Here's the situation...Here's the situation... When a teleconverter is used on a lens AT INFINITY FOCUS - the converter multiplies the focal length by a factor the the converter (a 2X converter will turn a 100mm lens into a 200mm lens with a 2 stop loss. When the converter is put behind a lens focused at minimum focusing distance (say, behind a 100mm Macro), it doubles the MAGNIFICATION - NOT the focal length. Look at it this way (please test it, if in doubt). Take a 100mm lens focused at infinity - then put a 2x behind it - you get 200mm focused at infinity. Take the same 100mm lens and put 50mm extension behind it, you get 1/2 life size on the neg.... Now, in this order, put the 100mm, then the 2x, then the 50mm extension - and you get 1/4X on the neg - but without altering the focus of the lens at all, you can swap position of the extension and the 2x and jump directly to 1X on the neg! the first case - putting the 2x directly behind the lens, doubles the focal length (to 200mm), and then the 50mm extension yields 1/4X magnification. The second option - putting the 2x BEHIND the 50mm extension doubles the MAGNIFICTION - so 100mm with 50mm extension = 1/2X...doubled = 1X (try it!) Another thing - converters have developed a bad rep - mainly because paople tend to use them with wide open apertures (have to due to the 2 stop loss of light). They actually perform pretty well when shot at a smaller aperture (required in Macro work) The loss in quality of a 2x doesn't come close to the loss of precision most photographers take given the additional magnification and loss of light. Excellent images can be obtained with some additional care with a 2X converter. (BUT I WOULD invest in a GOOD 2X - or preferably a 1.4X). Wolf