If the diaphragm is stuck open or sticky it will lead to overexposure
since it may not be able to close down at all or may not close down fast
enough before the shutter fires.
Remove the lens, set it on the smallest aperture (probably f/16) and
then look through the lens as you manually activate the diaphragm lever
on the back of the lens. The diaphrgam should close and reopen fully
and do it as fast as you can flick that lever with your finger. It's
behaviour should be "snappy". If you're unsure of what the diahpragm
action should be like compare it with another of your lenses that you
know does not have a problem.
If this is your widest lens it's also possible that you're just not
compensating for the wide field of view. What the meter sees with this
lens is not the same as any other lens of a different focal length.
Assume a hypothetical field of view with a normal brighness central area
and dark areas on either side. If you shoot this scene with a 50mm lens
which sees only the brighter central area you will get a shorter
exposure than with the 24mm which will try to lighten the darker areas
that only it can see.
It seems unlikely but it's also possible that your shutter isn't
behaving properely in manual mode.
But check the lens first. Sticky diaphragms on old Japanes made lenses
are pretty common. Many of them used a type of grease in the focusing
helicoid which breaks down with age and turns into a thick oil instead
of grease. It can then start flowing around inside the lens and getting
into places it's not supposed to be... like the leaves of the diaphragm
which should be bone dry.
Chuck Norcutt
Bao C. Ha wrote:
> Hello,
>
> I have just gotten the Tamron 24mm f2.5. It seems that I tend
> to overexpose while testing it out with the OM-4, manual mode.
> I just wonder if this is just an operator's failures, rather
> than the lens itself.
>
> Thanks for any comments/suggestions.
> Bao
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