Interesting info for the 7D mkII but even zoom lenses don't work well with the 7D mkii AF, I haven't yet found a pattern : is it dependent on the construction, the maximum aperture, the release date... Hard to say.
Back button focus is grea,t it allows me to leave the lens AF on and only use it when I need it.
Regarding the size of the focus box of the 7D, I measured the same thing on my 70D and obtained smaller and decentered focus box, some marks are only half covered :
I already had my first camera body exchanged by Canon because the first copy was worst than this. WIth this new copy it's better but it's still not perfect. I first though it was a manufacturing issue, but now I don't know if that's how the camera is supposed to behave.
If you look at the car's picture on the left, I made sure the plants were way outside the sensitive area (look at the position of the surrounding focus marks). The camera didn't obviously focus on the plants in the foreground, something must have fooled it and this is why some users say there is a focus issue.
I can't yet understand why something way outside the AF sensitive zone would do this. This is why I want to understand what the AF sees in different situations. For example, how big does the subject need to be, what color, what lightning conditions will give me good accurate focus. When I read the 7Dmkii manual I simply can't stop telling myself that the camera can't focus on something that small ! Look at the squirrel p22, it's eye simply isn't contrasted enough, the cars I aimed at where brighter than that.
I think we are more dealing with an AF precision issue not visible on a full frame sensor and with small aperture lenses. I also kind of wished Canon decided to include spot focus on the 70D. This camera is impossible to use if I have to be extra careful on how I frame the picture, making sure nothing else too distracting for the AF occupies 10-20% of the entire frame.
Also look in Andre's blog link you provided me, focus points number 1-2 and 4-5 are identical, how can I know where to aim to get good results ? The center of the focus sensitive zone ? The center of the box ?
On a 70D AF on fast wide angle lenses is simply unreliable, no amount of technique can solve that issue, I shouldn't have bought a 70D.
@Molybdo42 wrote:. . .
If you look at the car's picture on the left, I made sure the plants were way outside the sensitive area (look at the position of the surrounding focus marks). The camera didn't obviously focus on the plants in the foreground, something must have fooled it and this is why some users say there is a focus issue.
I can't yet understand why something way outside the AF sensitive zone would do this. This is why I want to understand what the AF sees in different situations. For example, how big does the subject need to be, what color, what lightning conditions will give me good accurate focus.
. . .
On the classic 7D there is a vertical extended zig zag AF sensor that runs from top to bottom in the center. It is supposed to speed focus when the lens is extremely defocused getting the camera in the ballpark. But, on the classic 7D, just like it appears to have done on your 70D and the plants it sometimes seems to extend the height of the center AF point when there is strong contrast just outside the normal AF area and less contrast in the AF area. It's just one of those things you learn about your camera. Once you learn how your cameras AF behaves all of this will really become second nature. But, it does take getting past believing it is not normal, and moving on to learning what it does do.
I also wonder if the size of the focus boxes vary at all with the focal length of the lens, that is something I haven't tested.
Of course that reducing the file size will help but I still want to be able to keep the full res picture to reframe in post if needed. I won't be trying to use autofocus with a wide angle lens anymore (at least on an APSC camera).
If you have any documentation detailing the elements taken into account by the camera during AF post them, I'm still trying to figure it out. But I would like informations about the 19 AF point sensor.
For example the lightning condition (stray light, shadows) seem to have an impact on my AF performance. Is there a way to configure it in the options ? Any information or tips will be welcome.
My dream camera would look like a full frame sensor (for lowlight : no noise at mini ISO 6400), as lightweight as possible, with touchscreen, swivel screen, 15-20 Mpx max (for noise issue), dual pixel AF and integrated flash (never use it but in a pinch...)... In short : a 70D with full frame sensor.
Last thing : I hope you are ready for the "5Ds focus issue" post 
