Why do some lenses "play nice" with certain Canon bodies and not others?

I've noticed recently that on my 5DIII my Tamron 24-70VC seems to wildly miss focus on easy "targets" while when shooting the same lens with either my 6D or even 7D, AF seems to be perfectly fine. Firmware on the 5D is up to date and I've tried eliminating any variables I could, but am still left scratching my head. The only thing I can come up with is that that particular lens doesn't like that particular body. Anyone have any experience with this sort of thing? Anyone find any solutions?

I have similar findings and don't understand why. My 100-400L II works perfectly on my 6D but is virtually useless on my 60D using autofocus. My 70-300L works terrific on the 60D. Very strange. Luckily I purchased the 100-400L II to primarily use on the 6D. Oh and it works well with my tiny M3!

Have you guys MFA'ed the lenses? I know you can't on the 60D but on other bodies you can. But the reasons are actually quite simple. All hardware has some level of slop. They are designed for perfection but during manufacturer anything that comes in some range, say 5% of perfect is considered fine. If you have a body and lens with are perfect, all is well. If you have a body that is on the +5% side and a lens on the +5% side, all might still be well. But if your bode is on the +5% side and the lens on the -5% side, it may be far enough off to notice it.

I had that experience with my 100-400 and 40D. It was so bad that I was redy to dump all my Canon equipment and switch. Both the body and lens made multiple trips back to Canon and they just couldn't get it acceptable.

But I got a good deal on a new 50D with MFA and was able to calibrate the lens to the body. The results were very noticeable. It turned a piece of junk 100-400 into one of my sharpest lens. Below is a sample test pic I took. The left is the lens with no adjustment and the right with adjustment applied.

At first I thought it might be a matter of microadjusting the lens, but then I looked at my misses and realized it wasn't a matter of a little back/front focusing, but rather wild misses. I would receive focus confirmation and the lens wasn't hunting, it was just off by a lot.

I've never run across this problem yet, either on NY T3i or 60d. I guess I've been lucky.

Frodge wrote in post #18083797
I've never run across this problem yet, either on NY T3i or 60d. I guess I've been lucky.

A lot of it is dependent on your final output or work flow. If you print big 8x12 or larger, crop heavily (for wildlife or such), or do a lot of post processing where you zoom in, you are more likely to see the problem. Most of the time it's close enough where it goes un-noticed.

dan.k78 wrote in post #18083488
At first I thought it might be a matter of microadjusting the lens, but then I looked at my misses and realized it wasn't a matter of a little back/front focusing, but rather wild misses. I would receive focus confirmation and the lens wasn't hunting, it was just off by a lot.

But if the lens is out on that body, then the AF becomes more erratic, less accurate in general.
So it's usually worth trying MFA.