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12-18-2008, 09:48 PM   #1
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Watching aperture blades move...

When I have a lens with an aperture ring, and I move the ring, I can see the aperture blades move. (Hang on, this IS going somewhere.) Once the aperture is fairly narrow, I can no longer see any movement. For instance, it looks the same at f/8 and f/22. Is this normal? Maybe the movement is so subtle at these small apertures that I can't see it... or is there something else going on? I understand that the movement from f/8 to f/11 is certainly much less dramatic than the movement from f/2 to f/4... but it truly appeared to be not moving at all when I observed this later today.

Curious.

I figure someone else has some enlightening remarks about this. I also figure that asking a lot of really weird (read: dumb) questions is a good way to get involved on the forum.

12-18-2008, 10:11 PM   #2
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They should move at all stops. The opening area decreases by half for every full stop number increase. I hope that makes sense. From f2.0 to f2.8, the opening becomes half as large, thus cutting the amount of light in half. Full stops are 1.0, 1.4, 2.0, 2.8, 4, 5.6, 8, 11, 16, 22, etc.

Why aren't yours moving? I don't know. There might be something about your particular lens; what is it? Are you looking at the lens on or off the camera? You can check to see if it works on the camera by taking pictures at all the full stops.
12-18-2008, 11:29 PM   #3
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I understand the 1/2 light per stop idea... but what I'm guessing is that 1/2 of f/8 is so small that I don't notice a change to f/16. I hope that makes sense. 1/2 of f/2 is a lot, so I can see it move to f/4. Once it gets down to f/8 it all looks the same, so I'm wondering if it's because the movements are so small at those apertures, or if my lens is closing up to early (closing to f/22 when I set it to f/8).

Maybe the change should really be noticeable, and mine's just messed up. I noticed this off the camera. I haven't played with enough lenses to know what's "normal" exactly.
12-19-2008, 12:41 AM   #4
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What particular lens do you have?

I just checked with a M50/1.7 and a FA35/2, and when I move the aperture ring from F stop to F stop, there is a distinct motion of the aperture blades and a change in size of the actual aperture (hole).

12-19-2008, 02:02 AM   #5
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Let me have a bet on this one:

It's a K-mount lens.

With the lens in your hand (separated from the cam), try this:
- Fully close aperture
- snap the aperture open and closed back and forth with the aperture lever on the lens' back (please make reasonable pauses in between snapping, there have been people jamming their blades with machine-gun like actuation - oh well)
- you will see the aperture fully close sometimes when you let it snap all the way and not fully close when you let it close very slowly

This is what happens when the aperture mechanism gets a little oily and/or dirty over time, also the colder the temperature the stiffer the blades. This could result in overexposed ('stopped down'-) pictures in wintertime and in the summer it's still OK. It could be that the actuation frees the blades to a degree were they become usable again.

The cure: CLA the lens :-) This spells clean, lubricate and adjust. Never mind, just go on using the lens. If you don't have a problem in your pictures, forget about it.

Best, Georg (the other)

Last edited by georgweb; 12-19-2008 at 02:10 AM.
12-19-2008, 10:44 AM   #6
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Not that I am necessarily going to try it, but how would one go about cleaning the aperture blades on a K-mount prime?

Some answers... more questions...

Intrigued.
12-19-2008, 10:53 AM   #7
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Does there appear to be oil on the blades? If so I'd recommend you send the lens to Home for cleaning. This is not an easy job for the inexperienced. Eric does good work and at reasonable prices.
12-19-2008, 11:22 AM   #8
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well, it certainly appears that after f/5.6, the blades don't close anymore. The blades appear perfectly clean, no oil or dirt.

the oddest thing (perhaps), is that the pin on the back of the camera that opens and closes the aperture apppears to be at the end of it's track at f/5.6. There isn't any more room for it to move. At f/1.4 it is at the far clockwise position, then as I stop down it moves counterclockwise, but it runs out of room to move at f/5.6.

I wish I had another lens to compare. I may be making mountains out of molehills, but this seems a little odd. If it turns out to be something normal, I'd at least like to know it isn't anything to worry about.

