Why is my camera focusing fine in liveview but getting it wrong with the viewfinder?

Question

When I first got my Yongnuo 50mm f/1.8 it would not focus through the viewfinder. It will always miss (95% of times). Yet it worked fine in liveview. And now, recently, I started noticing the same issue with my 18-55mm. I reviewed some pictures a friend took 2 weeks back (using my body) and the AF is always a little off. It's never really there!

I tested this with multiple focus points today, I placed two toy figures a few inches apart in the middle of the room against a white wall and took a bunch of test shots. It either grossly missed focused or came really close at smaller apertures. At best, toy figures had a soft look, whereas the same shots come out crisp when taken in liveview!

P.S my kit lens worked fine on a friend's 70D (I haven't tried it recently though)
Body: Canon SL1
Lenses: 18-55mm Kit; Youngnou 50mm f1.8
Summary: Camera won't focus through viewfinder; works fine when operated through liveview.

What can possibly be the cause, and how can I fix this?

Answer 1

It seems several things are going on here at once.

• You seem new to using PDAF through the viewfinder of a DSLR. There is a learning curve involved. It is just as significant a learning curve as the one encountered when moving from a compact, small sensor camera that yields almost limitless depth of field to a larger sensor camera that means the depth of field in most shots will be much narrower. You need to research and learn the unique characteristics of the PDAF system in your particular model. It will take some time and experimentation to master it.

• You are discovering the imperfections of Phase Detection Auto Focus (PDAF). When they first came out, PDAF systems were designed to be as fast as possible at the expenses of absolute accuracy. As technology has advanced over the past 25 years or so, the emphasis has shifted some to both fast and accurate, but this is mainly seen only in higher end camera/lens combinations. Entry level bodies and kit lenses still exhibit inconsistencies when PDAF is used. But the expectation of pixel-peeping-perfect-focus-every-shot in the digital age make them seem much worse than they did a few years ago when most photos were produced as small or medium sized prints and exact focus was nowhere near as critical!

• It appears reasonable to suspect that your EOS SL1 body needs the overall PDAF system to be calibrated. Again, higher end bodies allow the end user to do this via Auto Focus Micro Adjustment (AFMA) but the entry level Rebel/Kiss/xx0D/xx00D series do not. The camera will need to be sent to a Canon authorized service center to have the AFMA calibrated. Also send your most used Canon lens along as well.

• It seems your Yongnuo 50mm f/1.8 lens is even further out of calibration than your camera body, and in the same direction. It needs to be calibrated to your camera after the camera has been set at the factory service center. With third party lenses this can be a headache unless you are willing to send the body along as well. If one needs to send the lens all the way back to China for this, I would probably not recommend sending the camera along. For a lens this cheap it would probably make more sense to just: a) try another copy or b) get a Canon EF 50mm f/1.8 STM for practically the same price.

Answer 2

I don't think I'd jump right to "broken". The basic explanation is that live view and the viewfinder put you into different focus systems — the viewfinder uses phase detect, and live view contrast detect. In general, phase detect is much faster but may not be as precise.

So, one possibility is that the phase detect system just need to be better calibrated. With some cameras you have to do this yourself, but lower-end models (like yours) you need to send it in. I don't know about Canon, but usually this is done for free. Calibration is a matter of lens + body, so usually you send in your whole kit. (They might not want to mess with third-party lenses, though).

Another possibility is that the phase detect system is just grabbing the wrong thing. The AF area in such a system is usually much larger than that of the indicator in the viewfinder, and generally will focus on whatever has the most contrast within that area, even if it's not what you meant.

Answer 3

The AF (autofocus) system in your camera body is broken/damaged. Live view does not use the same AF system which is why you're seeing a difference.

You will need to take it to a professional service centre for repair, it's not something you could do for yourself.

Given that the problem started when you first attached the youngnou lens it's entirely possible that it has damaged your camera but proving that and getting something done about it may be difficult. I certainly wouldn't recommend putting that Youngnou lens on any other cameras unless you're prepared to pay for them to be repaired too and sticking to proven/reputable brands of lenses in future.