I suspect that the coupling mechanism in the Auto Tube is barely
catching on the camera's coupling ring just inside the lens mount, and
after a while it's slipping off. Less likely, but possible, is that
the aperture arm sticking out of the rear of the lens is barely
catching on the coupling arm inside the front of the Auto Tube, and
eventually slipping off there.
Start by mounting a lens onto the Auto Tube, mount the Auto Tube onto
a body with the back open, set the body on "B", wind and fire the
camera and hold the release button down to keep the shutter open.
Look inside the rear of the body to where the Auto Tube's coupling
mechanism mates with the aperture coupling ring of the body at the
upper right hand corner -- turn the lens' aperture slowly back and
forth, and at the smaller apertures (f16, for example) you should see
where they meet. I suspect you will see that the tab sticking out of
the back of the Auto Tube is barely touching the tab of the camera's
coupling ring. It may be necessary to bent the tab of the Auto Tube
ever so slightly _out_, away from the center, 1mm or so, so that it
catches the tab on the body's coupling ring more firmly.
If the problem isn't there, the same thing may be happening at the
other end of the Auto Tube, between it and the lenses. The tab inside
the front of the Auto Tube may be bent _down_ so that the tab sticking
out of the lenses barely catches. If that's the case, you may have to
bend the tab on the Auto Tube slightly _up_ so that it catches the
lenses tab more firmly.
>
> I'm looking for anyone who has experienced a similar problem.
>
> I've acquired an older 65-116 telescopic auto tube that seems to
have a
> linkage malfunction. It works fine for a while after attaching to
the camera body
> and lens, but changing the aperture setting on the lens causes the
tube to
> emit an audible "click", and the aperture ring becomes decoupled
from the
> body...it acts as if the aperture has a fixed setting at wide open.
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