Used the 85II on a portrait session yesterday and was getting a pretty high percentage of slight misses on a relatively static subject. I did pretty well with the same lens on my 5D3. Since switching over, the 85II has been pulled out twice for any significant amount of shooting and both times have been tough with regard to getting a lot of misses.
I'm going to go out on a limb and assume you have done a proper AFMA with the lens. I haven't used the 85II myself with a 6D or otherwise, but I don't have any issues with the 135L; I get consistent, repeatable results. The 85II should be usable on a 6D; plenty of people were shooting with it on a 5DII, which I can attest had a slightly inferior AF system to the 6D.
AFMA'd with Reikan FoCal three consecutive times yielding the same result. Figured it was good enough. I surprisingly have had the same findings as you regarding the 135 on the 6D. Seems to be just fine. But for some reason, the 85II on both occasions has been somewhat unreliable compared to when it was paired with my 5d3. The other ones that have given me the most problems is the 24II and the 24-70II. The zoom wasn't unreliable as far as AF goes, just seemed to hunt noticeably more in and in decent light which was odd. Most the other stuff has been fine.
That is interesting. I wonder if it due to the nature of the newer lenses being optimized for 1DX and 5III AF? I'm actually very surprised at more hunting with the center point, which has (on paper) greater sensitivity to anything else Canon has. I always use single point AF, myself, and in good light I don't have problems with any of my lenses on my 6Ds. And that center point can lock onto just about anything in any light.
This was actually during shoots so I was using all the points without paying too much attention to how often it was the center. Just based on feel/memory though, I don't believe I use the center point much more often than the outer points when composing if at all. I have definitely noticed that the center point has caught focus with a few of my lenses in very low light situations where I think I might have had a little more trouble previously. However, I am finding that doesn't help me all that much as I compose very frequently outside of the center which is probably more of an exhibition of how weak those outer non center points really are.
This little discussion may be key to at least some of the varying opinions on the AF of the 6D. I think it depends on one's shooting style. I tend to be a little more deliberate in my approach, and, frankly, the 6D works for me. I can't remember the last time I let a camera automatically select the AF point. I have had my most successful year commercially shooting 6D bodies exclusively. I don't have any problems at weddings and events, myself, but I do know my gear pretty well. IQ trumps all for me, and the 6D delivers as well or better than any other current Canon option.
But I have never recommended the 6D to those who shoot sports or a lot of action. They need a more robust AF. If they are a less deliberate shooter or prefer to use zones/auto AF the 6D may not serve them as well either because its AF points are not spread out super wide and there aren't that many of them.
Excellent advice as always, Dustin. Have you, by chance, used any fast primes like the 85 f/1.2 or 35 f/1.4 with either of your 6D bodies? I had the 6D for a limited time and found the outer points regularly inconsistent with such a shallow DOF, which is one of the main reasons I reverted back to the 5D3 (aside from dual card slots). What are your overall thoughts on using fast glass with the 6D, as you obviously know how to use it well - your images speak for themselves!
My fastest lens is currently only f/1.8. I have the new 35mm f/2 IS, 85mm f/1.8, 100L, and 135L primes. None of them are as wide as f/1.4, and certainly not f/1.2. I have owned and shot the 50mm f/1.4 but, frankly, it never did too much for me, and I ended up moving both copies I have owned within six months of ownership. That being said, it wasn't because I had focus issues.
I recognize from others that lenses like the 85L and the 50L are more challenging to get consistent focus from, but that seems to be true on most bodies (particularly with the 50L). I don't know what to tell you on that.
I can tell you that I have used both the 135L and several 70-200 f/2.8 variants without any issue on the 6D, and the depth of field is just as small (smaller @200mm f/2.
than either of the other primes. Those two just seem to be a little more finicky, and yet produce stunning results when they are on.
Proper AFMA is a big deal, and sometimes you don't get it right the first time. This becomes increasingly important the wider the aperture is. Technique is also a big part. I shoot events with a 6D/135L combo all the time (and sometimes with a 1.4x II on the 135L) and rarely miss focus.
I don't know what to tell for sure as I don't have experience with either of the lenses you are speaking of. All I know is that both pros and amateurs were getting stunning results with those lenses mounted on 5DIIs, and I know from experience that the 6D's AF is actually better/more accurate than the 5DIIs.