adjust aperture for Cosinon manual lenses (tip)

Started Apr 17, 2012 | Discussions thread
Contributing MemberPosts: 714Gear list
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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