In a message dated 4/23/2004 11:13:59 AM Eastern Daylight Time,
voxbongo@xxxxxxxxxxxxx writes:
> There has always been some confusion about how Olympus's numbering
> system for correction lenses worked--i.e. the "nominal" number was
> not the actual, measured, correction of the lens. For example,
> there's the mystery of what good a nominal 0 (zero) correction lens
> would be?
> I have a new working hypothesis, and I'd be interested to hear
> whether other people's experience backs this up. I think you're
> supposed to choose the nominal number equal to the correction your
> eye requires to focus AT INFINITY.
> Remember that without any additions, the OM finder appears to be at
> 4-5 feet. (Someone out there can remind me of the exact numbers.) And
> that's why there's a discrepancy.
> For my mild nearsightedness, 4-5 feet is just a *little* beyond where
> I can focus without glasses. My prescription for focusing at infinity
> is about -1.0. And I find Olympus's nominal -1 correction works
> perfectly for me. But what I suspect is that the actual correction is
> something more like -0.5 diopter.
> Meanwhile if you're farsighted, but can focus at infinity without
> glasses, you'd choose the 0 (zero) lens. I suspect that is is
> actually a weak positive lens (maybe +0.5 diopter?) which makes the
> finder's apparent focus distance equal infinity, not 4-5 feet.
> Olympus's published descriptions of the correction lenses say that
> -1, -2 etc are intended for myopia, while 0, +1, +2 are intended for
> hypermetropia (far-sightedness). So that does jibe with the theory
> that the nominal zero lens is actually a mild positive one.
There's a good description of how the dioptric correction lenses work in the
FAQ:
http://brashear.phys.appstate.edu/lhawkins/photo/olympus.faq.html#ques_P4
Bottom line is the Olympus dioptric correction lenses are marked with the
*effective* diopter adjustment when included in the whole viewfinder system.
Since the OM finders (OM-3/4 excluded) start off with a net of about -0.7
diopters, the "0" correction lens measures at about +0.7 diopters.
I'm no optician, but it makes sense to me. Personally, I feel lucky to be
able to use and wear contact lenses when I'm shooting.
Paul Schings
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