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Re: [OM] black flecks on lens

Subject: Re: [OM] black flecks on lens
From: Kennedy <rkm@xxxxxxxxxxxxxxxxxx>
Date: Mon, 23 Mar 1998 23:08:51 +0000
In article , BGA Services, Inc. <bga_svcs@xxxxxxxxxxxxx> writes
>Kennedy wrote:
>> The 'scientific' support for his one liner is two fold, but fairly
>> straight forward.  The black spec merely prevents a small part of the
>> lens from contributing to the image, it doesn't add any light of its
>> own.  That why if you scratch a lens the best policy is to paint the
>> scratch black so that is doesn't scatter light and reduce performance.
>
>Does this apply to internal elements as well?  On one of the lenses that
>comes with the cameras I'm considering purchasing (135mm f2.8), there's
>a
>small strange looking wrinkle on the second element (HA! I'm learning
>camera
>terms ;) ) from the front.  It looks almost as though it were a 2mm
>round
>piece of peeling transparent skin.

Whilst it is true that you wont see this defect imaged on the film plane
for the reasons I mentioned earlier, and it would probably be OK if it
were black, the fact that it is transparent will cause it to act as a
source of scatter, reducing the contrast you might expect from the lens.
The problem is that you dont have a reference against which to judge the
contrast loss, so you can't tell on your own if it is affecting
performance like this.  Also, if it is right in the centre of the lens,
a 2mm diameter defect is quite significant once the lens has stopped
down and could cause a change of effective aperture.

Its difficult to tell from the description if this is a serious defect
or not - it could, for example, be coating peeling off, but that would
be extreme. 

What you might try is viewing a darkish image with direct sunlight
falling on the defect.  Then gradually stop down - making sure that you
have the 'stop-down' button on the bottom right of the lens flange
depressed.  You should see multiple reflections of the iris, getting
smaller as the lens stops down.  You might also see multiple reflections
of the defect if it is a significant scatter source.  If the defect
coincides with the iris as it is closed then it will also be an
obscurant problem causing aperture errors at high f-numbers - for every
stop you close the lens down, the exposure meter in the OM-4 should show
a doubling of the shutter speed.  You might notice that at high f-
numbers this changes to 3x or 4x for every lens stop if the defect is
severe.

Even if it is a problem, it certainly wouldn't be a showstopper  - you
already have an unbelievably good bargain!  (Just to make the guys here
who don't know Bethany go green - $500 for 3 OM-4 bodies and a clutch of
Zuiko lenses!)  After you have it, take it to a lens shop for a
quotation on a clean of just that one element.  I am sure JH would give
you a price to look at the problem and assess the damage.
-- 
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers         (replace 'nospam' with 'kennedym' when replying)

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