[steveo]

I have a sigma 170-500 1.5-6.3apodg is there a teleconverter that is good for this lense and also what is a teleconsomething excatly?

[postcardcv]

With an slr a tc (teleconverter) go between the camera and the lens, basically they increase the effective focal length of the lens you're using. The commenest sizes are 1.4x and 2x, a 1.4x will cost you one stop of light and a 2x will cost you two. TCs tend to have a slight negative effect on image quality, though this will vary depending on the lens used, a 1.4x is normally the prefered option. With your lens you will loose auto focus with either size so you'll get a longer focal length, less light gathering and be left with manual focus.