Should Canon 5D mk II autofocus be accurate enough for a f/1.2 lens?

According to this Blog post by LensRentals.com owner Roger Cicala, the 5DIII has less than 1/2 the standard deviation of the 5DII with regards to Phase detection Auto Focus. But to get that additional benefit, one must also use Canon lenses introduced since about 2010. All of the testing was done using the center focus point.

In general, using the center focus point should yield faster, more accurate focus with faster lenses such as the 50mm f/1.2L and f/1.4. The 5DII can be an exception to this rule. When shooting in AI Servo AF, if C.Fn III-07 is enabled, there are six small AF Assist Points clustered within the spot metering circle that are active in addition to the Center Focus Point. The location of these six AF Assist Points are not indicated in the viewfinder! Yet the camera may be using one of them pointed at something that has higher contrast than what is located behind the square in the middle of the viewfinder when it decides the focus distance.

Here's the Map for the 5DII's focus system.

The entire series will tell you just about everything you've ever wanted to know about phase detection AF accuracy and consistency.

Autofocus Reality Part 1: Center-Point, Single-Shot Accuracy

Autofocus Reality Part II: One vs. Two, Old vs. New

Autofocus Reality Part 3a: Canon Lenses

Autofocus Reality Part 3b: Canon Cameras

Autofocus Reality Part 4: Nikon Full Frame

The Canon EF 85mm f/1.2L II USM lens is a very difficult lens to focus, for a few reasons. The biggest reason is that any lens at f/1.2 will have a very narrow depth of field. On your 5DmkII at 10ft you should expect right around 4 inches of acceptably in focus depth of field. If you step closer this window will narrow. The second biggest challenge is that this lens in particular is not a fast performing autofocus lens. It is known for being slower then average, but it is simply something that many photographers put up with because they enjoy the results of the lens so much.

Will AF performance improve if you buy the Canon 5DmkIII over the Canon 5DmkII, absolutely. The AF is without question better in the mkIII edition, hands down. Will the 85mm f/1.2 lens still be slow to autofocus, yes, will it still have a very narrow depth of field, yes. But It will lock on and find focus better and with more points.