Archived posting to the Leica Users Group, 2002/05/29

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Subject: Re: [Leica] depth of focus calculator
From: Dante Stella <dante@umich.edu>
Date: Wed, 29 May 2002 14:41:42 -0400 (EDT)

Thanks, Henning.  I found that formula.  My calculations are related to
infinity focus, so M=0.  If you start closing in, you need to take into
account the extension (via lens focal length) for effective f/stop and the
calculations become more messy.

Regards
- ------------
Dante Stella

On Wed, 29 May 2002, Henning Wulff wrote:

> >  >
> >>  Can someone point me to a formula for calculating depth of focus for a
> >>  given focal length and aperture?  This is NOT depth of field, but rather
> >>  the focus zone on and around the film plane.
> >>
> >>  Thanks
> >>  ------------
> >>  Dante Stella
> >>
> >>
> >
> >Dante,
> >
> >I think there is a formula for that in Jonathan Eastland's 'The Leica M
> >Compendium'
> >
> >sl
>
> If you are using the lens at low magnifications (ie, not macro) then
> the depth of focus (t) is t=2CN where C is the circle of confusion
> and N is the f-number. Depth of focus is independent of lens focal
> length, and is only aperture dependent.
>
> In general, t=2CN/(1+m), where m is the magnification. This is due to
> the fact that to focus closer, the lens has to be moved away from the
> film plane, causing the cone of light to narrow, which is the same as
> stopping a lens down.  So actually the first formula is always
> correct, as long as you use the effective f-number, taking into
> account lens extension.
>
> --
>     *            Henning J. Wulff
>    /|\      Wulff Photography & Design
>   /###\   mailto:henningw@archiphoto.com
>   |[ ]|     http://www.archiphoto.com
> --
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>

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