Oily diaphragm may eventually spray a fine coating of oil all over the 2
elements that face it. If there's enough oil on the blades, the mirror
will go up, the lever is pushed on the back of the lens, shutter fires
but does so without the blades stopping down (that's an extreme case).
___________________________________
John Hermanson | CPS, Inc.
21 South Ln., Huntington NY 11743
631-424-2121 | www.zuiko.com
Olympus OM Service since 1977
Gallery: www.zuiko.com/album/index.html
Carlos J. Santisteban wrote:
> Hi Chuck, John and all,
>
>
>> From: Chuck Norcutt
>
>
>
>> Anyone have suggestions for disassembly of the 300/4.5 to clean up an oily
>> diaphragm?
>
>
> As oir technician says, it seems to be a difficult job. However, your
> friend should check whether his lens is performing OK or not, despite the
> oily iris. According to my experience, non-OM lenses with oily diaphragms
> are nearly useless due to unexpected overexposure, but oil on OM lenses
> seems to be harmless.
>
> I have at least three oily Zuikos: 1) an MC 50/1.8, 2) a 50/1.2 and, guess
> which, 3) the 300/4.5. But the only malfunction they show is:
>
> 1) Iris closes OK, but after the shot opens slowly, in about a second. Thus
> the viewfinder doesn't recover its full brightness until that second --
> affects NOTHING.
>
> 2) Iris may close a bit slower, but shutter alwas opens when the diaphragm
> is already at the specified aperture -- that means a very slight delay with
> trigger action, but seems no worse than a modern wonderbrick.
>
> 3) Iris closes OK, but sometimes doesn't fully open after the exposure. Safe
> for a slightly dimmer vierfinder, doesn't affect OTF exposure at all. It
> will throw off 'standard' meter reading (ike the OM-1) but could be solver
> with a quick twist of the aperture ring to the widest setting.
>
> <begin of hardcore stuff>
> If you hold most non-OM lenses in your hand, they'll be already at the
> specified setting. If you move the auto-diaphragm pin at the mount, the iris
> will be fully open; and when you release that pin, a spring on the lens will
> return the pin at its 'closed' setting -- that is, the shooting aperture.
>
> When mounted on the body, the auto-diaphragm lever on that cameras will keep
> the pin pressed, in order to get the brightest viewfinder. When you shoot,
> the camera moves that lever, *releasing* the pin on the lens, letting the
> (comparatively weak) spring move it to the shooting aperture. Obviously, if
> that movement is braked by oil on the iris blades, it may not get to the
> correct aperture whe the shutter trips, leading to overexposure.
>
> OTOH, most (if not all) Zuiko OM lenses are constructed the opposite way: in
> your hand, the iris will be fully open, you have to move the auto-diaphragm
> pin at the mount in order to close it -- or press the DoF preview button,
> which does exactly the same. When releasing the pin, the (weak) spring on
> the lens will open the iris again, maybe slowly -- but that's *after* the
> exposure, so not much of a problem.
>
> In the camera, the auto-diaphragm lever normally doesn't touch the pin...
> until the camera is triggered, when a (rather strong) spring will raise that
> lever, together with raising the mirror and moving the auto-pin on the lens
> -- yes, that will close the iris simultaneously. A great thing on the design
> of OM bodies (the 2SP and the 40/PC being the exceptions) is that the
> shutter won't trip until the lever is fully up, which also means the mirror
> is up AND the iris is already at shooting aperture. In extreme cases, the
> lens may delay the rising of the body's auto-lever, but in any case the
> exposure won't start until it has reached its final position -- the shot may
> be sightly delayed, but properly exposed.
> <end of hardcore stuff>
>
> Please note that some aftermarket OM-mount lenses may be just a conversion
> of a standard mechanism, where "pressing" the auto-pin may just _release_ an
> internal lever with an even weaker spring, thus having the same problems of
> non-OM lenses...
>
> Cheers,
--
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