Henry Posner/B&H Photo-Video schrieb:
>
> At 02:39 PM 05/03/2001, you wrote:
> >This isn't entirely true. DOF depends on focal length whenever
> >you are not in macro range, IOW, if you start to approach
> >hyperfocal distance. The DOF of a 20mm and a 200mm, both at f/8
> >and at a magnification of 1:100 is extremely different.
>
> If you put a 20mm and a200mm lens on a camera, both at the same aperture
> and vary the camera-to-subject distance so the image is the same size on
> the film, depth of field will not differ.
I also thought this for a long time, because almost every text book
seems to teach this. But really, in my example above. Try a DOF
calculator with the following values:
1) c=0.025mm, f=20mm, N=8, d=2.02m
2) c=0.025mm, f=200mm, N=8, d=20.2m
where
c = acceptable circle of confusion size in mm
f = focal length in mm
N = aperture number
d = focusing distance (from front nodal point)
Both parameter sets give a magnification of 1:100, the first
with a f=20mm lens and distance d = 2.02m = 101*f, the second
with a f=200mm lens and distance d = 20.2m = 101*f.
Both times at f/8. Both setups will show the main subject at a
distance of 2.02 respectively 20.2 meters with the same magnification
on film.
DOF 1 (20mm lens): 1.01 meters to infinity = infinite
DOF 2 (200mm lens): 18.36 meters to 22.44 meters = 4.08 meters
Quite a difference ... The 20mm is already focused at its hyperfocal
distance, while the 200mm is still far from that. Notice that the DOF
in front of the subject is actually bigger for the 200mm lens, but
the rear DOF of the 20mm is infinite.
OTOH, for big magnifications, lets say 1:5, we get
3) c=0.025mm, f=20mm, N=8, d=0.12m
4) c=0.025mm, f=200mm, N=8, d=1.2m
DOF 3 (20mm lens): 0.114 meters to 0.126 meters = 12mm
DOF 4 (200mm lens): 1.194 meters to 1.206 meters = 12mm
And here we go, no difference, just like most text books
claim. But it's obviously not true for all magnifications. The shorter
lens approaches hyperfocal distance at much smaller magnifications.
Thomas Bantel
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