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Re: [OM] OM macro / closeups vs medium format / TLR

Subject: Re: [OM] OM macro / closeups vs medium format / TLR
From: "John A. Lind" <jlind@xxxxxxxxxxxx>
Date: Sun, 30 Dec 2001 08:29:21 +0000
At 11:48 12/30/01, John Robinson wrote:

--- Tom@xxxxxxxxxxxxx wrote:
> On Friday, December 28, 2001 at 13:20, Brian Swale
> <olympus@xxxxxxxxxxxxxxx>
> wrote re "[OM] OM macro / closeups vs medium "
> saying:
> ...
> > An old friend of mine who used Hasselblad used to
> laugh at me trying to
> > maximise DOF, and said "Why don't you get a Hassy,
> stand back with a normal
> > lens, get all the DOF you want, and enlarge the
> part you really wanted to
> > photograph in the first place.".
>
> She/He's fooling himself. You get the same depth of
> field with a given
> focal length, aperture, and image enlargement..
>
> tOM
  DOF's really the same for a given aperture and
Image/Object ratio regardless of focal length.  John Robison

And . . .
If I switch from the 50mm on the OM to the 80mm on the 645, and frame the subject approximately the same at the same distance I must stop down by about a half-stop to get the *same* DOF. Why? If you work out all the DOF equations the focal length is squared; the maximum diameter circle of confusion is not; both grow at the same rate with increase in film size. For the same angle of view, aperture and focus distance (magnification), an increase in film size shrinks the DOF!

-- John


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