I'm considering getting a teleconverter for my Sigma 70-200 2.8 USM. Two are offered, the 1.4 and 2.0. With the 1.4, the max aperture will be f4 while it'll be f5.6 with the 2.0 TC. I have two questions in mind from there. The first is regarding the relative aperture. Say, with the 2.0 TC, the relative aperture will be 5.6, but will the DOF behaviour of the lens be that of the "normal" lens at 5.6, or is it still that of the 2.8 max aperture of the front element ? I understand that 5.6 describes more the loss of light than the real lens aperture. Second question is about quality. Anyone here has a clear statement about the use of Sigma TCs ? I still have a Nikkor 70-300 ED (4.5-5.6 I think). How would it compare to using the the 1.4 TC on my Sigma 70-200 at max lenght (300 vs 280) ?