Agree with amk1977. I use both the 1.4 and 2x Nikon converters with great results. I have also used a 1.4 Sigma converter with my Sigma 50-500mm F4-6.3 but in manual mode only. To get the best from them you need lenses of F4 or less, and in some cases the AF will only work if you have a Pro or semi-pro camera. If the converters you seen on ebay are made by anyone else except you camera/lens maker then forget them unless made by Kenco, as they are usually rubbish and give you bad results. A 1.4 converter will cost you one stop, a 1.7 converter 1.5 stops and a 2x two stops.
