16-35 f/4 or f/2.8 for best off-center AF?

[rt]

Hi, I am having a hard time making a decision on which lens to buy: the existing 16-35 f/4L IS or the upcoming (?) 16-35 f/2.8 III. I am leaning towards the f/4 one as it fits my needs very well, I am just concerned about the autofocus performance difference between 2.8 and 4.0 maximum apertures.

I am currently looking at the following comparison of AF patterns: http://photo.stackexchange.com/questions/16399/what-apertures-are-required-to-enable-autofocus-including-cross-type-or-high-pr

For newer cameras, the only real differences between f/2.8 and f/4 would be in the high-precision sensors and dual cross-type points however, looking at the AF patterns, these points are typically in the image center. I do not happen to be using the center part very much for focusing so I am more concerned about the off center ones.

My questions are:
[list type=decimal]
[*]Am I missing any differences here? Even with the usual f/4 or f/5.6 sensitive points will the camera focus better with a f/2.8 lens rather than with a f/4 one (I wouldn't be surprised as there is more light available with a f/2.8 lens with maximum aperture, so the one that is used for focusing)
[*]Do you foresee that high-precision or dual cross-type points will be used closer to the edges in new cameras? The range seems to be growing (slowly) for the recents models, it's currently a column rather than one point.[/list]

I am mostly asking about the future direction (the camera is secondary to me really), but if this really matters, I will be using the lens with 7D (and 40D to some extent) now but I am planning on getting a 5D Mark IV (when it happens), will also consider 7D Mark III / 6D Mark II (if they happen).

AF precision is critical for me, it will be a deciding factor in the lens choice.

Thanks!

 

[neuroanatomist]

Am I missing any differences here? Even with the usual f/4 or f/5.6 sensitive points will the camera focus better with a f/2.8 lens rather than with a f/4 one (I wouldn't be surprised as there is more light available with a f/2.8 lens with maximum aperture, so the one that is used for focusing)

Unless the ambient light level is really limiting (e.g. below 0 EV), there's not going to a significant difference with a faster lens when used with f/4 or f/5.6 AF points. The aperture values specified for phase AF points aren't about the light intensity, but rather about a wider spread of light which is needed to support the wider baseline of the paired line sensors. In any sort of reasonable light with an f/5.6 AF point, you'll get the same accuracy with an f/5.6 lens as an f/1.2 lens.

Do you foresee that high-precision or dual cross-type points will be used closer to the edges in new cameras? The range seems to be growing (slowly) for the recents models, it's currently a column rather than one point.

I'd say it's unlikely. If you look at the 1D X AF sensor, you can see the limitation – the f/2.8 part of the 5 dual crosses are the diagonal lines.

In theory, they could possibly squeeze another set in a couple of colums away from the center, but they're not going to get much closer to the edges than that...and even so, I suspect there are other technical reasons to not locate them away from the center.

I am mostly asking about the future direction (the camera is secondary to me really), but if this really matters, I will be using the lens with 7D (and 40D to some extent) now but I am planning on getting a 5D Mark IV (when it happens), will also consider 7D Mark III / 6D Mark II (if they happen).

For current and the next generation of cameras, I would not expect much difference in focusing between the f/4 and potential f/2.8 versions of a 16-35 lens. The exception being if you typically use the center AF point, since with f/2.8 that's high precision whereas with an f/4 lens you're using the f/5.6 cross. However, with UWA lenses critical focus is generally not as evident due to the apparently deeper DoF.