Archived posting to the Leica Users Group, 2001/11/15

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Subject: Re: [Leica] Responsiveness of M6 meter at f/1.4 to f/2
From: John Collier <jbcollier@powersurfr.com>
Date: Thu, 15 Nov 2001 12:00:43 -0700

It is not that the Noctilux and Summilux are not true f/1 or f/1.4 lenses
but that, under certain lighting conditions, the inherent wide-open
vignetting will affect the metering area and you will not get a strictly
geometrical shutter/aperture relationship happening. The meter is not being
fooled if you do not follow it, it will underexpose. I have dug up the scans
from the last time this came up and if someone would like them, just let me
know.

John Collier

> From: Nathan Wajsman <wajsman@webshuttle.ch>
> 
> I had not noticed it in Osterloh's book before, but I now looked it up (p. 43,
> German edition) and sure enough, he says that, in effect, the actual maximum
> aperture of a Noctilux is not f1 but somewhere around f1.1-f1.2. I do not own
> one of those beasts anymore, but when I did, I never noticed the effect, and I
> cannot produce it with my Summilux. perhaps Ray should have his camera/lens
> combo checked after all.
> 
> Nathan
> 
> John Collier wrote:
> 
>> It is normal. The inherent vignetting of any optical system wide open causes
>> this effect. It is most noticeable with the Noctilux. See Osterloh's Leica M
>> book for a well illustrated explanation.
>> 
>> John Collier
>> 
>> PS: Try your local library for the book. I might be persuaded to scan the
>> relevant sections out of my copy if you cannot locate it.
>> 
>>> From: Ray Moth <ray_moth@yahoo.com>
>>> 
>>> I've just noticed a peculiarity in the behaviour of the light meter in
>>> my M6 TTL, using a 50mm Summilux-M lens. If I adjust the aperture from
>>> f/1.4 to f/2 or vice versa, the meter doesn't register any change,
>>> whereas a change in shutter speed does register.
>>> 
>>> For example, if the meter indicates that f/1.4 at 1/500 sec. will give
>>> the correct exposure, then I would expect f/2 at 1/250 sec. would give
>>> the same indication but it doesn't; it shows over-exposure, as if I'd
>>> selected the slower shutter speed but left the lens aperture at f/1.4.
>>> Conversely, if I started at f/2 and 1/250, a change to f/1.4 and 1/500
>>> would indicate an under-exposure. The lens aperture selector seems to
>>> function properly. Has anyone else experienced this problem?
>>>

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Replies: Reply from David Rodgers <drodgers@swiftnet.com> (Re: [Leica] Responsiveness of M6 meter at f/1.4 to f/2)
Reply from Guy Bennett <gbennett@lainet.com> (Re: [Leica] Responsiveness of M6 meter at f/1.4 to f/2)
Reply from Henning Wulff <henningw@archiphoto.com> (Re: [Leica] Responsiveness of M6 meter at f/1.4 to f/2)