adjust aperture for Cosinon manual lenses (tip)

Started Apr 17, 2012 | Discussions
Kerusker
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adjust aperture for Cosinon manual lenses (tip)
Apr 17, 2012

I have an EXAKTA 24mm 2.8 lens in P-A mount. I think it's a Cosina lens.
I had severe overexposure at all apertures except wide open.
I tried to fix that and found a little screw at the side of the bajonet:

Behind is an excenter, which adjusts the aperture lever. I adjusted it such that the aperture starts to close down without any play when activated by the mechanical lever at the lens back.
To do the same, you don't have to take the lens apart

If your lens has such a screw (with excenter), then you should be able to adjust the aperture for over- or under- or hopefully exact exposure.

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][.Kerusker
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brandrx
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Great tip.
In reply to Kerusker, Apr 17, 2012

Cool. Great find and great tip. Thanks.

Ron

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KentG
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Re: adjust aperture for Cosinon manual lenses (tip)
In reply to Kerusker, Apr 18, 2012

So how is this different from just using the green button to stop the lens down mechanically to the shooting aperture on any manaul lens like normal? All manual Pentax mount lenses except "A" lenses (when in the A postion) require you to use the green button as a stop down aperture lever because current Pentax cameras have no regular mechanical aperture coupling. So when you are using a manual lens unless you do this the lens is always at wide open aperture when you take a pictures regardless of what the viewfinder or aperture ring says it is. Please re-read the info on using manual lenses on Pentax AF and DSLR bodies.
Kent Gittings

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Kerusker
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Re: adjust aperture for Cosinon manual lenses (tip)
In reply to KentG, Apr 18, 2012

KentG wrote:

So how is this different from just using the green button to stop the lens down mechanically to the shooting aperture on any manaul lens like normal? All manual Pentax mount lenses except "A" lenses

I was writing about

lens in P-A mount

(when in the A postion) require you to use the green button as a stop down aperture lever because current Pentax cameras have no regular mechanical aperture coupling. So when you are using a manual lens unless you do this the lens is always at wide open aperture when you take a pictures regardless of what the viewfinder or aperture ring says it is. Please re-read the info on using manual lenses on Pentax AF and DSLR bodies.
Kent Gittings

Excuse me Kent, I was mainly talking about using the lens in A-Mode.

But you could also use this kind of adjustment for non-A lenses to get a correct aperture diameter or adjust for the non-linearity of the older aperture levers (pre A-lenses) which can't be perfect of course.

Since I'm not a native speaker, I keep it (too) short and often miss a better wording.
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KentG
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Re: adjust aperture for Cosinon manual lenses (tip)
In reply to Kerusker, Apr 19, 2012

Well your issue with metering just seems that it caused by the fact you are not doing the correct procedure to stop the lens aperture down before metering or taking the shot. That is what happens when you only get good images when the lens is wide open and the camera thinks it is wide open. It sounds like at every other setting the camera thinks the lens is stoped down but it is not because the green button is not down.That is because there is no mechanism in the AF/DSLR cameras to mechanically stop down any manual lens that either has no A setting or is an A lens being used in the non-A aperture modes. Except for the green button in DSLRs. One reason why although I have lots of M42 and other lenses from the early era I only use A series manual lenses on my DSLRs simply because I do not want to go through the stop-down metering mode needed to use any of the earlier lenses. DSLRs will not do open aperture metering with any lens that is not in A mode, manual or auto focus. that is what is meant by a "crippled KAF mount". They took out the mechanical open aperture metering bail in the lens mount area to save space. That means the camera can't change the aperture during metering and without the mechanical parts it has no idea what the lens aperture is set to. So since it can't stop it down the aperture stay wide open and the camera even if it is set to some other aperture value it ends up taking the image at wide open, overexposing every image except the one set for wide open.

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Kerusker
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Re: adjust aperture for Cosinon manual lenses (tip)
In reply to KentG, Apr 19, 2012

KentG wrote:

Well your issue with metering just seems that it caused by the fact you are not doing the correct procedure to stop the lens aperture down before metering or taking the shot. That is what happens when you only get good images when the lens is wide open and the camera thinks it is wide open.

Kent, as I said in my mail before, I use the lens in A-mode i.e. the aperture ring may be set to 5.6 and the camera mode dial to P. Then the camera measures with lens wide open and shuts the lens down to 5.6 during exposure. It does not expose with the lens wide open!

You use the same wording in your text below: " with any lens that is not in A mode" ???

It sounds like at every other setting the camera thinks the lens is stoped down but it is not because the green button is not down.That is because there is no mechanism in the AF/DSLR cameras to mechanically stop down any manual lens that either has no A setting or is an A lens being used in the non-A aperture modes. Except for the green button in DSLRs. One reason why although I have lots of M42 and other lenses from the early era I only use A series manual lenses on my DSLRs simply because I do not want to go through the stop-down metering mode needed to use any of the earlier lenses. DSLRs will not do open aperture metering with any lens that is not in A mode, manual or auto focus. that is what is meant by a "crippled KAF mount". They took out the mechanical open aperture metering bail in the lens mount area to save space. That means the camera can't change the aperture during metering and without the mechanical parts it has no idea what the lens aperture is set to. So since it can't stop it down the aperture stay wide open and the camera even if it is set to some other aperture value it ends up taking the image at wide open, overexposing every image except the one set for wide open.

I think all you say is correct, but you aren't correct about what I'm saying.
Maybe this makes it more clear:

Imagine I stop down the 2.8 lens to 3.5 with lens in A-mode, cam in P-mode and the lens stays fully open during exposure, because of the aperture lever at the back of the lens which is activated by the cam is loose (has play). With the described excenter I can get rid of the play and the lens aperture closes without play when activated by the cam.

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