Archived posting to the Leica Users Group, 2000/11/16

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Subject: Re: [Leica] UV filter for Noctilux-measurable light loss??
From: Robert Appleby <robert.appleby@tin.it>
Date: Fri, 17 Nov 2000 07:54:45 +0100

>>>>>

Date: Thu, 16 Nov 2000 15:09:53 -0700
From: John Collier <jbcollier@home.com>
Subject: Re: [Leica] UV filter for Noctilux-measurable light loss??
Message-ID: <B639A941.B66C%jbcollier@home.com>
References: 

If the Noctilux is wide open and you put a UV filter with a mount of normal
thickness on the lens, the exposure may be reduced significantly by the
increase in the amount of vignetting (caused by the rim of the filter) not
the extra glass of the filter.

John Collier
<<<<<

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.
As for the reduction in light intensity caused by the UV filter, that's
another one I just don't buy. Any difference is imperceptible and will be
proportional to the input intensity anyway. Or as near as dammit. The idea
that it subtracts a fixed quantum of light and therefore has more effect in
low than bright light is certainly wrong.
The only negative effect I can see a filter having on any lens (let's not
get itnto exotica like vignetting caused by normal mount polarisers on 19
mm lenses here) is increased reflection = flare. But I've never seen it
myself. Most flare IMO and IME is actually 90% bad exposure.
Anyway, I doubt you'll agree.
Rob.
Robert Appleby
V. Bellentani 36
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Italy
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