You often read this in lense reviews
“This lense is front-focussing” or “This lense does have an heavy back-focus issue” or things like that.
For those who don't know. Front Focus means the focus is actually before the subject and backfocus means the focus is behind the subject.
So either way, the subject is not sharp. What is the reason for this? Of course, the lense!
That is what most people think but it's not.
You don't beleive me? You want a proof? Set your camera to Live View and use Contrast Autofocus. Is the Autofoucus correct? There you go, the lense can correctly focus.
But the lense is not working with the Phase-AF, so does the lense have an issue with this? No, it doesn't.
So why do people think it does? Because they don't have any clue how AF on a camera works, so let me explain.
Phase-AF was invented because well… there was no other way to autofocus back then. You had an analog film, no image sensor, nothing.
So the idea was created to make the mirror of SLRs translucent, that means 70% of the light gets reflected up to the view finder and 30% go through the mirror, to a second mirror behind it, gets reflected down in the camera, goint through special lenses, another mirror which reflects it to the back of the camera, another lenses and then to an Phase Detection system.
Does that sound sane? You can imagine the limitation of this system, especially in terms of accuracy.
So where is the lense involved? You guessed it right, no where. The AF Sensor is getting the light and is telling the lense to change its focus until the _AF Sensor_ says, its sharp.
So the AF Sensor tells when it is sharp, not the lense. And when the AF Sensor says its sharp, when it is not sharp, that is not the fault of the lense. Bottom line, cased closed.
But you might ask, how does it come some lenses (especially 3rd party ones) are worse in autofocus then others.
One word: Software
There is a Software in the camera that knows the lense/body combination and knows how to adjust.
So the AF Sensor says “Now its sharp” but Canon knows that in reality it isn't and says “Hey, with this lense, take the value of the AF-Sensor and add 3 steps to it”
Simple as that and 3rd Party lense creators, especially SIGMA, just can't do that. They can't tell the camera “Hey you are an Canon EOS 80D, please add +5 to whatever you measured”
So what can you do? Send in the Camera + the Lense and SIGMA will measure how wrong your AF-Sensor is and programm it in the firmware of the lense.
As this differs from every camera body, they can't put all data into the lense always.
So you might want to blame SIGMA for AF Errors, but in reality, it is not related at all to the lense and you just have a bad camera body which doesn't know how to focus correctly.
If you use Live-View, Contrast AF, the Image Sensor (which is _way_ better in this) tells when it is sharp.
Because the Image Sensor can do it almost perfectly, your ““back-focusing 3rd party lense”” is suddenly turning into an lense that can focus perfectly on the mm
So myth busted, there are no wrong focusing lenses, there are just bad camera bodys and vendors who have the money and ability to put correction values into their cameras.