Why Mirrorless Autofocus Means Trade-Offs

Why Mirrorless Autofocus Means Trade-Offs

Mirrorless cameras like the Sony A7 don’t use a traditional phase detect autofocus. While this enables popular features like eye tracking autofocus, it brings a number of tradeoffs, which could be a deal-breaker for certain applications.

Two Systems Enter

The phase detect system, typically used in DSLRs, works by directing a small portion of the light which enters the lens away from the viewfinder and into a separate autofocus sensor array. After many years of development and refinement, this system is very polished. Cameras like the Nikon D5 and Canon 1D X Mark II can feature 50+ autofocus points and advanced tracking at high frame rates. This makes these systems the gold standard for sports and action photography, particularly when combined with the high framerates these cameras are capable of.

Mirrorless cameras can use a number of different methods, including on-sensor phase detection, contrast-based autofocus, and a novel twist on contrast AF called depth-from-defocus. Each offers different performance characteristics, but all rely on information from the camera’s sensor. This can enable better autofocus performance during video, face/eye detect autofocus, and a reduction in alignment issues. Most mirrorless cameras will use phase detect points built into the sensor, with contrast AF supplementing it. Depth from defocus is a particular implementation from Panasonic which relies on already known information about lenses characteristics to supplement contrast AF techniques.

With that overview of the different AF systems out of the way, I’d like to focus on the tradeoffs present in the two current, leading implementations: phase detect and on-sensor phase detection.

Under the Glass

The phase detection points on a mirrorless camera’s sensor sit, as indicated by the name, right on the sensor. This means they are underneath the filter stack, which includes an IR filter. This filter, which cuts out infrared wavelengths, is necessary for accurate colors, but causes the first major issue for low light use. In low light, AF performance drops regardless of system, since the sensors have less light and contrast to work with.

Wedding and event photographers have had the luxury of using AF assist, a feature that projects a mostly-infrared grid of lines onto the subject, typically from a portable flash. Since their mirrorless camera’s AF sensors sit below the IR filter, that grid is no longer visible to the system, unlike in a traditional phase detect system. Speedlights, like Nikon's SB-5000 prominently feature the AF assist light. Others, like Godox X1T-S, have an assist lamp closer to the visible spectrum, meaning better performance with mirrorless cameras.

Under that red plastic sits the AF assist emitters, which create a grid of points to help the camera focus in low light

This means mirrorless cameras are further disadvantaged in one of the most challenging AF scenarios, owing to the low light and fast movement. Some assist lamps rely on a more prominent visible light, which can be distracting to subjects, and still doesn't offer the same grid that phase detect systems can grab onto.

While some DSLRs combine information from their metering sensors with the AF sensors to provide face tracking or scene recognition, mirrorless cameras have access to all the information coming off the sensor. This is most recognizably implemented in eye AF, a mirrorless feature that enables precise focus on a subject's eye across the frame.

One advantage of sitting under the filter stack is how much easier it is to retain AF performance with less maintenance. Since the sensor and AF system are on the exact same plane, alignment shifts are much less of an issue. On traditional systems, the PD sensors are an additional location that could gather dust, while on mirrorless, cleaning the sensor also cleans the PD points. This can also mean greater reliability as the camera ages, since the sensor can't move independent of the PD points.

One last advantage is in the number of points. Nikon’s flagship D5 offers 153 AF points, while their mirrorless flagship has 493 AF points. These can’t be compared apples to apples, but a greater number of points spread further across the sensor means good things for landscape, still life, and portrait photographers. They can select the exact focus point across almost the entire sensor, preventing the need to focus and recompose. This is especially important with high megapixel bodies, as even slight focus shifts can show up in the final image.

The mature nature of the DSLR’s phase detect systems means they’re a virtual jack of all trades. While mirrorless cameras are catching up to flagship DSLR AF systems, particularly in cameras like the Sony A9, they are still typified by tradeoffs.

What’s the Right System for Me?

While I imagine few photographers will base their system choice entirely around autofocus, knowing the tradeoffs, benefits, and downsides to each AF implementation can be useful.

