Let's get right to the chase: two primary autofocus systems exist, contrast-based and phase detection. DSLRs use phase detection as their primary autofocus capability, while compact cameras use contrast as their sole autofocus capability. Mirrorless cameras can be either (and in some cases, both).
- DSLRs — one of the uses of the mirror contained in the traditional DSLR is that it splits off some light to a phase detect sensor. Phase detect sensors are interesting, because they can report not only which direction the focus is off (near/far) but also by how much. Thus, in a phase detect system, the camera looks at the focus sensor, sees direction/distance and tells the lens to "move there." It's very quick, and capable of following movement very easily.
- Compacts — compact cameras use the imaging sensor for everything. They look at a portion of the image and measure contrast. At the most simplistic level, high contrast means "in focus" and low contrast means "out of focus." But a contrast-based sensor has a vague notion of direction and distance the lens needs to move, so it iterates: it moves the focus a bit and re-evaluates and then uses that information to tell the lens more about where to go.
Phase detect sensors often operate with very high data streams (usually higher in a pro DSLR than in a consumer one, which is why the pro ones are better at focus on moving subjects). Contrast-based systems operate at (usually) the video frame rate of the camera, which is generally slower.
Mirrorless cameras are an interesting crossbreed, because we're seeing both types of systems evolve and get pushed in new directions. For example:
- The Nikon 1, Canon EOS M5/M6, Fujifilm X-T1 and X-T2, Olympus E-M1, and Sony A6xxx and A7 models — These cameras use phase detect sensors built into the imaging sensor. That usually means that some pixel positions on the sensor (73 for the older Nikon 1 models, more for newer ones) are dedicated to measuring focus information, not image data (Canon's approach is slightly different). It's easy enough to hide those focus positions because we've got millions of imaging positions and can easily interpret the imaging data that they might have gotten (Canon’s approach still collects image data from the focus positions).
- The m4/3 cameras — While the Panasonic and lower-level Olympus cameras are mostly contrast-detect focus, these two companies pushed three things that were different into their mirrorless cameras compared to their compacts: (1) faster imaging frame rates for focus (now at least 120 fps and as fast as 240 fps); (2) faster lens focus motors; and (3) more sophisticated check-and-jump focus algorithms. Panasonic now also uses a lens characteristic table to determine where to move the focus based upon the blur the sensor sees.
There's strong demand for mirrorless systems to have focus performance more like DSLRs than compact cameras, thus there's a lot of R&D tackling the problem. The m4/3 cameras were better than we were used to from contrast-based focus systems when they first appeared, and have gotten better with each subsequent generation. But most of them are no real match for the Nikon 1’s dedicated phase detect system in decent light, especially for tracking moving subjects.
The many phase-detect-on-sensor systems now appearing on mirrorless cameras are highly variable in nature. The Sony A7II is okay, but not close to DSLR performance, while the Sony A6xxx models come much closer. In order of performance, I’d call it this way from best to worst: Nikon 1 (all models), Sony A6500, Fujifilm X-T2, Olympus E-M1 II, Sony A6300 and A7rII, Sony A7II, and finally the Canon EOS M5/M6.
Where we stand today is here: almost all mirrorless cameras have focus performance somewhere between compacts and DSLRs, and the average is getting much closer to DSLR-type performance. The Nikon 1 cameras are very close and maybe even equal to DSLR performance in good light, while a few cameras—the older Canon EOS M3 comes to mind—are closer to the best compact cameras in focus performance.
Overall, here's how I'd characterize focus performance for mirrorless cameras today:
- Static subjects — Anywhere from very good to superb. Even the contrast-based cameras are getting very good and fast at focusing on static subjects. There's very little focus lag when you're in Single AF mode on any mirrorless camera. Worst performer: Canon EOS M3. Best performers: the Fujifilm X-T2, Olympus OM-D E-M1 Mark II, Panasonic GH4, and Sony A6xxx or A7 models.
- Moving subjects — Only the Nikon 1 cameras really get truly close to best DSLR performance, and then only in good light with 1 Nikkor lenses. The m4/3 cameras have certainly improved at Continuous AF, but frankly the "miss ratio” on erratic and fast moving subjects is still too high to rely upon them for this on anything other than the Olympus OM-D E-M1 Mark II and maybe the Panasonic GH5. Fujifilm, Olympus, Panasonic, and Sony are all closing the DSLR gap, though, and set and handled properly are usable for many continuously moving subjects. Still, your “hit rate” probably won’t match a good DSLR set properly. The laggard in autofocus is surprisingly Canon, and Continuous AF is unusable for motion on the EOS M3 in my opinion, and is still lagging a bit with the EOS M5/M6.
I expect these things to change (for the better) over time. But today the mirrorless cameras are pretty squarely between compacts and DSLRs in autofocus performance. Expect better than your compact camera (at least from the leading mirrorless cameras), expect worse than your DSLR (though the Nikon 1 may be an exception if you've got a very low end DSLR).