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[OM] Re: dust on lenses

Subject: [OM] Re: dust on lenses
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxx>
Date: Mon, 02 Oct 2006 16:49:09 -0400
See the secondary mirror in the front of this OM 500mm f/8 reflex lens?
<http://olympus.dementia.org/eSIF/om-sif/lensgroup/500mmf8.htm>
It acts just like dust.  It blocks light from passing through it and 
causes light at the edges to be diffracted around it.  Diffraction is a 
bending of the light rays and reduces contrast by scattering the light 
around and diverting it a little bit from its intended path.  But 
diffraction occurs at all edges contained in the light path.  That 
includes the diaphragm blades and even the edges of the lens mounting 
cells.  Your itty-bitty dust bits just don't count.

When you get enough dust in your lens that you significantly exceed the 
area blocked by the secondary mirror on this 500mm reflex then you can 
start to worry.  Until then, as Moose says:  "Don't worry, take 
pictures."  :-)

Chuck Norcutt


Moose wrote:
> Tim Randles wrote:
> 
>>I'm pooched...I have dust inside my lenses and on my viewfinder, I have tried 
>>everything.. any suggestions..can I fix this myself or do I have to take it 
>>in to get cleaned? How do I prevent this from happening again.. I use this 
>>thing in the field all day long..
>>  
> 
> First, dust on the mirror has absolutely zero effect on images. The 
> mirror is up out of the light path when the picture is taken. If you can 
> see any effect from it  in the viewfinder, which I doubt, you can clean 
> it off, but probably it doesn't matter at all.
> 
> Second, dust in lenses is essentially unavoidable except by not using 
> the lens. The good news is that it takes a LOT of dust to have any 
> effect on the images. IF, like many new to this stuff, you have peered 
> through your lens at a bright light and seen a few bits of dist here and 
> there, the solution is simple, stop peering through the lens except 
> through the viewfinder of the camera.
> 
> Don't worry, take pictures.
> 
> Moose
> 
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