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[OM] Re: focus and ? about lens filters

Subject: [OM] Re: focus and ? about lens filters
From: "C.H.Ling" <ch_photo@xxxxxxxxxxxxx>
Date: Wed, 9 Jan 2008 16:46:54 +0800
A black tape is totally different from dust, if you have hundreds of tiny 
pieces of dust (easily happen in real life if you left a filter on air for a 
few days), the contrast of the image will be reduced, it is due to the 
semi-transparency and color (usually very light color) of the dust which 
reflected the light and building of bright spots when sun light is shinning 
on the glass surface. Of course if there are only a few tiny pieces of dust 
will not affect image quality.

I don't need black tape for test, my 500mm mirror lens has a big one on the 
front element but it does not simulate the real life situation.

C.H.Ling

----- Original Message ----- 
From: "Chuck Norcutt" <chucknorcutt@xxxxxxxxxxxxxxxx>
To: <olympus@xxxxxxxxxx>
Sent: Wednesday, January 09, 2008 9:45 AM
Subject: [OM] Re: focus and ? about lens filters


> Ah, yes, the giant dust spec test.
> <http://www.chucknorcutt.com/dust%20spec%20test/index.htm>
> Both of these shots were taken with an A1 which has a front element
> approximately the size of a Zuiko 50/1.8.  One shot was taken normally.
>   The other is the same except that there is a piece of black
> electrical tape approximately 3/4" square (19mm) stuck on the front
> element.  Since the front element is about 40mm in diameter the tape was
> blocking about 29% of the glass area.
>
> The third shot is a comparison of the 1:1 pixel level detail from both
> images.  Can you tell which one is which?
>
> Yes, I did perform the test but at the request of Dr. Diffraction.  Dr.
> Focus said he didn't expect any impacts in his domain.
>
> Chuck Norcutt


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