Archived posting to the Leica Users Group, 1998/04/20
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]
On Sun, 19 Apr 1998, Nick Hunter wrote:
> Thibault Collin wrote:
>
> >Of course, but if we have free time this summer, we could try to calibrate
> >it with other lenses which will give other apertures!! It would be a tough
> >job!
>
> Thib:
>
> What an interesting idea! I don't know if I'll get around to doing anything
> about it, but I think the theoretical answer would be as follows: Two stops
> slower for twice the focal length. Therefore (approx.):
>
> 50mm= f:5.6/f:16
>
> 75mm= f:8/f:22
>
> 90mm= f:11/f:32
>
> 135mm= f:16/f:45
>
> I stand ready to be corrected by LUG optical wizards. Remember I did say
> approximate, and DOF is a tricky issue which can (and has) filled whole
> books.
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 ).