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10-05-2010, 11:29 PM   #1
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Focal 28mm f/2.8 iris does not stop down when unmounted

Hi all,

I recently acquired a Focal 28mm f/2.8 (52mm filter size) K-mount lens. It works great with my Pentax K1000 camera, but I also use K lenses with mount adapters on other cameras, and this is where I'm running in to a problem.

With the other K lenses I have, when unmounted, turning the aperture ring will immediately stop down the iris to the desired setting. Then when the lens is mounted on a Pentax body, it engages the "diaphragm release" arm to keep the iris fully open until the shutter is tripped.

But with this 28mm lens, even when unmounted, the iris always stays open, regardless of the release arm. So this means when I use it on my adapter, I can't stop down to the setting I want since it just stays open.

Is there some kind of modification I can make to the lens to allow me fully manual control to the iris when unmounted? Also, is there some term that differentiates this behavior, so that I can avoid purchasing this kind of lens in the future?

Thank you!

Last edited by mjordan; 10-05-2010 at 11:31 PM. Reason: Clarification
10-06-2010, 07:20 AM   #2
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Is there a tab or a pin that toggles this when mounted? Will it stop down to the set aperture if you tug on the aperture lever?

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10-06-2010, 02:41 PM   #3
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Some old lenses , usually after a long time of non-use, can have lubricant get on to the diaphragm, gum up, and cause the apereture to stop down slowly or not at all. you say it works with the K1000 though. I dont know why this is. does the aperture stop down when you take a picture?

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