12-19-2008, 11:26 AM   #9
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That's not normal. The lever should be around the middle of the slot at f5.6 with more room to move. What lens is this? You can easily remove the back plate (mount) with a jewelers screwdriver. Then you could look and see if there is something not lined up correctly.
12-19-2008, 11:30 AM   #10
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it's an M-50/1.4 in k-mount. I'm about to get out a phillips head and see what kind of trouble I can get into.

I bought it from a forum member, who said it came from KEH... so I only assume it been taken apart to be cleaned by them, but I also assumed it was put back together correctly.

This is... unfortunate.

EDIT: One of the screws is stripped. Woe is me. I guess I'll just have to use f/1.4-5.6 for now and hope for better luck on another lens later.

EDIT #2: Got the k-mount and aperture ring off. It looks like I could be in for a real job if I start going further without knowing what I am supposed to be looking for.

It appears that my aperture is not "wide open" even when the aperture ring is turn all the way. The ring is set all the way open, but the little pin in the back will still move some more. Also, I can see the aperture blades at 1.4. Is THIS normal?

Last edited by idig4phish; 12-19-2008 at 11:53 AM. Reason: new info.
12-19-2008, 12:54 PM   #11
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Alright... I'm guessing that the problem is that the mechanism for the aperture and the pin in the back is misaligned somehow. The aperture will not open all the way unless I manually move the pin with the K-mount off. The k-mount restricts me from moving the pin all the way open, and the mount under the k-mount is aligned such that the other little metal pin that interacts with the aperture ring will not move far enough to open the aperture completely. AND... at f/5.6 the aperture will not close anymore. I'm guessing that this is all part of the same problem with the misalignment of the aperture ring linking mechanism.

I hope this makes sense. I don't exactly know the names for all these parts, but I'm hoping that anyone who could help me would have enough camera knowledge to know what I'm talking about.

Anyhow... can someone tell me how to fix this? I got the K-mount off, and the screws that apparently do something with the focus ring, and decided I should wait for some instruction before going any further. I'm a pretty good DIY'er, but I just don't know jack about camera lenses. I think I could figure it out with a little help, but if someone knows what the problem is, you might can tell me if it's worth tackling or not.

KEH doesn't want it. They said it's not worth the repair since 50mm lenses are so abundant. So if I can't fix it by tomorrow afternoon, I'm just going to have them send me another one. They're much more expensive than private parties, but I would like the lens by Christmas, and they can ship to me in 1 day.

I might go the takumar route if I can find a screwmount adapter in time, since their k-mount 50/1.4's are $89 + shipping... not the bargain I was looking for. It would be awesome if somebody could help me repair the lens I have.



Geez...

Last edited by idig4phish; 12-19-2008 at 01:38 PM.
12-19-2008, 02:14 PM   #12
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This offers some help although a slightly different lens. Google Image Result for http://homepage.mac.com/jpelling/50mm/files/page11_3.jpg

But to adjust the aperture lever correctly, you have to go deep inside the lens. I suspect someone had this one apart before and put it back together wrong. maybe someone else can find a better link to help with the process.
12-19-2008, 03:36 PM   #13
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FIXED IT!

I can post details on the complete dismanteling and repair of this lens later if someone wants it.

Sorry for the heavy posting today, but I was sorta frantic. However, God is good... the lens is salvaged, in perfect working condition.

Thanks PZ for trying to help!
12-19-2008, 03:43 PM   #14
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That's not a bad price from KEH.

The aperture adjustment mechanism is doing a few things at the same time, and almost every lens is different. I might have some photos of the Pentax-M 50mm f2.0 but Pentax was never big on interchangeable parts, so it might not be exactly the same. Check back in a few hours when I have access to the right computer. Your lens probably has a part installed upside-down or not rotated correctly. If you bought the same lens from KEH, you would have a working version to copy. That's how I figured it out.
12-19-2008, 05:33 PM   #15
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QuoteQuote:
idig4phish said:
I can post details on the complete dismanteling and repair of this lens later if someone wants it.
Yes please,
might save a lot of work,
thanks, Georg (the other)
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