Archived posting to the Leica Users Group, 1999/12/01

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Subject: Re: [Leica] RE: 75mm f1.4 DOF
From: Eric Welch <>
Date: Wed, 01 Dec 1999 07:13:47 -0800

At 11:03 PM 11/30/1999 -0800, Jacques Bilinski wrote:
>"  By the "same magnification" I assume you mean the same
>magnification in the plane of sharpest focus. Say the 50mm is acceptably
>sharp X feet closer than the plane of sharpest focus, and Y feet beyond the
>plane of acceptable focus. The 100 mm would not have the same magnification
>as the 50mm at these 2 distances, so it does seem reasonable to me that
>maybe what matters is some king of 'range in magnification' rather than
>simply a set distance in front and behind the plane of sharpest focus.

No, by magnification, what I mean is that a 50mm showing the subject at the 
plane of focus of 1/2 life size is going to be X inches from the subject. A 
100mm macro lens at 1/2 life size is going to be 2X the distance. At the 
same aperture, the depth of field will be exactly the same between the two 
lenses. I have talked to many folks who know a lot more about optics than I 
that confirm this fact. To say that it is an approximation shows a lack of 
understanding of the nature of optics. Of course, focal length varies 
between two lenses, but not by that much.

Eric Welch
Carlsbad, CA

"There is nothing so stupid as an educated man, if you get off the thing
that he was educated in. "
      --Will Rogers