I've just bought a used Sigma 50mm macro lens for my 400d but I find I have to get really close to a subject to get the result I want. I also have a 2x tele-converter that I can add to the macro lens. Will this make it the same as a 100mm macro?? What are the dissadvantages of doing this?? I would have got a 100mm lens if it was affordable but I got a good deal on the 50mm.