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02-20-2009, 06:29 PM   #1
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Stuck Aperture

I just got a Pentax SMC M 50mm f1.7 lens online, and it looks to be in generally good condition. However, the aperture ring was hard to turn, and once it finally moved, the aperture is not changing at all, and is stuck wide open. It looks like it'll be a solid lens if the aperture works, but I'm not sure how solvable this issue likely is. According to the seller the aperture was snappy, so I have no idea if something would have happened in transit due to weather changes (Hawaii to NH). Any advice?

02-20-2009, 09:48 PM   #2
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Where did you buy the lens? Notify the seller as soon as possible, describing to them what you tell us here. Give the seller the chance to make good on the deal.

I honestly doubt the aperture became stuck in the shipping process---highly doubt. Ask the seller if he/she tried stopping the lens down. Best of luck.
02-20-2009, 09:55 PM   #3
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If the aperture remains stuck wide open with the lens off the camera as you stop down, the lens will need repair. Turning the aperture ring while the lens is on the camera will not stop it down until you take the picture. To check that, use the optical depth of field preview with the lens set down to f/8 or f/11 and see if the viewfinder darkens.
02-20-2009, 10:16 PM   #4
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QuoteQuote:
CanadaRockies: Turning the aperture ring while the lens is on the camera will not stop it down until you take the picture. To check that, use the optical depth of field preview with the lens set down to f/8 or f/11 and see if the viewfinder darkens.
Yes, thanks Al for bringing more clarity to the discussion.

pygmalionundone: You also can take two pics, at different enough apertures, and that will tell you if the aperture is stopping down on camera. From your discussion above, I got the impression you were working the lens in your hands.

02-20-2009, 11:19 PM   #5
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It's not stopping down off the camera or on it. I let the seller know that I was having some trouble with it, but frankly with the cost of shipping it won't be much savings versus just holding onto it as a F 1.7 only lens, although that's obviously not ideal. Might explain how I got a good deal on it on ebay.
02-21-2009, 12:21 AM   #6
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QuoteQuote:
It's not stopping down off the camera or on it. I let the seller know that I was having some trouble with it, but frankly with the cost of shipping it won't be much savings versus just holding onto it as a F 1.7 only lens, although that's obviously not ideal. Might explain how I got a good deal on it on ebay
Why would you get a good deal on it--no one would have known the blades are faulty--right? You say the seller advertised the blades as "snappy." Do you have a link to the original Ebay auction? I think more people than just me would like to know more about the seller.
02-21-2009, 11:47 AM   #7
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On lenses with aperture rings, there are two semi-independent mechanisms that move the aperture blades. The ring is one, the aperture lever is the other. Your problem turning the ring points to the ring mechanism, but I'm not sure how it might have been broken. I took mine apart for some photos. Here, the mount has been removed with a Phillips #00 screwdriver and the 5 chrome screws. The aperture lever is sticking up in the upper right. On the left center, there's a copper colored rectangular tab that is connected to the ring mechanism.



Now I have removed the aperture ring. Be careful not to lose the small ball bearing trapped between the ring and the lens body, located just under the orange diamond marking above the ring. You can see it in the photo, stuck there by a bit of grease. I make sure to set this aside in a safe place - they disappear easily. Now you can see another rectangular tab connected to the one in the first photo, which engages a slot in the aperture ring.



And here's the aperture ring, with the square slot for that copper tab at the top and a little to the left.



I know I have taken many lenses several times from Hawai'i to Connecticut without problems, but I can't rule anything else out for you. If you're up for some disassembly, it might be repairable. Or, priority mail return shipping ought to be somewhere around $5.
02-22-2009, 03:26 PM   #8
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Dave, thanks so much for the pictures. Even after shipping I only paid $35, and so I will only gain about $15 to send the lens back for a refund, which the seller has agreed to do. My sense after looking at your post is that somehow the ring mechanism got detached. Now would it be possible a piece broke in there (the initial resistance with turning the ring that finally gave way), and if so would it be difficult/impossible to repair?

02-22-2009, 03:56 PM   #9
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That lens is small enough to send priority mail for about $5 (I've done it)... I'd return it.
02-22-2009, 08:50 PM   #10
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That lens is small enough to send priority mail for about $5 (I've done it)... I'd return it.
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02-23-2009, 12:47 PM   #11
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QuoteOriginally posted by pygmalionundone Quote
Dave, thanks so much for the pictures. Even after shipping I only paid $35, and so I will only gain about $15 to send the lens back for a refund, which the seller has agreed to do. My sense after looking at your post is that somehow the ring mechanism got detached. Now would it be possible a piece broke in there (the initial resistance with turning the ring that finally gave way), and if so would it be difficult/impossible to repair?
If some part is broken, the only source is another lens or making it yourself, which is not that easy. That's somewhat rare, though. Loose screws are the biggest problem I've seen, and one possible culprit here. But I was curious, so I did some further disassembly on my lens. I removed the three phillips screws going into the lens body, which are exposed when you remove the aperture ring. One of the heads is right under the yellow "m" on the distance scale. Then you can pop off the whole aperture adjustment assembly. On my lens this part was kind of stuck on, but came off without a hammer or violence.



Then I could remove the ring on the right, which is upside-down in this photo.



These parts form the semi-indepent aperture movement I referred to in my first post. I'm not sure if I can describe it with words but here goes, referring to the second photo. The lens aperture has a round pin connected to it which fits in the small fork riveted just inside of the aperture lever, top center of the mechanism on the left. There's a return spring, mostly hidden at the bottom of the mechanism. A stamped steel not-quite round piece has the aperture lever and fork riveted to it, and is attached to the mechanism body with one screw at the bottom that also holds the spring. That allows the aperture lever to move sideways and spring back.

The ring on the right has four attached protrusions to move the lens aperture to the aperture ring setting. On the bottom is the key that fits into the aperture ring. Sticking up is a part which goes through the lens mount and should engage a lever in the camera mount. I'll skip it for brevity, it doesn't matter for our purposes. Then there are two tiny pins on the left and right, one shiny and one dark. These fit inside the stamped steel part on the left (remember this part is upside down). The stamped steel ring has an arc in its shape that engages one of these pins. The spring holds it against the pin. When the aperture ring moves, the pin moves the stamped steel ring and through it the aperture blades, while still allowing the aperture lever to move the blades between that setting and wide open.

Whether anyone can follow that or not, here's a theory about what has happened to the broken lens. The tiny pins were broken off when the lens was set to f1.7. That would prevent setting any other aperture and the lever could move without any actual effect. The broken pics could maybe be replaced with small screws, filed down after installation.
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