Sorry if this has been answered before
The activity in this forum would stall if people would get used to using the search option, so ... :-p
but I wonder if someone could explain to me how to derive the relation between DoF and f number.
Smart question, because you noticed that the f-number is important, and not the physical aperture (I only learned this recently)
Say that I have determined that using a 50mm lens and f/6.3 I have enough of the picture in focus to do a family shot (let's not debate this, it's just an example). If I move twice as far away from the family and use a 100mm lens (to achieve the same framing of the subject), can I still use f/6.3 or do I need a different setting?
To just find out the result, you can simply use your trusty dof calculator like here: ... http://www.dofmaster.com/dofjs.html
... for your example:
Family @5m with 50mm = standard dof 4.4m (in front 1.37 m (31%) / behind 3.03 m (69%))
Family @10m with 100mm = standard dof 3.91m (in front 1.59 m (41%) / behind 2.32m (59%))
Which proves the rule of thumb that with shorter focal length the dof distribution is 1/3:2/3, with longer lenses it gets nearer to 50/50. And it demonstrates why small f-numbers with long lenses gets tricky, the dof is becoming very thin - for example with my 70-300/f5.6 I often need to step up to get the subject in focus, no matter how bokehlicious it would look If I had a f2.8.
Beyond a simple yes or now, can someone explain the basic optics here (as at least a quasi-math rule of thumb). I would even be happy with a pointer to a detailed analysis.
I'll let the CR heavyweights post the maths behind it, but from a simple users' pov you should read a thread like here why dof is just the subjective(!) "acceptable" sharpness and depends so much on print/view size: http://www.canonrumors.com/forum/index.php?topic=15884.msg293901#msg293901