On Sun, Dec 30, 2001 at 02:10:09PM +0000, John A. Lind wrote:
> At 17:43 12/30/01, John Robinson wrote:
> [snipped out portion about insomnia]
> >What I tried to say was that if you set up your OM to photograph say a
> >head and shoulders portrait with a leafy background and as an expirement
> >you first
> >mounted your 24mm, set apature at f4 and placed camera 2 and 1/2 feet from
> >subject, exposed frame 1, now mount the 50mm at f4 at 5ft from subject,
> >frame 2, now mount your 100mm at f4 at 10ft from subject. frame 3,
> >woulden't all 3 frames have about the same DOF and the background have
> >about the same degree of fuzzyness?
>
> DOF? Absolutely. You have kept magnification constant on the same film
> size (format). The DOF remains the same. However, the circle of confusion
> grows faster as you move farther outside the DOF. The circle of confusion
> at "infinity" will be much larger from the 100mm lens than it will be from
> the 24mm lens. This was a revelation some time back when I started playing
> with the DOF equations. Something about a common (and incorrect)
> recommendation to "use a longer lens to reduce DOF" didn't make sense if I
> then moved to keep the subject the same size. What does change is
> perspective and the size of circles of confusion well outside the DOF! Not
> certain I would want a tight head/shoulders done with a 24mm (unless I was
> a circus clown and wanted the big red nose to look even larger)!
>
Hi John(s),
I don`t agree here. John A. Lind wrote in an older mail, that DOF changes with
(focal length)^2.
But to keep the same magnification, the distance only changes with f^1.
There is some compensation, and there might be situations where the influence of
focal lenght isn`t really much.
But I´m sure DOF changes with focallenght even when you keep the same
magnification.
Altough I don`t do the math now. Just my thoughts.
Frieder Faig
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