Originally posted by Reportage ...The target was locked but still could not freeze the moment. Using a Tamron 75-300 at f/5.6 for the night match as it was all i got as still deliberating the 50-135, 70-200 and 300mm fast lens.
No offense, but I think this post exemplifies a lot of the confusion about what AF is supposed to do.
Firstly, the job of your AF isn't to freeze the moment - that's the job of your shutter speed.
Secondly, the AF did find focus - just not on the subject you wanted - it focussed on the high contrast areas at the back of the scene, not the guys in front and centre. How the AF behaves under those circumstances depends on how you have set the focus area and also how 'intelligent' the AF is. But AF intelligence is over-rated - just try using AF on any system when birding, and try to obtain focus on a bird sitting in the middle of a bunch of foliage to find that out. You can't expect AF to be as intelligent as you in figuring out what is the real and interesting feature of a scene. Even on a high end Nikon.
Third, if you wanted more of the scene to be in focus, you would have taken steps to boost the depth of field as much as possible by using a way smaller aperture - eg f11 and up. Which isn't practical in poor light with a telephoto even on a K-x, but with a D700 or 5D2 (with their enormous usable high ISO headroom) is entirely workable. But if you managed to pull this off, it wouldn't have been an achievement of the AF, but an achievement of the wider DOF and high ISO capabilities of the camera.