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,
--
Carlos J. Santisteban Salinas
IES Turaniana (Roquetas de Mar, Almeria)
<http://cjss.sytes.net/>
--
_________________________________________________________________
Options: http://lists.thomasclausen.net/mailman/listinfo/olympus
Archives: http://lists.thomasclausen.net/mailman/private/olympus/
Themed Olympus Photo Exhibition: http://www.tope.nl/
|