In article , Dr. Matthew J. Cordery <cordery1@xxxxxxxx> writes
>I'll just say that my 50/1.8 has one black flec on a rear element and I
>haven't noticed it in any of my slides.
>
>
Of course you won't, as one of my old lens designers says :
"Lenses are for looking through, not loking at!"
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.
The second part is that the image on the focal plane of any object at
the stop of the lens is the fourier transforn of the object itself. The
fourier transform is an odd process where small objects are converted to
large features and large objects convert to small features. So small
objects, such as specs of dust cause large area deficiencies in the
image - not the small shadows you might expect. The lens iris - as
large an object as possible, defines the exact geometry of every point
of light in the image.
Opaque flecks on the lens wont make any difference to the image unless
they happen to lie close to an intermediate image plane, and I don't
know of any lenses in the OM series which have this sort of design -
someone may know better though!
--
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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