Archived posting to the Leica Users Group, 1998/04/21

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Subject: Re: [Leica] DOF indicator
From: Nick Hunter <nhunter1@mindspring.com>
Date: Tue, 21 Apr 1998 23:54:34 +0100
 
Duncan:

Oops! I screwed up on this one...sorry. Looks like the DOF notches will be
of little use for longer lenses. However, 35mm lenses would work out to
f:2.8/f:8 (I think). That might be helpful.

Nick

>DOF is proportional to aperature i.e. F4 has 1/2 the depth of field of F8/
>
>DOF is proportional to the inverse square of the focal length. i.e.
>100mm is 2 times the focal length of a 50mm, therefore has only 1/4 the DOF.
>
>
>Based on this
>
>   50mm = f:5.6/F16
>
>   75mm = F:12.5 ( say approx F11 )/F:36 ( say approx F:32 )
>
>   90mm = F:18 ( say approx F16 )/F:51 ( say approx F:45 )
>
>  135mm = F:40 ( say approx F32 + 1/2 )/F116 ( say approx F90 + 1/2 )
>
>
>    Duncan ( who has also double checked his assumptions on a depth of filed
>             chart ).