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Re: [OM] ISO may not be accurate

Subject: Re: [OM] ISO may not be accurate
From: Ken Norton <ken@xxxxxxxxxxx>
Date: Fri, 30 Jan 2009 09:20:11 -0600
The ISO standards used to mean something.  It is my understanding that the
analysis was specifically to mid-tone (18% or 12% pick your poison).  If DXO
is using highlight rating, this is a completely different story.  Imagine if
they rated films and used the shadow point for slide comparison.  Fujichrome
Velvia, in their analysis would be ISO 8 whereas Agfachrome 50 would have
been rated at ISO 200, yet the midtone ratings are truely ISO 50.

The ONLY valid comparison is midtone accuracy. Testing to any other point is
going to be affected by the toe/shoulder characteristics as well as the
entire dynamic range of a given imaging system.  When I'm using my
flashmeter or ambient dome, it is vital that my ISO ratings are true from
one camera, film and meter to another.

My Minolta A1 is 2/3 a stop off in it's midtone ISO rating.  When I have it
set to ISO 100, it's actually 160.  The E-1 seems a little slower (ISO 80
instead of ISO 100), but it's close enough to not worry about.  But that
differential on the A1 is tough to work around.

AG (100 speed) Schnozz
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