaning ancient metal lenses does not require professional work

you take it apart, you clean it, you put it back together

its like making a peanut butter sandwich, only you cant eat it.
08-11-2008, 01:14 PM   #7
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Good Lens Repair Shop !

I tried to exercise the lens diaphram with no luck. I did notice that when the manual/auto switch is in auto the lens stops down when I turn the aperture ring and remains there until I turn the aperture ring back to maximum aperture. In other words the diaphram is being controled by the aperture ring alone and the pin has no effect and when the switch is in manual mode the diaphram remains open when I turn the aperture ring until I press the stopdown pin at which time the diaphram closes down to the selection on the aperture ring and will not open until the aperture ring is returned to maximum aperture. Sounds like something is definitely wrong. Does any one have any suggestions as to a good reputable repair shop. I have used one in the past that did more damage than good. Thanks for all the advise.
08-11-2008, 01:19 PM   #8
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Made my last post reply before I got yours. I only gave $5.00 for this little beauty so I guess I don't have much to loose. Would still like the name of a good shop just in case things go sour on my repair attempt. Thanks

08-11-2008, 01:25 PM   #9
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I think you confused the switch settings, cause your description is just swapped.
When the M can be seen, the lens is in manual mode and the aperture should react to the settings of the aperture ring. When the A can be seen, the lens is in automatic mode and should close the aperture to the value selected on the ring by pressing the lever and should open immediately when you release the lever. From your description i think the aperture is simply stuck in the closed position and disengages if you turn the aperture ring. As Gooshin stated, if you are familiar with fine mechanics and optics, try on your own. Use alcohol or methanol for cleaning up the blades of the aperture - be careful though, the blades are often very delicate thin and easily bend.
08-11-2008, 01:41 PM   #10
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repair place

Home

This is Erics website he does alot of repairs on pentax lens and camera. I just sent two of my Taks to him to do some work on so hopefully when they come back they will be good for another 30 years
08-11-2008, 01:56 PM   #11
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Just a side note - if this is a SMC or S-M-C era lens there's a tiny additional pin that needs to be pushed in (ie. when the lens is mounted) for the A/M switch to work. Though that doesn't sound like your issue.

You could try googling for the a/m switch linkage, I'm sure someone's posted photos of how it works and how to fix it. You can decide from there if it is something you want to tackle or not.
08-11-2008, 03:07 PM   #12
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Robert, Just a suggestion, but why not give gooshin a PM and see if he can fix it for you.
Seems like he knows what to do.
08-11-2008, 03:52 PM   #13
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Eric's Website !

Thanks for the suggestion (gokenin). I will be sending the lens to Eric in the morning. I live only about an hour and a half from him. Good Deal and thank you all very much.
08-12-2008, 12:42 AM   #14
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5 bucks? Almost as lucky as I am. The normal going rate is about 100$ .

I recently dismantled a 50mm. It's rather different to other takumars. If you can, try removing the cap from behind and then the rear element. Because if you go the front route, you might to have to completly dismantle the lens. I had to remove everything to get into all the elements. I am not sure if you can get the cap off without taking the lens apart completly, but worth a try. It just screw off like a ordinary bottle cap, under it you will find the normal lockrings to remove the rear elements, which will let you get into the back of the aperture blades.

However, if the back of the blades is not enough, you will probably have to take the whole lens apart. Which is nice, because then you can clean it all up and relubricate it. Mine had mould in the heli.
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