5d mkII vs 50D in terms of AF

[QNeX]

just use the center point and f2.8 lenses or faster and you'll be fine

I think F/4 as well?

Well, with f/4 you get only horizontal sensitivity, vertical line has
sensitivity with f/2.8 or brighter lenses only.

In other words, with f/4 lens you won't have a cross center focus point,
you'll get plain old horizontal f/5.6 point.

[skitron]

just use the center point and f2.8 lenses or faster and you'll be fine

I think F/4 as well?

Well, with f/4 you get only horizontal sensitivity, vertical line has
sensitivity with f/2.8 or brighter lenses only.

In other words, with f/4 lens you won't have a cross center focus point,
you'll get plain old horizontal f/5.6 point.

Now that is a fly in the ointment for my plan to buy a 70-200 f4 IS...

[neuroanatomist]

just use the center point and f2.8 lenses or faster and you'll be fine

I think F/4 as well?

Well, with f/4 you get only horizontal sensitivity, vertical line has
sensitivity with f/2.8 or brighter lenses only.

In other words, with f/4 lens you won't have a cross center focus point,
you'll get plain old horizontal f/5.6 point.

Briansquibb is correct. The 5DII's center AF point is an f/5.6-sensitive cross-type point, with additional f/2.8 sensitivity in a single orientation.

[QNeX]

Briansquibb is correct. The 5DII's center AF point is an f/5.6-sensitive cross-type point, with additional f/2.8 sensitivity in a single orientation.

I've had an impression that a single line can be either higher precision (f/2., but will only work with f/2.8 or faster lesnes, and that is the reason why xxD bodies (IIRC starting with 40D) have center point that is composed of horizontal and vertical lines sensitive to f/5.6 and additional diagonal lines sensitive to f/2.8.

I did look for more authoritative source and found this:
http://cpn.canon-europe.com/content/education/infobank/autofocus/autofocus.do

EOS 5D Mark II AF system
(...)
The centre AF point has a special hybrid design. With f/2.8 or faster lenses, focusing is a two-step process. First, the f/5.6-sensitive cross-type sensor components are used to focus. When focus is almost achieved, a switch is made to the f/2.8-sensitive vertical line-sensitive sensors for high-precision focusing.

Additionally, the centre AF point’s f/5.6 sensors each have two lines with individual pixels arranged in a zigzag pattern, making a total of four lines for horizontal line-sensitive focusing at the centre.

With this improvement, the centre AF point can do a better job of detecting difficult-to-read, low-contrast subjects, improving the overall performance of the autofocusing system.

So, I stand corrected.  Thanks for pointing this out!

[neuroanatomist]

xxD bodies (IIRC starting with 40D) have center point that is composed of horizontal and vertical lines sensitive to f/5.6 and additional diagonal lines sensitive to f/2.8.

I think the reason for that arrangement is so that the center AP point is acting as cross type with all lenses for the entire focusing process.