I may be in minority here, but I like shooting Portraits with TC's since the depth of field is reduced by the TC's multiplier... e.g. if the DoF of the 135mm is 1.4 inches for a given length, you can slap on a 1.4x TC and reduce the DoF to 1 inch (1.4/1.4=1) and melt the background even more. This assumes similar framing.
I don't think this is correct. If you put a 1.4 TC on a 135mm f/2 lens it will behave exactly like a 189mm lens at f/2.8 (135mm x 1.4 = 189mm). To get the same framing you will need to stand 1.4 times as far away from your subject. So for example, if the subject distance is 100 inches with the 135mm at f/2.8, you will get a DOF of about 2.2 inches on FF. Adding the 1.4 TC means you need to step backwards to 140 inches from your subject to get the same framing. The DOF at 140 inches with a 189 mm f/2.8 lens is - 2.2 inches.
But...with the 135mm lens you still have the option of shooting wide open at f/2.0. At 100 inches you'll now get a DOF of 1.6 inches, considerably narrower than you were able to get using the TC with the same framing.
I used the DOF calculator at http://www.outsight.com/hyperfocal.html for these calculations.
From my unscientific tests... it seems that OOF blur is always more with the TC since you need to stand further back.
Also I am not 100% sure you can use standard DoF calculators for TC calculations (Maybe I am wrong again).
You are right though, for the same framing I will need to step further back , but the DoF will not be 2.2 inches as in this case, but maybe 2.2/1.4 = 1.57inches?
For the most Part adding a 1.4x TC does add a stop of light, but no one has been able to convince me that the aperture (which determines DoF) magically closes down by adding a TC. Ye sthe system overall will lose a stop of light and "act" like a f/2.8 so that the camera can compensate for lower light transmitted, but the Aperture will stay wide open.
I need to do some tests on my own to prove or disprove this, but at the moment, I am not convinced that standard DoF calculators work correctly with TC's...
This is not scientific but I came across this... why is Kenko claiming the DoF will be lower pls. see their description: http://www.bhphotovideo.com/c/product/674567-REG/Kenko_PRO3002XDGXC_Teleplus_PRO_300_DG.html
Will anyone back me on this? I am out on a thin limb here... Neuro!!