Many (all?) lenses have a (slightly) curved focal plane. In general it's close enough to flat that you don't really notice it, but under certain conditions it may become apparent. As I understand it the curved focal plane is most likely to cause issues with wider and/or wide-aperture lenses (e.g. LensRentals on the 50/1.2).
I've seen various threads (e.g. here, here, and here) with people having trouble getting such lenses to focus correctly using the outer AF points, yet the centre AF point works fine. Seems to come up with lenses like the Sigma 30mm f/1.4 on APS-C, or Canon 50mm f/1.2 on FF. Many responders put it down to the lenses having a curved focal plane, but my basic understanding of the way AF works makes this kind of hard to believe.
Is it normal to be unable to use the outer AF points on a normal to wide angle lens at very large apertures (~f/1.4)? Or should all AF points get the focus right for that point, no matter the curvature of the focal plane?
It seems to me that unless the AF points are trying to do something 'smart', using the known curvature of the focal plane, for example, they should all only provide good contrast at their specific location on the frame. So I can't really see how the curvature of the focal plane should affect focus achieved from an AF point provided the shot isn't recomposed after focusing.
To complicate the various discussions on the matter, some people use poor techniques, like testing in poor light, small or low contrast targets, without a tripod, etc. This makes many of the forum threads on this very hard to follow, and makes me doubt what everyone is saying (on both sides). However, there's plenty of cases of people using sensible targets (large, 2D regions with vertical lines, daylight, tripod, no recompose) and finding their camera+lens front focuses consistently with the outer AF point but is consistently correct with the centre AF point(s). Seems to happen only/mostly on these normal-wide, large aperture lenses. So I'm wondering if it's a normal thing, or if it's just a lens alignment/calibration/tolerance issue that shows up more easily with these lenses?
I just bought a Sigma 30mm f/1.4 (the new Art one) for my 7D and have experienced exactly this problem. My first tests were using the centre AF point up close and it worked perfectly. I assumed I had a good copy and started shooting. Went to shoot a subject from about 6m away using the far-left AF point and could not get it to focus correctly; the focal plane was over a metre in front of the subject. So I thoroughly tested it with a well lit high contrast focus target (black & white printed A4) and am finding I need to apply autofocus micro-adjustments of about +4 using the centre AF point, +8 for many of the outer AF points, and +12 on the far left AF point (this is at about 2-6 metres away, it's less dramatic at 30cm, where +0 is fine for all AF points with the target within the 5mm DoF).
My other lenses (including 50 f/1.8) don't exhibit noticeable AF differences with different AF points, so I'm fairly sure it's not a problem with my AF sensor.
And it's not a pixel-peeping problem, it was clear from just looking at the 3" photo on the back of the camera that it's not focusing properly (DoF is about 1m wide, and it was front-focused by about 1m).
If it was vaguely consistent (+/- 3 say) across the whole frame & across all focal distances, I'd just pick an adjustment and get on with it. But no matter what MFA setting I pick, some AF points will be unusable. Even if it was just consistent across the frame, but varied with focal distance, I'd get the Sigma USB dock and recalibrate, but that won't fix AF variation across the plane, only for varied focus distances.
So is this normal? Is it reasonable to expect that I can only reliably use the centre AF point on a normal-wide, large aperture lens? Or have I (and others) got reason to return lenses performing this way, expecting usable AF across the whole frame?
Update: Sent the vendor AF test shots, they claim its just normal AF variation, like @MichaelClark suggests may be the case. Could be I need to calibrate it with Sigma's dock and/or learn to AF.