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[OM] Zuiko lenses effective speed, WAS: 28/2.8

Subject: [OM] Zuiko lenses effective speed, WAS: 28/2.8
From: "Ing. Radovan Faltus" <faltusr@xxxxxxxxx>
Date: Thu, 13 Jan 2005 00:31:35 +0100
So the conclusion is: The effective speed of
Zuiko lenses is half stop lower than rated.

Can anybody refute this with calibrated lightbox measuring?

EV4 from light box (if possible to set such low light level),
f1.4 setting on Zuiko 50mm/f1.4, 6 ASA setting on OM-4Ti body,
2 sec. resulting shutter speed measured with digital stopwatches
confirms the effective shutter speed is f1.4, 3 sec. shutter speed says
effective shutter speed is f1.7. (half stop lower than rated). Or any
effective lens speed can calculated from measured shutter speed with
accurancy given by sum of lightbox accurancy, time measurement accurancy
and camera lightmeter + integrator accurancy. I guess the total experiment 
accurancy
can be better than 0.2 stops (absolute value).

I don't know the light range of lightboxes but for f2.8 lens
we need EV6 under conditions above and I beleive its within the range, am I 
right?

Hope I counted right.

Radek Faltus


----- Original Message ----- 

> I have noticed a similar issue when testing bodies for long exposure 
> capability.
> Basically, set up a camera on a tripod in a dimly lit room pointing at a 
> blank wall
> about 2m away.  Unexposed film in the film gate, time the actual exposure 
> with a stop
> watch.  I found using a 50/1.4 initially that within a certain lighting 
> range the timed
> exposure always doubled as I stopped down (expected result), except for 
> f1.4 to f2
> where the time was less than double (about 1.5 from memory, details are 
> filed
> somewhere @home).  This was confirmed using several OM bodies (OM-2N, OM-
> 4).  I also tested another lens (can't remember which, possibly 100/2.8) 
> with the same
> result.
>
>
> Wayne Harridge


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