[eeyore]

Originally Posted by Chris Hawes

Hi,

I use a 1D MkIII, so the sensor crop doesn't help me get as close as some other bodies, so the 600mm is tempting. I understand it is also fully compatable with both the 1.4x and 2x converters, whilst maintaining autofocus - I don't know whether or not this is also true of the 500mm.

both are f4 lenses so on your camera this is the case - be aware that if used on a consumer body (for example if you got a 40D for back up) the af might not focus in dim light as the consumer series cameras need a max app f5.6 as opposed to the f8 of the proffesional models.

You might also want to consider the 400mm f2.8 (which is in the same price bracket) used with a 1.4x this would equate to 560mm f4)