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

[John Collier <jbcollier@home.com>]

Rob,

I do not agree or disagree, I merely speculated.. Someone asked if anyone
might know a reason why a Noctilux's light transmission might be effected by
using a filter. The Noctilux is a special case as the designers used
vignetting to control some aberrations when the lens is wide open (source:
Erwin). I proposed that the addition of a normal thickness of a filter ring
might increase this designed in vignetting to the point where it would show
on the light meter. As you can see by my previous post, the glass of a
single filter has no measurable effect on the amount reaching the film plane
(discounting flare, we are talking about transmission). I do not have a
Noctilux to test (donations for the furtherance of Science will be
cheerfully accepted) but our learned friend Byron Rakitzis does. As you will
have read in his post, one can see the filter when looking through the film
gate. At no time did I mean that a filter will affect a regular lens, I was
just referring to the Noctilux.

John Collier

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