gigabellone said:
I'm about to spend a significant (for my finances) amount of money on a new Canon full frame body, it is going to be either a 6D or a 5D3. The plan is to get the camera body and 2 very fast primes: Sigma 35/1.4 and Canon 85/1.2. The 6D has got what i want and costs about 1000€ less than the 5D3, but given the fact that the DoF with fast primes is going to be tiny to say the least, how is the supposedly "simplistic" 6D AF system going to perform? The main fields of application will be travel and street photography, family portraits, some landscapes, the occasional event, and everything will happen just for the sake of it, i'm not trying to make a living out of my hobby (or else i wouldn't have such quantity of money to squander on gear
). I just don't want to spend a significant amount of money and be disappointed by a quirky AF behavior. I would like to hear from owners of these cameras and lenses, lots of online researches and reading still aren't enough to make up my mind.
I've written on this forum extensively about this, and am getting tired of it. I've tried the 5D3, the 1DX, the 1Ds3, the 1D4, the 5D2...and I recently passed 11,000 shutter count on my 6D.
If you're a perfectionist about razor sharp focus accuracy as many on here claim they are...(but you are somehow also not caring a wit about luminance noise above ISO 1000, because the 5D3 has it in spades and the 6D's is far better)...and you will be spending much of your time using the outer focus points to achieve focus with a fast prime (something that is asking for trouble with the 6D's AF sensor)...then FORGET THE 6D and buy the 5D3.
You claim this is a lot of money for you...it was for me too. But apparently you're still going to spend more than I did over such a short amount of time. 6D + those two lenses = approx. $4650. Therefore, you might as well go ahead and get the 5D3, because if you've got the cash to blow on the 85 f/1.2, then you've got plenty of disposable income in my opinion, to spring for the 5D3. At nearly $5000 spent in one day, what's another grand or so?
I made the right decision for me, because I didn't need the snob appeal of owning the higher priced body, and I cared more about image quality in the ISO range between 1000 and 10,000...than perhaps a portrait photographer would (who uses a 5D3 in a studio with strobes and fast primes...at ISO's under 1000...where the 5D3's noise level and DR are easily as good or better than the 6D). I mostly shoot landscape and wildlife, and I get by fine with the 6D. Frankly if I were a wedding photographer who somehow always wanted to shoot running children at the wedding with only outer AF points on an 85 f/1.2 (you know, so I could get the framing just right along the rule of thirds and blah blah blah), I would have bought the 1DX, and forgot all about the 5D3 (as many have).
I've only used two fast primes on my 6D so far (three actually, but only two of them can autofocus). The 135 f/2 cannot AF as fast or as accurate as I would like on the 6D, at least in low light. It works, but servo AF with all points active, is asking for trouble, and results in inaccurate AF at or near wide open aperture. The reason is the outer points are basically crap, they only sense phase differences in one direction, and are especially upset by a lens whose maximum aperture is wider/faster than f/4.
Worst of all, the 6D's processing seems to give these outer points priority while in servo mode. Even though there's plenty of phase to detect at the center point, somehow it's not lighting up. Eventually Canon or ML will fix this, or they won't...I don't know.
This is one of several reasons that so many people gleefully bash the 6D and/or its autofocus ability. Another reason is they need to chest thump about how proud they are of themselves for owning the 5D3 and earning $15k a week with it shooting weddings, Vogue covers, and the Miss Universe pageant, etc etc.
The fastest and most accurate AF with my 6D, is achieved with my 70-200 f/4...even in very low light. Still, about 90% of the time I am in servo AF mode, I leave center point only active (no matter what lens is attached). The other fast prime I've used, is the much maligned Sigma 24mm f/1.8. It exhibits similar behavior to the 135. The biggest difference with this Sigma, is that the autofocus micro adjustment changes every time I use it. I suspect the focus motor(s) are somehow going askew every time I put the lens cap on...so the solution is to leave it off! It's not a "usm" motor so I'm sure that is a factor. It's still a very fun lens to use, though.
My third fast prime is a Cosina Voigtlander 58mm f/1.4 Nokton in Nikon mount, with a chipped adaptor that allows focus confirmation. All I can say here, is the focus confirmation seems to work very accurately even with the outer points...but the final results are really still not as good as an autofocus lens. So for critical focus with this lens, I use live view.
If any of this has helped you at all, then you need to lend me those two lenses, one at a time...for three weeks each. Thanks and you're welcome!
. I better put it up for sale now and start looking for a 5D3... 
