Archived posting to the Leica Users Group, 2003/06/09
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At 12:20 PM +1000 6/10/03, Alastair Firkin wrote:
>Interesting thought: it was very dusty, and I did not clean the lens
>between takes. Never noted dust to do this, but with longer
>exposures etc, maybe, but then, they were not in the same place all
>the time.
>
>Hmm
>
>anyone know if dust can do this ?
>On Tuesday, Jun 10, 2003, at 11:54 Australia/Melbourne, Mike Durling wrote:
>
>>Could these be from dust on the lens surface that the light is
>>hitting? With a lens that wide they might be just out of focus.
>>
>>Mike D
These are lens design/coating parameter conditions, and dust wouldn't
be enough of a factor. Also, dirt, dust and fingerprints would only
produce 'fuzzy' diaphragm images. Under very contrasty images with
multi-element lenses, you'll always get ghost diaphragm images with
any lens, even the best.
It's mostly a matter of coatings. What's required is a coating that
will absorb all visible or recoradable wavelengths on a surface of a
given refractive index. If you where recording a single wavelength at
a fixed angle of incidence, you could eliminate reflections entirely,
but not with a wide range of wavelengths at a wide range of incident
angles. Compromises are necessary. Since the refractive index is a
constant, and the wavelengths aren't, you can't get complete
absorbtion with the present technology. If the contrast is high
enough, you'll get ghosts.
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* Henning J. Wulff
/|\ Wulff Photography & Design
/###\ mailto:henningw@archiphoto.com
|[ ]| http://www.archiphoto.com
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