Olympus-OM
[Top] [All Lists]

[OM] Re: OM eyecups & dioptric correction

Subject: [OM] Re: OM eyecups & dioptric correction
From: Pschings@xxxxxxx
Date: Fri, 23 Apr 2004 12:07:25 EDT
Cc: voxbongo@xxxxxxxxxxxxx
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

The olympus mailinglist olympus@xxxxxxxxxx
To unsubscribe: mailto:olympus-request@xxxxxxxxxx?subject=unsubscribe

To contact the list admins: mailto:olympusadmin@xxxxxxxxxx?subject="Olympus 
List Problem"
        

<Prev in Thread] Current Thread [Next in Thread>
Sponsored by Tako
Impressum | Datenschutz