f/2.8 telephoto lenses have better AF than slower lenses?

[FarQinell]

This I have culled from www. learn.usa.com (with thanks) about 7D auto focus:

"... the center AF point has a separate diagonal pair of high precision line sensors which provide even greater precision - automatically used with lenses f/2.8 or faster....."

When considering an investment in a big lens is this a consideration ie go for the f/2.8 lens rather than a longer f/4?

[neuroanatomist]

IMO, it's definitely a factor to make one consider an f/2.8 lens over an f/4 lens, provided the trade offs (cost, size) are acceptable. 

The 5DIII and 1D X have five of those high-precision f/2.8 AF points.

If it helps, I wrote up a description of EOS AF Systems, which includes a discussion of the high-precision points, as well as the differences between cross, dual cross, what f/2.8 vs. f/5.6 'sensitivity' really means, etc. 

[FarQinell]

IMO, it's definitely a factor to make one consider an f/2.8 lens over an f/4 lens, provided the trade offs (cost, size) are acceptable. 

The 5DIII and 1D X have five of those high-precision f/2.8 AF points.

If it helps, I wrote up a description of EOS AF Systems, which includes a discussion of the high-precision points, as well as the differences between cross, dual cross, what f/2.8 vs. f/5.6 'sensitivity' really means, etc.

Thanks for the very useful background information - worth reading for anyone contemplating a big lens investment - f/2.8 better than an f/4?