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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73 Comments

EL PIC's picture

Very Interesting..
FWIW I often AF twice on my 5D.
Pre focus by half way pressing the shutter and then repeat with fully pressing.

The worst picture you can take is OFF (Out of Freaking Focus).

Mirrorless is infinitely better in this regard, because, as stated in the article, the imaging plane and focus plane are the same singular part. As opposed to DSLRs that use two separate parts at different angles, using mirrors to reflect light at the AF sensor, which creates alignment issues. You never have to AF twice on most OSPDAF cameras to get it right the first time. The biggest reason I switched from DSLR to mirrorless, and I'm still amazed how accurate these cameras are.

Alex Coleman's picture

Have you tried back button autofocus? I've found that it is easier to control than that half-press option. https://fstoppers.com/originals/back-button-focus-what-it-and-why-you-sh...

Peter Bakos's picture

Although this is personal preference, I don't see the point configuring my camera for *constant* focus & recompose (back button focus) rather than using the default for *ocassional* focus and recompose (configuring focus hold). Back button focus actually slows you down considerably in sports/wildlife photography since you have to press two buttons within short period of time to take a picture in focus, whereas with focus on shutter button definitely reduces response time.

EL PIC's picture

Yes I have used back focus but as above user says ... it slows you down.
I m old school and have done half press then full press many many years.
We all have preferred work flows and where we want to put our fingers.
Focus Twice .. Expose Once is my long term words of wisdom.
It’s kinda like Measure Twice and Cut Once.

Funny how some in this thread disagree with your statements about Mirrorless Focus ..
I m under the impression that double focus and expose once is improved and more accurate but has less effect these days than the old days of auto focus lenses.
It is still valid for exposure and re comp.
You need to write more on this.

You have this completely backwards, back-button focus speeds things up. It's how most pro sports and wild-life shooters operate. It allows them to separate AF and shutter into 2 independent operations. Activating the AF button eliminates the half-press operation, which allows you to operate the shutter instantly when a minor delay can be the difference in capturing the moment or losing it. The other way works fine for portraits or landscapes, but really slows you down in high-speed situations.

Just because you shoot a certain way doesn't mean it's the only use-case scenario.

Marcus Joyce's picture

It took me time to learn back button focus but you set your camera to constantly focus.

It's less stressful as your not doing a micro adjustment on your index finger for the half press.

Peter Bakos's picture

Some can argue, it is more stressful having to press two buttons instead of one..

Peter Bakos's picture

Can you back your statement "It's how most pro sports and wild-life shooters operate" up ? Because I don't trust it. Pressing two buttons instead of one will slow you down most definitely. Also, there is certain latency when pressing shutter button and not using the half press at all, since you STILL have to get through the half press anyway.. just think about it, instead of following some trends online.
Check this reasoning out - https://www.essence-of-light.com/debunking-back-button-autofocus/

Jozef Povazan's picture

Peter, back button focus does not slow you down at all. What it does it allows you to select a focus by pressing a thumb on the back of camera and lock an that subject part precisely. Then you simply wait for right moment and action happening within your frame and if your most important part of the frame is still where you back button focussed you can simply keep holding your thumb on AF ON and fire with your other finger shutter single or multiple times... For predictive actions in sport wildlife weddings etc... this is a must and it saves you lots of missed shots :) you do not need to believe it but when you try and learn it you will be amazed how things change in your favour :) happy shooting guys... and yes I find Z7 focus more precise in AFS, but AFC still does not compare to D4s or D850 IMO :) cheers

How does pressing 2 buttons simultaneously slow you down? Back button focus is the best thing since sliced bread lol. So many people get stuck in learning one method of things and cling to it with such a narrow mind. I've used a 1dx a d5 an a7iii an neither one of those cameras leave me wanting to use it over my a7iii not even the a9. " Pro's "only use dslrs because they either work for a company not willing to reinvest in upgrading gear, dont know about cameras just know photography and will repeat "its all about the photographer not the camera" or someone who's personally invested in thousands of dollars of high end glass that have no need to upgrade their gear absed on what they shoot. The DSLR is dead... i really wish people would just let it go... this is the same thing as people making an argument for a flip nokia after the iphone 3gs has come out... times change.. tech evolves lets stop the dslr vs mirrorless .. it's realllllllyyyy not honest for those who are looking for information on cameras. Nobody buying a camera in 2019 needs to buy a dslr over a mirrorless camera. Theres no justification what so ever... this isnt 2012

Alex Coleman's picture

Muscle memory can definitely make switching to back button a trade off. If you're happy with your current technique, keep it up!

