Bob Talbot wrote:
>
>
> Using the exact formulae shows you to be correct after all (but IMO to
> a small extent and nothing like the magnitude of the popular
> statement).
> Even at 1:20 magnification the 20mm lens only give 1.5 times the DOF
> of a 500mm ... and those are extremes of F for most people.
> [Formulae snipped ...]
>
> Anyway: for various example magnifications at f22 with object distance
> adjusted for equal magnification ... compare the differences in DOF
> for a 500mm and 20 mm lens ...
>
> M 500mm 20mm
> 5:1 0.2640 0.2643
> 1:1 2.200 2.207
> 1:5 33.001 33.924
> 1:20 462.247 693.180
>
Yes, this seems to be consistent with what I get out of my little proggy.
Your listed DOF is in mm I guess. Now, I agree, that for a whole lot of
shooting the difference is negligible. But, in your example above, the
difference really explodes if we decrease magnification a little further.
At 1:50, about the magnification of a full body shot in vertical format,
the corresponding line would read:
M 500mm 20mm
1:50 2813.511mm infinite (0.43m to infinity)
For my purposes, mostly cityscapes, landscapes and such, a magnification
of 1:50 is enormously big ;-) Most of my pictures have much smaller
magnification (at the point in focus). So the difference is somewhat
relevant at least. OTOH, most of the time the focal length to use is
determined by other, stronger needs than DOF anyway. When I want to shoot
from a specific point and angle and with a specific framing, I have no
choice of focal length anymore. So I will have to live with whatever
DOF this focal length will give me and to stop down a little more if
needed (and if diffraction and the lens itself permit).
If you're into macro or portraits, a 20mm is no valid option most of
the time and your magnifications are also mostly big enough that the
different DOF of different focal lengths is negligible anyway. I'll
gladly admit a 20mm wouldn't be my favourite lens for a full body
shot, although it would allow me to have infinity in sharp focus. ;-)
Thomas Bantel
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