Marsu42 said:
dgatwood said:
Second, as I understand it, the 6D's center point has increased accuracy when used with f/2.8 and faster lenses, so there's a pretty significant benefit to the faster lens even in normal light, assuming you're using the center point.
Afaik you've got that wrong: If you're shooting with slower lenses, you most likely don't profit from the f2.8 precision as your dof is deeper, but the enhanced precision slows down the af.
Sure, with 2.8 being the edge condition, I'd expect it to have the least benefit from the added precision. That said, presumably, they chose f/2.8 for a reason, so if the added precision were only useful at f/1.8 and wider, they would have picked f/1.8 as the starting point. Either way, if a single f/2.8 precision focusing point has no benefit with that lens on the 6D, then I'd expect multiple f/2.8 precision focusing points to have no benefit on the 5Dmk3, so the argument that the 6D doesn't benefit from it is still wrong. Either they both benefit or they both don't. Perhaps the 6D might have more focus misses, but that's a separate argument, IMO.
Marsu42 said:
Last not least, speaking of the 6d (but not opening the can of worms again) @ f2.8 the af is more precise. but also more unreliable since it isn't a cross sensor anymore just like on the 5d2. Imho that's part of the reason why Canon issued the 24-70/4 which works much better with the af system of the 6d.
That's certainly plausible, giving the timing, but if true, I would have to question the competence of the management of Canon's camera engineering team for not putting a single high-precision f/2.8 cross point instead of a single diagonal, and thus forcing the lens team to design a whole new lens to work around the camera's inadequacies. That's incredibly wasteful, resource-wise, and for a few extra cents per 6D unit sold, that time could have instead gone towards something more useful, like improving the focus speed of the 85 L II and adding IS to it.
That would be like Apple's laptop team forcing the iPad team to design a whole new model of iPad because of a hardware bug in the USB port on one model of MacBook Pro.... Their answer would almost certainly be "No. Fix your f**king laptop design," only without the self-censorship.
I think it's far more likely that the f/4 version of the lens was caused by someone trying to hit a price point so that they could have a reasonable alternative kit lens to the 24-105 for folks who cared more about image quality than reach. That's consistent with what they have historically done in the EF-S world, so it makes perfect sense that they would do the same thing for a prosumer full-frame body like the 6D. I'd be shocked if the lack of a cross-type f/2.8 high-precision point factored in significantly, because if it did, the lens team's response should have been, "No. Fix your f**king camera design."
Then again, the camera team's managers could just be @$$holes.