I'm using an EOS 750D with an EF-S 18-55mm IS STM lens. I often have trouble getting accurate focus using the viewfinder autofocus. Live view autofocus works fine.
Here are two crops of the same scene, the first is taken with live view autofocus, the second one using viewfinder autofocus. In both cases, I used point focusing on the Nine Men's Morris board on the right, which is the center of the full image. The images were taken on a tripod in manual mode with the exact same settings (250ms exposure, f/3.5, 18mm, ISO 100). I manually defocused the lens before taking each picture to make sure the autofocus system actually did something.
You can see that the focus in the second image is significantly in front of the target. The curtain on the left is in much better focus than the board, which was the actual autofocus target. The curtain is about 1.5m from the camera, the board is about 3m from the camera, so the focus error seems quite significant to me, relatively speaking. Sometimes the focus is a lot better, but it's never quite perfect, and as this example shows, it varies quite a lot. About 2/3 of the shots are this severely out of focus.
This seems to happen with all sorts of targets and also with other autofocus points than the center one. However, it only happens at 18mm. If I zoom to 55mm, the viewfinder autofocus is spot on. It also doesn't happen with any of my other lenses (EF-S 55-250mm IS STM, EF 50mm STM), although the EF-S 18-55mm IS STM lens doesn't have issues at these focal lengths either.
Any ideas what could be causing this? Am I doing something wrong? Should I send back my camera and/or lens? Or am I simply expecting too much from the autofocus system (although 1.5m focus offset on a 3m target seems quite a lot to me)?
At this point, I'm ready to send my camera kit back, unless this is the sort of performance that is expected from viewfinder autofocus.
The thread Why is my camera focusing fine in liveview but getting it wrong with the viewfinder? does not really answer my question. I already know the possible reasons for why PDAF might be less accurate than CDAF. What I would like to know is if the severity in my example shots is normal and why it only happens at 18mm. It would be nice if someone with a similar camera could report on their experiences.