I already busted that myth, DOF is the same with or without a TC, teleconverters don't magically make your lens longer or mysteriously decrease the depth of field, they just blow up the image - it's just like cropping but keeping the same image resolution and having more optical resolution than a crop. The same shot, without adjusting zoom or repositioning, will have the same DOF with or without a teleconverter. The same shot, with zoom or position adjusted to get the framing of the subject the same, will have different DOF because you changed the conditions.
Canikon makes 200 f/2's btw, they just cost *if you have to ask you can't afford it*
on teleconverters Bob Atkins says
[link, btw]
(1) They work no "magic" with depth of field. The depth of field of a 300/4 lens plus a 2x multiplier (= 600/8) is exactly the same as the depth of field of any other 600/8 lens. For a lens system of any given focal length and aperture, the depth of field is essentially the same, whether or not a multiplier is used.
he (and most everyone else answering the question) agrees with what I'm saying (DOF DOES NOT CHANGE, DOF does not magically become shallower), but is following a common misconception on the depth of field. Do the DOF calculations and you'll see, as I've already posted, that with a 1.4x tc the resulting equivalent focal length has an equivalent DOF of 2 stops less than the lens (4 stops less for a 2x tc, making his 300/4 with 2x TC have the DOF of a 600/16). I even used Bob Atkins' own DOF calculator and got the same results. These people, they say the same thing as what I'm saying (depth of field is the same with or without TC) but don't bother to check the DOF at X focal length + Y aperture before throwing numbers at you, so I present to you a giberish function of DOF, where X is focal length and Y is aperture:
DOF(X + Y) ? DOF(X*1.4 + Y*1.4)
DOF(X + Y) = DOF(X*1.4 + Y*2) = DOF(X*2 + Y*4)
/post took a long time because I was googling and reading wikipedia, trying to disprove myself, finding people on forums who just repeat what other people say without doing any research themselves