It was my impression that the DOF depended not on f/stop, but rather on the
ratio of the width of the effective aperture to the distance of the object from
the lens. If that is so, shorter focal length lenses will give give more DOF at
the same marked f/stop at any distance. Witness the extended DOF of wide angle
lenses compared to telephotos.
Tom
> Date: Fri, 30 Apr 1999 00:29:08 +0200
> From: "Hans van Veluwen" <mail@xxxxxxxxxxxxxxxx>
> Subject: Re: [OM] RE: macro questions (and a few more for the experts!)
>
> Jan Steinman wrote:
>
> : In particular, the girth of the 90/2 makes it difficult to
> : operate the mount release and the depth-of-field preview. Also, the
> 50/3.5 : takes cheaper/lighter/smaller 49mm filters, and fits into the
> slide : duplicator, and has more depth-of-field.
>
> and:
>
> : wide angle lenses and short extension tubes -- the 21/2 and 7mm of
> : extension make you feel you're actually "inside" the macro world, and :
> there's much greater depth-of-field, as well.
>
> I don't think this is correct. DOF depends on magnification ratio and
> aperture. With non-macro setups, the focal length and distance determine
> the magnification ratio and therefore DOF. With macro setups (macro lenses
> with built-in extension tubes like the 50 or 90 or bellows mounted macro
> lenses), the extension and focal length determine the magnification ratio
> and thus DOF. The larger the extension and the smaller the focal length,
> the larger the magnification ratio, and the smaller the DOF. Since the
> 50/2, 50/3.5 and 90/2 are give a magnification ratio of 1:2 at their
> closest focusing distance, their DOF should be identical. And an 21mm on
> an extension tube probably exceeds lifesize magnifications (?) and should
> have a very narrow DOF.
>
> Hans
------------------
From: Tom Trottier, President, ACT Productions Inc.
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