Doing google search for information that will give me better understanding about focal length and DOF, I found two opposite, battling views about the effect of focal length on DOF at the same aperture, IN A SITUATION WHERE THE PHOTOGRAPHER MOVES CLOSER TO OR FURTHER AWAY FROM THE SUBJECT IN ORDER TO MAINTAIN THE SIZE OF THE SUBJECT SAME IN THE FRAME.
One view says the focal length in that scenario won't have any effect in the DOF (http://luminous-landscape.com/tutorials/dof2.shtml), while another view says the focal length absolutely has an effect (http://theonlinephotographer.typepad...he-sequel.html). Our own trusted Cambridge in Colour tutorial is siding with the former (http://www.cambridgeincolour.com/tut...h-of-field.htm).
I'd like to put things in perspective here: so if you are changing the distance from the subject in order to maintain the size of the subject similar, (1) the DOF is pretty much the same across different focal lengths, (2) the distribution of the DOF changes such that longer focal length has more DOF in front, and (3) the blurred area (not the focused area) may have different characteristics such that longer focal length will afford more "enlarged" blurs. (I hope (1) through (3) are correct. I've spent quite a while to get to the bottom of these.) The second article above was incorrect, because it only ran test on an object that stood behind the subject (which would go more quickly out of focus as the focal length goes up) and it conflated the DOF with OOF (the rear subject appeared relatively sharp"er" in the wider angle, but may not have been in focus after all in the technical context of measuring DOF.)
The implication? If you are concerned about the focus (which is what the DOF concept is for), then the absolute "depth" may not change but the distribution of the DOF may matter - shoot wider if you want more focus behind the subject, and shoot longer if you want more focus in front of the subject. For this reason, longer would work better if the subject is moving towards you like in sports. If you are concerned about the BLUR (or bokeh), then shooting longer would give you more pleasing effects, not because the DOF changes but because the OOF blurs have more pleasing appearance in the numerically identical, physically larger aperture of longer focal length.
Can someone confirm that my summary of understanding is correct? I'm raising this because the information available on the internet is still confusing and conflicting, there being no final arbiter to judge who's right or wrong.