I had the same or similar problem on a similar lens. The problem seemed to
have
been caused by a slight binding of a ring which transfers the stop down action
from
the mount to deeper in the lens where the aperture blades are.
This ring is easy to get at. One first removes the cross head screws in the
face of
the mount to remove the mount from the rear of the lens. Further in, There
were a
couple more screws to be undone, a spring had to be unhooked from the ring and
a
brass spring retaining ring which holds the ring in place had to be gently
eased
from its groove to allow the ring to be removed.
I polished some spots on the ring, the recess it rotates in and the retaining
ring where there seemed to be some rubbing. This improved matters greatly for
a
while but a permanent cure necessitated gently rubbing a pencil point over the
surfaces the ring touches to leave a minute surface coat of graphite which did
the
trick. This way there is not enough lubricant to find its way where it
shouldn't.
I would try the pencil before anything else. The ring is anodised aluminium
and some of the anodising had worn off. I suspect the anodised surface has
inherently low friction so polishing may only have served to remove slightly
more of
the low friction coating. In hindsight I should have tried the pencil first
but I
wanted to avoid any lubricant because of the usual prohibition.
Giles
> this. My 50mm 1.4 lens has developed a sluggish diaphragm. When I
> removed the lens mount the diaphragm blades seem fine. The parts of the
> mechanism in the mount are sluggish. What is the best way to rectify
> this situation. I have NOT added lubricant.
>
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