Is it normal for phase-detect autofocus to be inaccurate with a Canon EOS 750D and EF-S 18-55mm lens?

Question

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.

Answer 1

PDAF systems work based on the amount of contrast that they can detect. Your test scene includes a poorly lit and fairly low contrast target in the same frame with a much brighter and much higher contrast area at a different distance from your target. Even if both areas were equally in focus the area to the left would appear, to our eyes, to be better focused. So your test, as constructed, is pretty much designed from the start to cause PDAF to fail.

Is it normal for phase-detect autofocus to be inaccurate...

Yes. Even the best systems aren't perfect. There's no such thing as a perfect AF system, and certainly not a perfect PDAF system.

Roger Cicala at lensrentals.com recently wrote a series about focus performance that is pretty detailed and touches on several of these issues. It is a lot of material to go through, but I found it interesting reading.

Autofocus Reality Part 1: Center-Point, Single-Shot Accuracy
Autofocus Reality Part 2: One vs. Two, Old vs. New
Autofocus Reality Part 3A: Canon Lenses
Autofocus Reality Part 3B: Canon Cameras
Autofocus Reality Part 4: Nikon Full Frame

What I would like to know is if the severity in my example shots is normal and why it only happens at 18mm.

The reason it is more pronounced at 18mm is simply because the area of sensitivity for each AF point on the PDAF array buried in the floor of your camera covers a greater area of coverage of your target as you increase the angle of view of your lens. And those areas of sensitivity are often much larger that those little squares in the viewfinder!

The PDAF system in your 750D/Rebel T6i is pretty much a clone of the PDAF system introduced in the EOS 7D back in 2009. A few features available in the original 7D are not available in the 750D. This is not due to a hardware difference but rather a limitation in the firmware that reflects a choice by Canon to only include certain features in their higher tier cameras. As such, this answer to another question contains information highly relevant to your question. Pay particular attention to Roger Cicala's AF Reality, part 3b blog entry. The 7D (and 70D, 750D) PDAF system, though very configurable, is not exactly Canon's most consistently accurate one.

Answer 2

It is going to be either extremely difficult or incorrect for anyone to make assumptions based in a single example image in less than perfect testing conditions.

My advice would be to send the pair to Canon for adjustment and fine tuning if you desire the utmost in AF accuracy. Since your model does not offer AF microadjustment, having Canon do it is the only way to my knowledge.

Generally speaking, that much out of focus is not desirable but we all have different levels of tolerance to this type of accuracy.

See this question for a very similar situation: Why is my camera focusing fine in liveview but getting it wrong with the viewfinder?

Maybe something in Rogers lengthy posts will help you, very good info here: http://www.lensrentals.com/blog/2012/07/autofocus-reality-part-1-center-point-single-shot-accuracy

Answer 3

After some more testing, it seems that high contrast areas can affect the PDAF, even if they are far away from the selected autofocus point. However, even in the very best conditions (flat, high contrast target on a flat, uniformly colored surface), some inaccuracy remains.

I also tried a different EOS 750D with the same results, so it seems unlikely that it is just an issue with my particular bodies.

I guess I'll just have to live with these inaccuracies of the PDAF and use the CDAF when I need perfect focus.