i have a canon 70-200mm f4 L with no IS. But i want more reach because I do alot of Motorsport and Sports photography. The canon 100-400mm was something I was considering but a friend said just to buy a 2x converter because the 100-400mm wont be as sharp?
I’d only go with the TC idea if you can live with the 2-stop loss in aperture, i.e. the practical speed of a 400mm f8. Motorsports would be about the last subject where a teleconverter would come in handy. Perhaps think about getting the EF 1.4x II, It maintains the IS of your lens and it becomes a 100-280mm f5.6 which isn’t that bad. I’ve taken very nice shots with TCs: Of the full moon at night with a 2x TC stacked on a 3x converter on a 300mm lens (=1800mm, f32) – on a tripod and with a remote, and also occasionally use a 1.6x converter as an extension to my 105 and 60mm Macro lenses, to me, pic quality CAN be actually very good, esp. stopped down a bit. However, for action they don’t really cut the mustard for me. Others may differ of course. What you really want is the 400mm f2.8 :). Fast pics need fast glass. Consider upgrading your 70-200mm f4 lens to an f2.8 if you’re serious.