Originally posted by XATN3q I also have a question
what happen if I attach a 2x teleconverter on a 500mm lens ? my K200D only reconizes up to 800mm.
I 've once put two "2xTC" on a 55-300mm lens an the images were awful
That is entirely expected. Generally speaking, teleconverters work best on single focal length lenses. There are two considerations to keep in mind. The first is that whatever the defects in the lens, they will be multiplied by the converter, and the converter will add its own defects to the mix. The other consideration is that when you have, for example, a 400/5.6, the first 2x converter will give you an 800/11 and the second would create a 1600mm f/22 lens. The 1600mm on its own is problematic, and the slow f/stop just makes your life worse. Rather than using two doublers on the lens, use one if you must, and then simply crop the image to create the larger view.