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