I'm pretty happy with the sharpness of the 300+TC, but haven't really tried it wide open. Guess I should go do that. I've been practicing on lizards in the backyard but not wide open. DOF even stopped down is very shallow.I would think that is going to depend on how sharp the lenses (+TC re the 300) are at infinity. Does the 300+TC need to be stopped down to get best sharpness? How does wide open on the 300+TC compare to the mirror?