My understanding and first hand experience is that adding a 2x tele-extender to a lens, results in a loss of 2 stops of light. For example, I can take a shot with a Canon FDn 200 / 2.8 and note the shutter speed. Then add a 2x extender, and in Aperture mode see that the light loss slows down shutter speed a couple steps. (The equivalent of an aperture being stopped down 2 stops). As we also know, a certain focal length and aperture setting gives a certain amount of DOF at a given distance. Using the same 200/2.8 as a reference, at 50 feet it gives a 2 foot DOF. (per an online DOF calculator) Now for my question: If a 2x extender is added, I know there is the light loss...BUT...does it effectively make the aperture a 5.6 as far as DOF is concerned? Would the configuration become a 400/5.6 as far as DOF is concerned...or would it be a 400 with DOF still that of a 2.8?