[Leica] Re: UV filter for Noctilux-measurable light loss??

[Byron Rakitzis <leica@rakitzis.com>]

Robert Appleby <robert.appleby@tin.it>:

>John, I'm afraid I just don't believe it. The Noctilux is a 50 mm lens, not
>a 19. I have two B&W filter rings on my 35/1.4 as a shade and they cause
>absolutely no vignetting. How can a single filter ring on a lens with a
>smaller angle of view cause or promote vignetting? Vignetting can only
>happen when the ring blocks a portion of light from the area covered by the
>lenses angle of view. At least such is my common sense view.

Robert, we can argue about the angle of view and the geometry of
mechanical vignetting but since it's very hard to pass diagrams back and
forth on the LUG, I'll tell you about a little test I did for myself 5
minutes ago:

I put a regular B+W filter (not an ultra-slim model) on my Noctilux
(last generation E60 with fixed hood) and looked through it with the
back of the M camera off and the shutter fixed open (on B).

Now it's my understanding that if you can tilt the camera back and forth
and see the corner of the film gate all the way to the edge of the front
element as you tilt, then your shade is not vignetting -- this is a test
I customarily use to see if I've got the right shade on a lens.

With the 60mm filter on I have to say that the filter ring does nick
the corner of the film gate. I don't know how much vignetting this would
cause but I would not rule it out arbitrarily as a factor.

As for angle of view: the Noctilux is a large lens. The front element
is far away from the principal point of the lens. If you draw out the
lines of the angle of view for this lens, you can see why it wouldn't
be hard for the rays to intersect a narrow shade.

Okay, so much for my half-baked optics knowledge. I'm waiting for someone
in the know to correct me.

Byron.

PS The Noctilux has so much falloff in the corners at f1.0 anyway that
I'm not sure if a little vignetting matters so much. But this is what
I saw so I thought I'd contribute...

For what it's worth, I use my lens without filters and with the fixed
shade in place at all times instead of a lenscap. Once or twice I've
done a "Rabiner special" (actually, I claim the inspiration for this)
and have shot APX25 with a B+W 061 green filter in broad daylight at f1.0.
I can't say it's my cup of tea but I probably haven't tried it often enough!