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02-26-2013, 11:51 AM   #1
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Problem with Cosinon-S 55mm F1.2

Hi all,

I just received my very first F1.2 lens and was very excited to give it a whirl. I've used a number of older manual lenses to good effect and never had any problems before.

Until now. I've got the Cosina Cosinon-S MC 55mm 1:1.2 (the 8-aperture blades, F1.2-F16 variant) and am mounting it on my K-r. Something mechanical seems to be interfering with the shooting process. I'm reasonably experienced with using the camera and a variety of lenses but am not so technically proficient with the mechanics to explain this easily but will try. Essentially, it will generaly take between 1 and 10 shots before refusing to take any more. It continues to click like it's taking a shot but sometimes sort of hangs like it has a very long shutter speed, but doesn't take an image. At this point I have to swap out the lens for another (any, it seems) and the camera returns to normal. All I can think is that it's something to do with the fact that the portion of the lens that sits inside the camera is considerably longer than any other lens I own and it must be impinging on some of the moving parts within the camera. Either the mirror or possible the small lever on the left hand side of the mount as you look at it (I'm sorry, I don't really know what this does).

Does anyone have any ideas why this might be or has anyone experienced anything similar? If so, any ideas what to do to resolve it? Is it a duff lens and should I return it or is this something that can be worked round? It was a reasonably expensive purchase for my budget and I need to decide what to do fairly soon so i can return it to the Ebay seller if necessary. The lens is in otherwise pretty good condition, other than a slight stiffness of the focusing ring at close distances. It'd be a shame and a pain to have to return it, particularly after doing all the research, finally plucking up the courage to go for a slightly more "exotic" lens and spending a few quid getting it.

Any help would be greatly appreciated.

Cheers,

Nick

02-26-2013, 12:24 PM   #2
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Can't help but Im curious as to where you got the lens - was it japan Fuji? I only ask as I had my eye on that one and when I finally got paid, it was gone
Thinking about the rikenon 55mm now, same as you, it will be my first 'exotic' lens purchase and now you have me worried...
02-26-2013, 12:47 PM   #3
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Hmm... I think it was Korea actually, so probably not "your" one ;-)

I guess it's always a bit of a concern when you spend a fair chunk of money on a used item, particularly a high spec one with the potential for being faulty. I'm just keeping my fingers crossed someone will give me a nice easy fix...
02-26-2013, 01:00 PM   #4
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During focusing, the lens moves more inside and outside of the body. Does the problem persist regardless of how it is focused (near vs. infinity)? Just be careful so you dont damage the lens or mirror. I kind of doubt the actual lens recedes further into the body than other K mount primes. Can you compare it to some other lens?
Oh, also, try cleaning the lens contacts. and make sure the aperture shield is not too large.

02-26-2013, 01:21 PM   #5
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QuoteOriginally posted by EasyLuckyFree Quote
Hmm... I think it was Korea actually, so probably not "your" one ;-)

I guess it's always a bit of a concern when you spend a fair chunk of money on a used item, particularly a high spec one with the potential for being faulty. I'm just keeping my fingers crossed someone will give me a nice easy fix...
I agree, but although a concern, it is always exciting as well, I have spent the last two weeks thinking of the pictures I want to ty.

here is hoping you get your issue sorted...
02-26-2013, 01:22 PM   #6
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It seems to trigger the problem when focus approaches infinity. There's a slight but perceptible resistance between the 30ft and infinity marks on the ring. It's smooth through this section when the lens is not attached. That would point to it hitting something I guess, which is a bit worrying. There's not much else there bar the mirror...

I've compared it with my old manual SMC Pentax-M 1:1.7 and my modern 40mm DA Limited and it is perceptable longer from the plate to the furthest point in. It may only be a couple of millimeters but it is obvious when looking.

How would I know if the aperture shield is too large? Is that the metal bit that sticks out adjacent to the small lever?
02-26-2013, 01:40 PM   #7
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Further experimentation seems to indicate that is is problematic at either end of the focusing, but fine in the middle. Odd.
02-26-2013, 02:22 PM   #8
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A long shot, but you never know....

Is the mount of the lens (the flange against the camera's lens mount) bare (chrome) or anodized (black)? Can you please post a photo of the mount of the lens.

If the mount is black, I think you need to scratch it to bare metal to ground the contact on the camera. If this contact is not grounded, the camera thinks there is no lens and refuses to take photos.



02-26-2013, 02:31 PM   #9
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Obviously it's a K mount lens but what variety, A, M, or stop down mode - Av? The small lever on the left side of the lens mount opening on the body actuates the pin on the lens that closes the diaphragm down momentarily on the A or M lenses to the aperture value you are shooting at. In the stop down mode (Av), typically with M42 lenses, that lever has no relevance because whatever aperture value you set on the lens, will physically darken or lighten the lens until you move the aperture ring once more.

It's possible the aperture actuating arm on the lens is not being hit perpendicularly by the lever inside the camera and over a period of so many shots, the small lever on the camera jams against the actuator arm on the lens, causing the camera to malfunction. It could be a simple matter of straightening the actuating arm on the lens to resolve this issue. Visually compare the actuating arm of the problem lens to the one that's functioning normally and see if the former looks bent. If so, gently try straightening the arm.
02-26-2013, 02:34 PM   #10
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Interesting. It is black. How much would I need to scrape back? My only concern is that if it isn't that, then the seller could rightly say I'd "damaged" the lens. Would a small, fairly inconspicuous patch suffice?

Photo attached.
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02-26-2013, 02:46 PM   #11
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It's an M and I would intend to shoot in fully manual mode with it, more often than not.

The lever looks pretty straight though. Compared it to my functioning Pentax M 1:1.7 50mm and there's no obvious difference.
02-26-2013, 03:01 PM   #12
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QuoteOriginally posted by EasyLuckyFree Quote
How much would I need to scrape back?
See the photo in my post above as where and how much to scrape. You need to ground only one contact (the last one I think).

If you don't want to scrape the lens's mount just yet, try a piece of aluminum foil between the lens and the camera to ground the contact.

Try this:

- Remove the lens.
- Try to take a photo. The camera will not.
- Use a piece of aluminum foil to ground the 7th contact on the camera's lens mount (just keep it with one hand). Still without a lens attached, try to take a photo.
02-26-2013, 03:23 PM   #13
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Err, the camera does take pictures with the lens detached (and the front open). It's not something I've ever tried before but I kind of assumed it wouldn't, as you say. Is that a problem?

Though back to the original problem, I've just had a thought. I've used plastic Holga toy lenses on this camera which have no metal parts at all and they've always worked fine. If they didn't need grounding, would a normal lens?
02-26-2013, 03:30 PM   #14
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QuoteOriginally posted by EasyLuckyFree Quote
Err, the camera does take pictures with the lens detached (and the front open). It's not something I've ever tried before but I kind of assumed it wouldn't, as you say. Is that a problem?
No, that's not a problem. It's just that my assumption about your camera model was incorrect.

Since K-7 (and K-r I believe), a Pentax camera won't take photo if it thinks there is no lens attached. Models before that will fire regardless.

What model is your camera body?
02-26-2013, 03:40 PM   #15
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Yeah, it's a K-r so that makes sense...
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