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07-09-2012, 08:59 PM   #1
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MC Cosinon-T 200mm f/4 Cosina Lens Made in Japan

I just bought this lens (MC Cosinon-T 200/4 Cosina), I paid $7 plus $8 shipping, it looks in great shape, clean, almost new and when I move the aperture lever, it opens it fine. It focus well, but when I try to stop it down with the green button, it cannot be stopped down. It always shoot with wide open. I know the aperture is working because it manually open and closes. I a using the Pentax K5. Any advice?

PS. I also bought the same lens from a pawnshop for $1, but it was damaged, the focus was loose, and the aperture lever did not open the diaphragm.

I really appreciate your advice, the photos wide open are OK, as far as i can see in the short period of time at night.

Here is the lens.

Last edited by Pepe Guitarra; 07-09-2012 at 09:11 PM.
07-09-2012, 09:20 PM   #2
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I believe that you are going to have to operate the lens as a fully manual lens. Which means to stop down, you have to do it the old fashion way, and actually turn the aperture ring to the desired f stop, then meter with the green button (setting the shutter speed and ISO depending upon your selected mode - essentially AV or M) and shoot. It looks like a very nice lens and you really can't beat the price.

I think that this is your lens...Not the world's best optics - but a 7 rating at a couple of dollars is not bad. You can have a lot of fun with it and it looks very clean!

07-09-2012, 09:26 PM   #3
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I usually shoot manually. However, when I set up the aperture with the aperture ring, focus and push the green button, it does nothing. So, it does not stop down. Therefore, the shot is taken wide-open. I compared it with my other (most of them) manual lenses and it looks fine, when out of the camera, it does open the diaphragm with the lever. I tried using the Pentax 1.7xAFA to see if I get some traction but nothing. It is like if the lever never engages with the camera.
07-09-2012, 10:15 PM   #4
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I had a similar problem with a 50mm f1.7 (M) lens that I purchased on ebay. After a while it exhibited the symptoms you describe.I pushed (by hand) on the aperture setting lever--either outwards or inwards--forgot which and that did the trick.

07-10-2012, 04:07 AM   #5
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Dear Pepe.
I had similar thing with Porst 55/1.2. The thing is that the lens mount is covered with some sort of dye that did not conduct electricity. So, camera can not recognize the lens to be mounted. What I did is I scratched the stain from the mount where the contacts on the body are located. Some people are using foil just sticking into this place. I tried that but it did not work.
Hope this would be helpful.
07-10-2012, 04:31 AM   #6
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I have the same thing with my old Cosina 50/1.4, aperature works fine on my *istD, but not on my K-5 or K-7. I think its the crippled mount of the newer camera's.
07-10-2012, 04:33 AM   #7
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Check the EXIF from photos taken with the lens. If it says "M42 or no lens", rather than "K or M lens", it means that tin foil, conductive paint, scratching or whatever will make the camera accept the lens as K-mount.
Had the same thing with a Takumar which the camera believed was K-mount.


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