Mathmans wrote in post #17959170
When doing tests in live view I always place focusing rectangle in the middle of the screen. There is nothing wrong with the camera because all my other lenses are doing fine in live view.
There is something about Sigma. I have read on the net that some other Nikon shooters have similar troubles with some Sigma lenses.
OK - I've heard of Sigma issues, many with needing their chip to be udated for changes in AF systems for which they reverse engineered the communicaions to e.g. Nikon or Canon. Do you have other lenses with the same FL and use the same distance to the same target? In every case the area on your vertical focus target plane is much bigger (on the screen) than the focusing rectangle*, so there's no possibility of something of high contrast at a different distance that might "fool" the AF, right? Just checking...
(*This is waaaay, waaay bigger than the AF squares on the focusing screen for PDAF)
Now, if you use manual focus and zoom in with LiveView, you should be able to get optimal focus with no "AF assist" - might need a deft touch on the focus ring but "you can do it" Now, defocus a lot and push the "AF On" button after making sure the LiveMode is operating (i.e., the mirror should not move when you push the button). The camera should AF, albeit slowly. When it's done, it is obviously not as well focused as the best you can do with MF? (That seems to be what you are saying - and for other lenses it is as good). That would be hard to understand, given how the system works - the AF system moves the lens back and forth, based solely on the information from the sensor, until the contrast peaks at edges are maximum - it does not need to know what is going on in the lens movement mechanism/control (like it does for some PDAF systems). In your case, if your technique is good, the contrast peaks are not being maximized and I have no idea how that might happen, especially with only one lens