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Re: [OM] urban legends #2435 & #3528

Subject: Re: [OM] urban legends #2435 & #3528
From: frieder.faig@xxxxxxxxxxxxxxxxxxxxxxxxx
Date: Sat, 20 Jul 2002 14:32:45 +0200
On Sat, Jul 20, 2002 at 12:59:26PM +0200, Carlos J. Santisteban Salinas wrote:
> I might be wrong, but I think this is the reason of that behaviour:
> 
> The OM-1, like most other cameras, does light metering off the focusing
> screen. If you look closely at a FS, you'll see a lot of concentric circles
> -- I believe it's some kind of 'Fresnel lens'.
> 
> So the screen is a not-so-simple optical system, which may be sensitive to
> the angle of light rays -- and sure it is. You may check it by looking thru
> an OM-1 viewfinder and then turning half-way the mirror lock-up knob: the
> image not only  gets defocused and distorted, but also darkens noticeably.
> However, if you only turn the knob slightly, you get the focus shift but no
> darkening.
> 
> Since in an SLR the lens can't be too close to the film (less than 35-40
> mm) because of the room needed for the mirror, the angle of light rays is
> "never" too steep (max. about 30 dgrees, which may be the limit that FS's
> are designed for). However, shifted lenses may _increase_ this angle, so
> there's darkening and subsequent metering error.
> 
> On the other hand, cameras like the OM-2 do light metering off the
> curtain/film surface -- much simpler optically. They shouldn't be affected
> by incidence angle, so Auto exposure in the OM-2 is OK. I'm not sure about
> OM-2S/P, OM-3 & OM-4, but they don't meter of the focusing screen, so maybe
> they aren't affected, even in manual.
> 
> Let's hope I'm not too wrong ;-)

Good explanation. This are my conclusions too. See my older posting.
OM-2S/P, OM-3 & OM-4 are affected too, because the semi-mirror-element
also uses something like a fresnell element.

Frieder Faig 

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