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[OM] Arcane Question No. 1: 50/1.8 as loupe

Subject: [OM] Arcane Question No. 1: 50/1.8 as loupe
From: Chuck Norcutt <norcutt@xxxxxxx>
Date: Mon, 10 May 1999 06:38:08 -0400
Joel Wilcox wrote:

Oly Shooters:

Lots of us are using an extra 50/1.8 as a loupe.  Anybody have any idea
approximately what magnification ratio this lens would have when used as
a loupe?

---------------------------------------------------------

To the best of my recollection a 1" focal length (25.4mm) is considered
a 10X magnifier.  Not sure why but my guess is that it's the ratio of
the focal length to 10" which is considered normal reading distance.

So, used as a loupe, you have (25.4 x 10)/ 50 = 5.08... call it 5X.  

If "normal reading distance" is truly part of the equation then we
shouldn't try to be terribly precise about the computation.  Therefore,
rounded to the nearest full unit we get:

35mm = 7X, 28mm = 9X, 24mm = 11X.

Shorter than this I would have to shift to my telescope eyepieces which
go down to 16mm = 16X, 12mm = 21X and 8mm = 32X. 

Chuck Norcutt

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