A lot of people don't realize that when you "fire the shutter" on an SLR,
you're not really releasing the shutter -- you're releasing the mirror, and
the mirror trips the shutter once it moves up out of the way. And on OM's at
least, the same mechanism that lifts the mirror also stops down the lens'
aperture. When you "work" the DOF button, it may be jiggling the aperture
stop-down arm enough to make it "fire", releasing the mirror then firing the
shutter.
I would suspect that the "catch" that holds the mirror/aperture lever is
either "sticky", worn out, or set so that it barely holds on to the notch it
drops into. If it's worn out, the only solution is to replace it. The good
news is that both the OM-4 and OM-3 use the same parts (though not the "T"'s)
so parts aren't too hard to find. The bad news is that one of the parts that
sometimes wears out is riveted to the mechanism plate that forms the side of
the mirror box -- and the rest of the levers are rather difficult (and messy!)
to remove.
All that being said, I'd first try to clean the related lever, especially if
there's any signs of "liguid damage" (rust or "sticky stuff" like soft
drink). Then I'd try to adjust it to lock as deeply as it could. Only then
would I resort to disassembly and replacement, and I'd probably replace the
whole assembly completely rather than piece by piece, just to save time and
effort.
Kam Cheung wrote:
> Hi all,
>
> My old friend OM-3 has developed a strange behaviour. Sometimes (it does
> not happen all the time) I can fire the shutter by simply pressing the DOF
> preview buttom for one or two times. Any idea what's going on?
>
> Regards,
> Kam
>
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