falta a distancia focal.
28mm a f/2.8 são diferentes de 200mm a f/2.8
Err....além do que o
Fatalit1-4 disse, a distância do sensor ao plano de foco tem influencia, e no exemplo que deste essa distância varia de foto para foto. Depois ainda falta isto:
"
CLARIFICATION: FOCAL LENGTH AND DEPTH OF FIELD
Note that
focal length has not been listed as influencing depth of field, contrary to popular belief.
Even though telephoto lenses appear to create a much shallower depth of field, this is mainly because they are often used to magnify the subject when one is unable to get closer. If the subject occupies the same fraction of the image (constant magnification) for both a telephoto and a wide angle lens, the total depth of field is virtually* constant with focal length! This would of course require you to either get much closer with a wide angle lens or much further with a telephoto lens, as demonstrated in the following chart:
Focal Length (mm) Focus Distance (m) Depth of Field (m)
10 0.5 0.482
20 1.0 0.421
50 2.5 0.406
100 5.0 0.404
200 10 0.404
400 20 0.404
Note how there is indeed a subtle change for the smallest focal lengths. This is a real effect, but is negligible compared to both aperture and focusing distance. Even though the total depth of field is virtually constant, the fraction of the depth of field which is in front of and behind the focus distance does change with focal length, as demonstrated below:
�
Distribution of the Depth of Field
Focal Length (mm) Rear Front
10 70.2 % 29.8 %
20 60.1 % 39.9 %
50 54.0 % 46.0 %
100 52.0 % 48.0 %
200 51.0 % 49.0 %
400 50.5 % 49.5 %
This exposes a limitation of the traditional DoF concept: it only accounts for the total DoF and not its distribution around the focal plane, even though both may contribute to the perception of sharpness.
Note how a wide angle lens provides a more gradually fading DoF behind the focal plane than in front, which is important for traditional landscape photographs.
Longer focal lengths may also appear to have a shallower depth of field because they enlarge the background relative to the foreground (due to their narrower angle of view). This can make an out of focus background look even more out of focus because its blur has become enlarged. However, this is another concept entirely, since depth of field only describes the sharp region of a photo — not the blurred regions.
On the other hand, when standing in the same place and focusing on a subject at the same distance, a longer focal length lens will have a shallower depth of field (even though the pictures will frame the subject entirely differently). This is more representative of everyday use, but is an effect due to higher magnification, not focal length.
*
Technical Note: We describe depth of field as being
virtually constant because there are limiting cases where this does not hold true. For focal distances resulting in high magnification, or very near the
hyperfocal distance, wide angle lenses may provide a greater DoF than telephoto lenses. On the other hand, at high magnification the traditional DoF calculation becomes inaccurate due to another factor: pupil magnification. This reduces the DoF advantage for most wide angle lenses, and increases it for telephoto and macro lenses. At the other limiting case, near the hyperfocal distance, the increase in DoF arises because the wide angle lens has a greater rear DoF, and can thus more easily attain critical sharpness at infinity.
http://www.cambridgeincolour.com/tutorials/depth-of-field.htm
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