I have and regularly use three 100mm lenses: 100/2 Zuiko with 35mm, 100/2.8
Zeiss Planar with 6x9cm Graflex XL, and 100/5.6 Schneider APO Symmar with 6x9cm
Crown Graphic. At any given aperture, I am certain, and my experience bolsters
my certainty, that all three have exactly the same depth of field, or depth of
focus, whichever you prefer to call it. (Don't confuse me with circles of
confusion.)
However, it might seem that the two 6x9 lenses have a greater depth of field,
but that's only because a 6x9 negative or transparency doesn't have to be
enlarged nearly as much as a 35mm negative or transparency to make an equal
size print. But if we snip out a 24x36mm section from the center of the 6x9cm
negative or tranny and blow it up to the same degree as the 35mm, it will look
exactly the same in terms of depth of field. The more you magnify the slightly
out of focus parts, of course, the more out of focus they look.
Wait a minute! I think I used this same "snipping out a piece" analogy a while
back in a discussion about focal length and perspective. But what the hell!
It applies here too. Focal length is focal length, and aperture has the same
relation to focal length no matter what the focal length happens to be: the
hole in the diaphragm has a diameter 1/x the focal length of the lens, where x
equals the f/number. (Don't confuse me with T-stops.)
I may be wrong, but I'm not really uncertain. However, I await correction.
Old as I am, I keep learning new stuff and unlearning old stuff I throught I
knew.
Walt
--
"Anything more than 500 yards from
the car just isn't photogenic." --
Edward Weston
-------------- Original message ----------------------
From: Fernando Gonzalez Gentile <fgnzalez@xxxxxxxxxxxxx>
>
>
> > From: "Piers Hemy" <piers@xxxxxxxx>
> > Reply-To: olympus@xxxxxxxxxx
> > Date: Sun, 19 Dec 2004 13:23:10 -0000
> > To: <olympus@xxxxxxxxxx>
> > Subject: [OM] Re: New member with question about 100/F2
> >
> > All 100mm lenses have the
> > same DoF. It depends how D0F is defined -
> [snip]
> > (and it is
> > also showing the shortcomings of a principle (DoF) defined in the 1920s (if
> > I remember correctly) .
>
> Could you elaborate or point towards more web published material on this
> subject, Piers?
> I supposed that the wider the aperture, the shallower the DOF; the longer
> the FL, the shallower the DOF.
> Wrong, isn't it?
>
> Thanks,
>
> Fernando.
>
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