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01-21-2011, 08:24 AM   #1
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Manual lens does not stop down when metering or shooting.

I have got a couple of old K-mount lenses and most of them work just fine on my K5 (metering via green button or preview..., manual mode etc.). Now I do have two lenses that do not stop down when metering or shooting, i.e. the aperture stays wide open all the time when mounted.
The Lens I care about is a Cosina 135mm f/2.8 (Cosinon-T MC) which appart from this problem does take nice pictures.

What I have done already:
- I checked and compared the aperture (and the trigger) to my other lenses and couldn't detect any differences. The trigger seems to be at the same place, it triggers, the aperture opens and snaps back.
- When I take out the trigger so that the aperture is not coupled to the camera the lens works fine (except of course, that my viewfinder gets darker when closing the aperture...).
- Mounted on an old Pentax ME it works just fine !?

Maybe someone has an idea what the problem could be?


Last edited by FH_le; 01-21-2011 at 08:38 AM.
01-21-2011, 08:42 AM - 1 Like   #2
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Maybe it is this:
The pins on the camera have to be shortened. If the bajonet of the lens is non conductive, e.g. due to paint, it does not work.
You can either scrap off some of the paint or add some alu foil between lens and camera or glue some foil on the lens.
01-21-2011, 09:10 AM   #3
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QuoteOriginally posted by blende8 Quote
Maybe it is this:
The pins on the camera have to be shortened. If the bajonet of the lens is non conductive, e.g. due to paint, it does not work.
You can either scrap off some of the paint or add some alu foil between lens and camera or glue some foil on the lens.
Wow, thanks!
That was easy ...
I took some aluminium foil and now it works as it should!
(so I will simply glue or solder some conductive material to the lens)
01-21-2011, 11:59 AM   #4
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Your easiest, cleanest bet is aluminum tape from the hardware store.

One roll will cover about 3,000 lenses.

I can't do catch in focus with any of my M42s, but the tape will fix that. One day, I'm gonna splurge, but I think a roll was like 12 bucks at Home Depot.

01-21-2011, 12:32 PM   #5
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Some M42s are so small that they don't even cover the pins.
I hope that Pentax will change this.
There is no necessity for this behavior.
01-21-2011, 01:23 PM   #6
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QuoteOriginally posted by blende8 Quote
Some M42s are so small that they don't even cover the pins.
I hope that Pentax will change this.
There is no necessity for this behavior.
What do you mean?
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