I have a Sigma 16/2.8 lens. The front section detaches. At the rear of the
front section are 22.5mm threads for a filter. There is a glass filter in there
now, UV I think. How can I obtain different filters of that size? I asked
Sigma, and they don't (anymore). The Tiffen catalog seems to include virtually
all commercial lenses, except the wide angle lenses like mine or the Zuiko
16/3.5.
In particular, a company named Lumicon makes special interference filters for
astronomy that are made to be relatively selective about certain ranges of
wavelengths. These filters pass the wavelengths emitted by nebulae (H-beta,
OIII, H-alpha). Effectively they screen out light pollution. The response of
such is filter is sensitive to incident angle, so in a lens like the sigma,
I'm not sure it would perform well. Any guesses?
BTW I think most wide angle lenses that have internal filters require a filter
to be in place in order to focus correctly. John Hermanson says he has seen
this with his Zuiko 16/3.5. This is true even though the filter is a piece of
flat glass. Incoming light rays hit the filter at differing angles and emerge
at affected angles, as a function of the optical density of the filter. The
Sigma will not focus at infinity without a filter.
Matt BenDaniel
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