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Playing with my DoF/FoV calculator, I've come to realize that what depth of field really depends on is field of view, sensor size and lens aperture: lens lenght and distance to subject don't matter, as long as the field of view, defined as "height and width of image at focus distance", remains constant.
Let's see an example.
Suppose I want to take a head-and-shoulders shot of some friend. He's going to be in focus, and picture height at that distance has to be around 64cm (2 feet).
Besides that, I want my friend to be in focus, over a very-out-of-focus background. I want my depth of field to be around 22cm (9 inches).
My camera is a 1.6x-crop canon, my target aspect ratio is 1.77:1 (16:9), and I have access to any lens I want to use (lucky hypothetical-me).
What I've found is this: no matter which lens I use, in this example I want to set aperture to f/2.8. Choosing different lenses will mean I have to get closer or farther from the subject, and perspective will change, but if I want DoF to be around 22cm, aperture has to be f/2.8.
Any of the following combinations will provide me with an image that is 64cm tall in the focus plane, and has 22cm of DoF:
17 mm at 0.85 m, set at f/2.8
24 mm at 1.20 m, set at f/2.8
28 mm at 1.40 m, set at f/2.8
35 mm at 1.75 m, set at f/2.8
50 mm at 2.50 m, set at f/2.8
80 mm at 2.80 m, set at f/2.8
120 mm at 6.0 m, set at f/2.8
180 mm at 9.0 m, set at f/2.8
DoF is not exactly constant, but at f/2.8 the DoF difference between the 17mm lens and the 180mm lens is less than 4mm, or 2%.
What does change, a lot, is perspective: the size of stuff on your background will change dramatically if you go from 17mm to 180mm. And that affects bokeh big time: at 17mm, you'll see a whole building behind your subject, and it will be out of focus but you'll know it's a building, and recognize its features; at 180mm lens, with the background at the same distance, all you see behind your subject is a small patch of wall, so it will not be just out of focus, it will be blurred beyond recognition. So depth of field is the same, but the background will be a lot softer on longer lenses.
You probably knew all this already, but I didn't. I'm happy I now know.
And so, here's my simplified DoF/FoV calculator.