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08-24-2013, 10:51 PM   #1
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Pentax K 120mm f2.8 issue

I have recently acquired this lens and, with much enthusiasm, have started testing it.

In testing it I have realised that there may be an issue with the lens.

The lens is snapped on to my K30 and does so as you would expect. However, when you reposition the aperture ring, the iris remains open - so no variable aperture.

When the lens is of the camera I can easily manipulate the aperture blades without a problem although I can see how, when mounted, the blades would not operate.

Is this a known issue and if so how do I rectify? Help in other words


This issue extends to all my K and M lenses (shock) on both my K30 and K01 - I do not understand???

Thanks in advance.



OK I put the camera in bulb setting and did a selfy ...... wha la the aperture closes to the stopped position on shutter activation. Cool didn't realise this.

I think I am sorted but help/ comment on the topic is still requested ....

Last edited by Wild Mark; 08-24-2013 at 11:21 PM.
08-25-2013, 12:26 AM   #2
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You need to set the camera to allow using the aperture ring and then use M stop down exposure mode as eplained in this article.

Enjoy your K lens!
08-25-2013, 08:21 AM   #3
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This is the expected behavior. All K-mount lenses have a feature called automatic aperture (auto aperture for short) where the aperture remains open until time of exposure. With a very few exceptions, all modern lenses for all makes have auto aperture. At one time, this was a big design feature to the extent that many vintage lenses made it part of the name. I have several Auto Vivitar primes, for example as well as a few Auto Rikenon and an Auto Mamiya/Sekor. Other vintage lenses had fully manual iris diaphragm (always stopped down) or pre-set diaphragm where stop-down is managed by a switch or ring on the lens body.

Where things get confusing is that Pentax lenses from the A-series forward support a feature where aperture may be controlled by the camera body for Tv and P modes. Pentax calls this feature "automated aperture control" and lenses that support it have a position on the aperture ring marked with an "A" as well as electrical contacts on the lens mount face.


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