Wiki - http://en.wikipedia.org/wiki/Autofocus#Contrast_measurement

That is a decent description.

Contrast = determines the focus point with the change of light to dark areas in the selected focal point. i.e. features of the eye - eyebrow to white of the eye.

Phase = like a rangefinder focus system but utilizing the sensor. There are dots that line up on the sensor to the image. Fuji's site has a great description to this and a video that makes since of this one. The J1/v1 I think uses this type of system.

Cross type/51-point = In addition to the 1 -up/down/left/right there are speedy sensors going at diagonals out as well that are not quite as good as the main ones, but better than the "supporting" sensor points.

It is not that they "have" a greater surface area but they Create more areas to focus quickly on. Evidently the D4's new AF sensor creates a whole new standard.

The cost adds much more to the system. One of the reasons I really didn't like the D7000 and wouldn't consider anything lower than a D300 is due to the AF system. It is better for how I like to shoot. It alone is worth the $500 premium for the next body up. For instance - I was just playing around with my Tokina 300 2.8 (AF-D - screw type) with the 51point 3d tracking on, and shooting pigeons in flight that were about 200 yards away. Now they were not much more than black dots, but the outline edges were sharp and the focus automatically just follows which ever birds were in the "speedy" spots and tracked them from far to close and out of 25 shots, only about 3 were out of focus. That was in daylight. In low light, the slower AF lens shows as well as more missed shots. I also have focus priority set to On as well.

Yes thanks TTJ, I wasn't too sure how it worked. Two more questions (in laymans terms please) what is going on in the camera when focusing with phase as opposed to contrast AF, and I know that they are better, but how do cross type AF points practically differ from standard? Is it that they have a greater surface area of receptors, and so are more sensitive? I presume they are much more expensive to make otherwise why not have 51 cross type sensors?

Nice explanation, TTJ. (At the risk of sounding pedantic, I'll mention that the AF sensor is below the mirror. ;-)

So many places one could start - I'll try to keep it logical - others maybe able to fill in some gaps I may miss. Not sure which body you have, so I'll keep it general.

There are 2 types of AF points - Main (speedy - center + 1 up,down,left,right) and supporting (all the other points). As bodies become more advanced, the more speedy points there are. Center point is always the fastest.

When the light entering the lens dims, the further out points don't get enough light to be accurate and MAY NOT grab focus. The speedy ones, almost always grab focus but may take longer. This applies for the AF sensor in the prism.

Live view I think uses contrast focus now - I have the D300 so the mirror slaps down to focus. With less light, contrast systems (live view - non mirror slap) fail much more and are less reliable.

F8 and birds - there are way too many factors that can cause this. Wind, branch move, low light, camera shake, higher iso, etc, etc, etc.

So are you stopping the lens down to F8 or is that the equiv light loss. Big difference there. If the lens is stopped down to f/8 + 2 stop light loss, that is very dim in average light which could cause the AF to miss some especially if you are not using the center AF point. I usually focus on the center AF point and AF-lock it and recompose.

Personally at those distances, I don't even attempt MF - I loose that battle every time.

Does that make since?

So I see mentioned that only some af points work at smaller apertures.

Firstly: why?

Secondly: how do I know if a point isn't working, when it behaves as though it does, ie; it locks on and beeps.

Is this why so many of my f8 bird shots are not sharp?

I find it too hard to manually focus an AF lens without live view, as the plane of focus is so narrow (if the bird takes up a good portion of the frame at 600mm), with live view I often miss the shot as the birds move in and out of focus contantly on the branch (the D4 looks as though it locks on MUCH fater in live view, which is great).

It would be handy to have the MF 600mm 5.6. it's small, easier to manually focus and a stop brighter than my 300 2.8 with tc-20eiii (stopped down to f8), but I don't, so help would be appreciated.