I recall an article in an astronomy magazine some time ago that dirt on a
lens or telescope objective mirror will not affect image quality noticably
until it covers an area equal to a blob one-tenth of the lens diameter
across. So, for a 50/1.8 lens, diameter of the glass is 28mm, so you could
have a lump of muck 2.8mm wide stuck to the lens without it affecting image
quality.
I don't recall their criteria for determining what loss of image quality was
acceptable. If I can dig out the article I'll provide more info.
Roger
-----Original Message-----
From: Daniel J. Mitchell <DanielMitchell@xxxxxxxxxxxxx>
>
> How can this be? If something's right on the surface of the lens
presumably
>it's not going to be in focus, so I get the basic idea -- but if so, why
>does anyone bother cleaning lenses? How dirty can a lens get before the
>change is visible? (well, I guess this depends on how large you want to
make
>the final image, film quality, etc, etc, but in general?)
>
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