Re: [Leica] WAS; UV filter and 21mm now 15 mm!

["GC Capital & Finance SA" <swissfin@worldcom.ch>]

Dave,

The degree of skylight polarization depends on the angle viz-ā-viz the sun.
Light coming from a 90 degree angle from the sun is polarized maximally and
as the angle photographer-sun/incoming light decreases, the level of
polarization moves to the minimum level (0 degree). The blue of the sky will
become darker (specially on slidefilm) as lightrays come in from angles
increasingly closer to the 90 degree maximum point.

The effect is not very visible with tele lenses as their angle of view is
small. With a 15mm Voigtländer (Cosina) Heliar, with an extreme wide angle
of 110 degrees, it is almost impossible to produce skies with even density
as light comes in from many different angles within the 110 degrees,
producing many different density levels. The fact that you have a problem in
bright sunlight while the problem is gone under overcast skies seems to
indicate that the various levels of polarization of the sky cause the
darkening, specially visible on high-contrast slide film.

Gerard Captijn.




of
- ----- Original Message -----
From: <drodgers@nextlink.com>
To: <leica-users@mejac.palo-alto.ca.us>
Sent: jeudi, 13. juillet 2000 21:47
Subject: Re: [Leica] WAS; UV filter and 21mm now 15 mm!


>
> Robert
>
> >>Or even a $450 Cosina 15mm Super Wide Heliar, which has no filter thread
> <<
>
> Somewhat related to the topic of filters (UV and otherwise) and the 15mm
> Heliar, yesterday evening I noticed some really strange illumination
issues
> in some negatives I just developed. I scanned about 8 rolls of film and
the
> issue existed in a couple of dozen images.  At first I thought I'd taken
> the faulty shots with my 28/2.8 Elmarit R and a filter caused vignetting.
I
> use a 55mm B+W polorizer with step up ring. I've never had problems with
> that in the past. After closer examination I realized the lens used was a
> Heliar and the camera was my M6. It sort of looked like shutter bounce,
but
> I ruled that out.
>
> The issue/problem is a very noticable darkening on the right and left
> borders of the frame. It only occurred in photographs taken in bright
> sunlight. Nearly identical photographs taken the day before under overcast
> skies didn't show the problem. Perhaps it had to do with the aperture
used.
> I'm at a loss to understand the problem, but and I plan to get to the
> bottom of the problem. Anyone else out there experience unusual light
> fall-off with the 15mm Heliar?
>
> Dave
>
> PS I'm not ready to blame the Heliar, yet. I've used it a great deal and
> I've never had problems before. I still need to do some research.
>