Using a longer focal length does not make the background "softer"; it changes the perspective of the background.[a href=\"index.php?act=findpost&pid=188360\"][{POST_SNAPBACK}][/a]
Tony, this statement is wrong. Actually, using a longer focal length (and backing up accordingly) will make the background softer
and change perspective, i. e. make the background objects appear larger. With regard to relative sharpness of the background objects, these two effects cancel each other out, i. e. relative sharpness remains the same (more or less). But DOF does change, because DOF is about absolute sharpness, not relative. You, and Michael R., mustn't confuse these two things.
If with the shorter focal length an object in the background that is just outside DOF can be recognized despite the blur, then with a longer focal length (from an accordingly longer distance) it will be recognizable, too. It will be rendered more blurred but also bigger, i. e. relative sharpness will be the same. But this fact doesn't entitle you to say DOF was the same! DOF is
not relative sharpness. DOF is defined via the diameter of the circles of confusion, and those diameters will increase due to the larger magnification of the background objects so DOF will decrease.
What is important to understand for the purposes of this thread is that longer focal lengths have the same DOF as shorter focal lengths at the same aperture and magnification.[a href=\"index.php?act=findpost&pid=188360\"][{POST_SNAPBACK}][/a]
This statement is wrong again, generally! It's true only as a first-order approximation so it will hold only in those cases where DOF is small in relation to focusing distance, i. e. when comparing long lenses to longer lenses, or in the macro range. But when comparing a, say, 28 mm lens to a 100 mm lens (for 35-mm format) then the difference in DOF, caused by the second-order effect of non-constant magnifications across the range of DOF, will become obvious.
Increasing focal length will not decrease DOF ...[a href=\"index.php?act=findpost&pid=188360\"][{POST_SNAPBACK}][/a]
It will---particularly when starting out with a short focal length. Less so when starting out with a lens that already is fairly long.
Let's just use a depth-of-field calculator (there are several floating around on the Internet) and compute a few examples:
35-mm format, COC = 0.030 mm17 mm lens, f/11, focus distance 0.61 m:
DOF is
1.35 m, from 0.37 m to 1.72 m (or -0.24 m to +1.11 m);
28 mm lens, f/11, focus distance 1.00 m:
DOF is
0.92 m, from 0.72 m to 1.64 m (or -0.28 m to +0.64 m);
50 mm lens, f/11, focus distance 1.79 m:
DOF is
0.81 m, from 1.47 m to 2.28 m (or -0.32 m to +0.49 m);
100 mm lens, f/11, focus distance 3.57 m:
DOF is
0.78 m, from 3.22 m to 4.01 m (or -0.35 m to +0.44 m);
300 mm lens, f/11, focus distance 10.71 m:
DOF is
0.77 m, from 10.33 m to 11.10 m (or -0.38 m to +0.39 m);
1,000 mm lens, f/11, focus distance 35.71 m:
DOF is
0.77 m, from 35.33 m to 36.10 m (or -0.38 m to +0.39 m).
The focus distances are adjusted to the focal lengths so at the plane of focus the magnification will be always the same. As you can see, DOF decreases rapidly in the wide-angle range (high influence of the 2nd-order effect) und then slower in the telephoto range (2nd-order effect hardly shows up here). Between 300 mm and 1,000 mm the change is only millimeters---or fractions thereof. So for practical intents and purposes, we can ignore the 2nd-order effect for lenses longer than portrait telephoto ... or maybe even for anything longer than a standard lens if we're not too finicky. But we definitely cannot for wide-angle lenses.
Contrary to intuition, foreground DOF
increases with longer focal lengths but background DOF decreases, and at a faster rate so the sum of the two also decreases. (By the way, those who still believe DOF always extends 1/3rd towards the camera and 2/3rds into the background should give this (wrong) notion a thought or two.)
Relative sharpness of background objects will be the same, more or less, across all focal lengths (i. e. rendered more blurred but also bigger with longer lenses) ... but that's not what DOF is about.
-- Olaf
