Thanks for all the replies. None of them have responded to my specific question, but I appreciate you taking your time anyway.
David Beard, if you do your research you will quickly discover that there is - as yet - no such thing as the perfect autofocus camera. And that is not what I was asking. My clearly stated question - asked of people who have used both the 5D3 and 1DX in the motorsport field - is whether there is any notable difference between the two in terms of in-focus results
the operator is always a variable, but a skilled operator will be able to notice a difference between them, IF there is one. And this is the only issue I'm interested in i.e. is there a compelling reason to spend substantially more on a camera which appears to be substantially the same as it's cheaper sibling? None of the respected review sites I've yet found have tested either camera shooting racing cars and it is perhaps the most taxing environment for auto focus I know of, especially if one pushes the limits as I do, for example, a lot of 3/4 panning work on corner exit (and entry) using slow shutter speeds. There are all sorts of considerations which come into play in this regard - efficiency of the camera's auto focus system is the easiest to control, IF one knows what one is buying in the first place.
Kayemod - I can't comment on the equipment you are using, as I've never used Nikon. My instinct is that what you are seeing as 'soft focus' - especially if it's consistent, is perhaps not a focus issue, but rather that you are expecting too much from your 200mm lens with this kind of photography. I don't know of any zoom which works best at either extremity and, to me, 200mm is just too short for the camera sensor to be able to capture the detail of this sort of action. Another factor which may be coming into play is that your auto-focus may be 'hunting' to find - and keep in focus - the subject, against the low contrast sky. Almost every camera/lens combo would find this a challenge. What I would do with your existing gear is as follows (and please excuse me if you are already doing some of this!) :
1 - make sure that you have a good quality UV filter on the lens,
2 - make sure that your camera is set to continuous auto focus and not 1-shot,
3 - IF the camera has a setting where you can change the autofocus reaction speed, slow it down
to around 1/2 to 2/3 of it's maximum capability, rather than just bang it onto the maximum as some do,
4 - if your camera settings have a sharpness adjustment, it is likely to be set around mid way of the range. Turn it slightly sharper (Canon has a 7 click adjustment - for motorsport, I use setting 5 of 7 and have found this to be about optimum since I 'went digital' in 2005 with the 20D,)
5- shoot with image stabilization OFF unless you are in lighting conditions which are forcing lower shutter speeds than you would like to use,
6 - shoot prop-engined planes at 1/200 to 1/250 so that you capture some movement of the props; shoot everything else at 1/400 to 1/800 of a second ... or faster,
7 - use the centre focus point ONLY for darker planes and the smallest selection of available centrally located focus points for lightly coloured and/or silver/grey planes,
8 - and one of the most important factors imo, don't leave your shutter button part-depressed while you casually track the subject. Follow it until you sense that you are getting in the zone, THEN part press the shutter to get a focus lock and then pretty much in the same second, take the pic. I have found this approach - with each of the 6 digital auto focus SLR's I have used over time - to give a consistently better hit rate.
9 - if possible, try and get hold of a 400 mm zoom lens. You will likely find that you are shooting around the 300 to 350 mm mark and be able to fill the frame with enough subject detail, so as not to expect the camera to capture detail which it is just not capable of seeing. If you can't do this, use a 1.4x converter BUT you may experience a hit in image quality and auto focussing speed. How much depends on the camera/lens/converter combination you are using.
Thanks again for all the feedback.
Edited by Speedy27, 23 May 2013 - 11:17.