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[OM] ( OM ) Dust and other foreign bodies in lenses - follow-up.

Subject: [OM] ( OM ) Dust and other foreign bodies in lenses - follow-up.
From: "Brian Swale" <bj@xxxxxxxxxxxxxxx>
Date: Sat, 15 Jun 2002 08:52:58 +1200
Hi folk,

It's not just zoom lenses (push-pull zoom or not) that behave this way.

Many / most macro lenses have a lot of helicoid travel and therefore there is 
a lot of air displacement during use. Lenses that focus close may in some 
instances have the same larger movements (eg 90, 100 f/2).

Operate your lenses in a quiet place and listen. Feel the resistance to the 
focussing movement as it forces air past the seals and the mounting flange.

Also, some lenses have a yawning gulf at the back when opened up. I 
noticed this first when swapping my Zuiko 100 f/2 for another when doing 
closeup shots in the field among leaves, pine needles and what have you. I 
noticed a little grass seed had flicked into the cavity and was (was!) stuck on 
the grease on the helicoid.

Just imagine what happens when you are changing lenses in a desert with  
fine sand and there's a wind blowing (usual ?) Or a sandy beach. Or ... ...  
almost any place. Or focussing a lens in a sandy desert - or construction 
site etc etc.

The point Moose made to Jochen about the effect of the little hair on  photo 
quality is very valid. Imperfections, especially dark-coloured ones and near 
the front, have little or no perceptible effect on the picture quality.

However, in my opinion, beware dirt etc on the rear element; and any kind of 
light-coloured haze which will bounce light around, reducing contrast and 
detail .

Brian

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