I've been thinking of a way to add more magnification to a Vivitar 100 2.8 in canon FD mount. Not wanting to shorten the working distance with tubes and diopters, I'm looking into a 2x teleconverter. I understand I'll lose 2 stops in light, but I figured its not an issue considering that I need a large DOF anyways and I'll be useing flash. Does anyone have any experience using the teleconverter with a macro lens? How are the results? Any brand recommendations?