Thanks Dylan I appreciate your reply.
The 400 f2.8 seems to be a more versatile option as it can be used bare, or with the 1.4 and 2xTC’s
So all this from the same lens;
400 @ f2.8
560 @ f4.5
800 @ f5.6
Only thing is that I’m always looking for more telephoto properties to help fill the frame, and already have the 100-400, so straight away I’ll be using it with my 1.4xIII, then I’ll also need to buy the 2xIII; but on the same hand I will have a seriously powerful lens for my track activities………this ain’t gonna be an easy decision!!!
If anyone wants to share their findings/feelings with using the 400 f2.8 with a 2xTCIII then please feel free to jump in! I’m feeling the pressure of this decision
Im in the same boat really.
If your instantly looking at the lens to use it with a tele and its not long enough your looking at the wrong lens. Teleconverters are extremely useful but if you find 400mm on the 100-400 short it will be short with the 400F2.8, the 500 or 600 might be more useful for you, you can still put a 1.4 and get a F5.6 lens.
Both teles will work very well on the 400 with good technique with not a huge amount of slow down certainly more than manageable. Have you tried the 1.4 on the 100-400mm to see the extra reach? I use it on my 100-400mm when I need to and it still focuses quickly. Only downside is subject isolation but at 560mm DOF its still good but obviously noting like F4 would be.
For me the 400DO MKII looks more attractive because of weight and size makes it more useable in more situations imo. but the 200-400mm is even more versatile but again weight. 500 is great too but then I think will 100mm make the difference? Probably not.
I will use mine for motorsport and wildlife, so F4 will be fine for motorsport and ok for wildlife can never have enough DOF with wildlife but again usually poor light. The other combo I've been looking at is the 300mm F2.8 because of its size and cost then maybe adding something else later. But it will be used with teles all the time probably which puts me off.
For the time being the 100-400mm is proving the best bang for buck and its IQ is really very impressive 80% of the 200-400mm half the weight and size and again 1/3rd the price. 1.4 in a pinch, used on a 7DMKII because again its performance has really impressed me.