Now I have the 1DmkIII I also have its powerful AF, and the possibility to AF down to f8. This of course means I can now add the 2x converter to my 500 f4. Admittedly this probably gives me less length than the 7D with 500 and 1.4TC (700mm x 1.6 = 1120 with 18million pickles!) but it makes up for some of the lost resolution. I tested it out on Sunday as the snow adds a bit of light to the scene. Obviously 1/160 sec was not fast enough but the results were suprisingly good, and so they should be for a lens that cost more than my car.