Phase detect in DSLRs:

  • Leading performance at high frame rates
  • Mature implementations with well established modes
  • Quicker initial response and confirmation
  • Smaller area of the total image covered by AF points
  • Alignment issues can require repair or recalibration
  • Potentially less information available for features like eye AF

On sensor phase detect

  • Near DSLR levels of performance, particularly in bright light and with cameras like Sony’s a9
  • Higher accuracy for still subjects
  • Face/eye recognition means easier use with human subjects
  • Inability to use IR AF assist
  • System may fail to find focus, falling back to slow contrast AF
  • DSLR AF features and modes may be missing

With the newest generation of mirrorless cameras, the gap in AF performance has definitely closed. What still remains, however, are a number of key differences. Depending on your subjects of choice, mirrorless cameras may be a perfect fit. The consistency and accuracy of the system, combined with the higher number of selectable points and new features like eye AF are a natural fit for landscapes and portraiture. Meanwhile, action and event photographers who rely on tracking and AF assist devices may need to hold off.

Have you found your perfect match in an AF system? For what I shoot, I've been very happy with the performance of my Nikon Z7, but I definitely can see a step back in tracking compared to my past DSLRs.

Lead image courtesy of Tom Pumford

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Previous comments

Well I came from a 5d2 so a any workable AF system would make me happy!

Interesting article. I hadn't considered the effect of the IR filter. But, that also made me wonder if the time during exposure when the mirror is up has the inverse effect on DSLR focusing capability? I'm curious, when you bought the Z7 you must have known the a7rIII would have better AF performance? Was your choice a personal preference for ergonomics or menus or use of existing Nikon glass?

Alex Coleman's picture

Number of factors- similar controls and menus to my existing gear, able to overlap most lenses with flawless adapter performance, good prices, fewer image quality concerns (star eater, raw compression issues).

The mirror up time most likely does. Nikon claimed the D850 had the exact same AF system as the D5. However, AF tests showed that the performance in sports shooting conditions didn't live up to the claim. It's theorized that the time the mirror was up negatively affected the D850's AF, due to having a slower fps, and hence, using a slower mirror mechanism. The AF system had less time to "see" the scene due to the mirror being away from its rest position for longer periods of time between shots, giving the AF system less data to analyze from the scene in the same amount of time vs the D5's faster moving system.

Alex Coleman's picture

Arguably, the slower FPS would give the mirror more time in the "down" position, resulting in the AF system having more time to work.

I think either view is an oversimplification, since the D5 may have more processing power available to the AF system, the D850's AF may be geared to accuracy for the higher MP sensor, etc....

I think that what many studio shooters think is poor low light AF for the Sony cameras, is actually very largely an issue that could so easily be solved with a firmware update.

When shooting in a studio, you have to turn "settings effect" off (otherwise, you're looking at a preview of what the camera estimates the exposure to be based on ambient light.... which is a completely black frame)

The problem is, with "settings effect" off, the lens stops down to it's shooting aperture to acquire focus.

So if you're shooting at f/11, the lens is stopped down to f/11 during AF. That results in a lot of hunting and very hit or miss AF.

For this reason (and ONLY this reason), I've hung on to all my Canon gear (IE, I only use it for studio shoots). So it may be a bit of a nuance to what the author described, but I totally agree with the overall gist of the article.

Alex Coleman's picture

Hi Jacob, good idea. I'll have to do some more reading on that, since it seems each brand implements low light/stopping down behavior differently.

Uh, correct me if I'm wrong but Fujis have Phase Detection.

Alex Coleman's picture

They do. Can you give some more context for your question?

nice video showing comparison between best mirrorless and best DSLR AF systems (and some other things) ... spoiler .. A9 is not there yet but still damn impressive (that buffer unloading is joke thought :) )


Colin Robertson's picture

And then there's dual pixel AF on Canons... I'm surprised this wasn't mentioned in the article as it's used on all of Canon's mirrorless cameras. Anecdotally, I find the AF on my EOS R to be more accurate than my 7D (using the phase detect sensor), but slower to acquire. I think a lot of that is due to lens selection (I don't have any native RF lenses yet), and processor speed.

Alex Coleman's picture

Dual pixel is on sensor phase detect, so there's no need for a separate entry. It carries all the pros and cons inherent to that that system.

"...retain AF performance with less maintenance..."

And that, right there, is why I'm absolutely done with optical PDAF, and oh-so-ready for on-sensor hybrid AF.

Sick and tired of my well-used lenses and bodies needing to be calibrated (micro-adjusted) every few months. Sick and tired of not being able to totally trust my camera within even just a few weeks of my most recent calibration.

If anyone has ever tried the Sony A9, they'll know that the future of reliable autofocus is absolutely mirrorless. The Canon EOS R is no slouch either, it's way more reliable with both of the new f/1.2 RF primes compared to a 5D4 and the older f/1.2's. Nikon needs to really put some R&D $$$ into this, though they do show a decent amount of promise.