The point of my remark is that shorter lenses are typically faster (say, 50/1.4 vs. 200/2.8), so one could exploit this, with all the implication about the DOF.
I understand your point now. In the same example, am I right in supposing that the out of focus background will be smoother for 200/2.8 than 50/1.4?
There is a simple approximation for background blur for an infinitely distant background: and by "infinity", that is any background about ten or more times as far as the distance from the camera to the subject. So if you are five feet from your subject, "infinity" is approximately fifty or more feet beyond your subject.
Background blur = subject magnification x focal length / f-stop value.
Subject magnification is equal to the size of the object on your sensor divided by the size of the subject in real life. More magnification, such as getting closer to your subject, will increase background blur, so half the subject distance means twice the blur. Larger sensors are good for giving more background blur, as you can get closer to the subject with the same focal length, increasing the subject magnification..
Doubling the focal length, or halving the distance to the subject, or halving the f/stop will all double the background blur.
Focal length / f-stop is the measure of the lens' "entrance pupil diameter" and a larger diameter will give you more background blur on a distant background. So for your two lenses:
200 mm / 2.8 = 71.4 mm
50 mm / 1.4 = 35.7 mm
So the longer lens will give you double the background blur, but only for a suitably distant background.
For close backgrounds, you'll have to go back to the depth of field calculators, and eventually for a really close background, the lens with smallest f/stop will give you the maximum background blur.
