Hugh,
Adding the 1.5x teleconvertor will lose you one stop of light. As Derek has said, this will make your lens effectively f/5.6, but this is only at the 50mm end of the focal length. The maximum aperture of this length varies as the lens is zoomed, which is why it is described as 1:4-6.3
This means that the wide open aperture at the 50mm end is f/4 but at the 500mm end, the maximum aperture (without the teleconvertor) is f/6.3.
Adding the 1.5x convertor loses you one stop of light which makes your lens f/5.6 at the 50mm end, but a rather poor f/9 at the 500mm end.
Duncan
|