Tony Politi's picture

With BBF you set the camera to continuous focus and hold the back button down. If you want to focus and recompose you simply release the back button. In general your finger is always pressed and you simply press the shutter button at the correct moment. This in no way slows you down. It takes a bit to get used to but I find it vastly superior. Additionally most of the time I am half pressing the Shutter button. It is also nice when you are shooting a still subject as half press will activate IS without changing your focus.

Studio 403's picture

Interesting post. I have a fujifilm XH-1 I bought the godox camera flash to use. I find the flash will fire when not pressed at times and when I use manual mode, the godox sometimes does not sync with camera, and when I use TTL, I don’t get an accurate TTL , mostly overexposed. Perhaps this is some of what you are saying...I know I am in a quandary for predicable results

Eric Salas's picture

With your Godox while using TTL have you changed your metering modes at all? I steer away from using TTL 90% of the time now but found that unless I was using spot metering my results would be fluctuating drastically.

Studio 403's picture

Good info, did not know this....so your saying if I use spot metering when going TTL? I have the same thing happen when I use manual mode. Godox does not always duplicate the manual settings or read properly. Can’t figure out if the body or Gordon unit, Ken thanks for the tip

Eric Salas's picture

Yes, metering modes can throw your TTL into WTF very quickly which may be why you’re experiencing problems.

If it’s metering across the entire frame instead of just for your subject your results will be all over the place.

As for the unit not “duplicating settings” I’m not sure what you mean by that.

Motti Bembaron's picture

I am no expert but here is how it works (somewhat); When you are on TTL and you press the shutter, the flash shoots twice, once to illuminate the scene -for the camera to read- and one to actually light the scene after the camera "told" the flash how much light is needed.

The camera reads the first 'flash' and determines how much light is needed to have perfect exposure, relates it to the flash and the flash shoots a second time.

This happens so fast that for our eyes it looks like one flash. But it is actually two.

Therefore, when the flash goes off the first time - so the camera can read the (and calculates) how much light is needed - it is better if the camera reads the light reflected from ONE SPOT. If the camera relays the information it reads from a few spots, like a white shirt, a black-tie or a dark red dress, the flash has no idea how to translate it and that's why it's different each time.

I personally almost never work in TTL, for that reason and also because A) It drains the battery quicker B) Recycle time is way longer and C) The flash heats up faster. All because it actually shoots twice for every time you press the shutter.

I hope I was accurate in my explanation :-). Any input is welcome.

I heard from some Sony and Fuji users that the flash is not consistent. I use Nikon (D750) and a colleague of mine uses Canon (D5 Mark IV), we both use the V860 II and either TTL or manual, it is extremely accurate. But it could be the flash unit itself.

Might just be poor firmware. No issues on my Sonys with the TT685S flashes except a bit of lag before the camera shoots (TTL preshot metering).

Alex Coleman's picture

Interesting issue. It could be a firmware problem, given the different brands involved. I've been happy with TTL when using Nikon speedlights with my Nikon cameras, but I've found manual to always be consistent.

michaeljin's picture

In bright light the a9 definitely beats any DSLR on the market today by a good margin in AF speed, accuracy, and tracking so it's not "near DSLR performance", but rather above DSLR performance in that scenario. Low light performance has gotten a lot better, but it is still an issue for most MILC's so that would be the DSLR's primary strength at this point although the difference isn't nearly what it used to be.

Rk K's picture

If the a9, a6400 and a7r4 didn't exist this article might have a point. But they do, and the a9 has a faster, better af system than top of the line dslrs for almost every situation.

Umm... No.

Colin Johnson's picture

Umm, yes. I sold my D5 and D4s and bought 2x A9s when they were released. I compared them side by side in a number of scenarios, including low light and found the A9 AF to be as good as the D5. The AF has now improved to better the 3D tracking on the D5.

I'm not saying it can't be as good, or even better, in certain situations. Rk K stated it's "faster, better … for almost every situation" and that is NOT true!

Pat as someone who shot with the a9 and top-end nikon dslrs prior to that, I can tell you for a fact the a9 is superior. And that was before the real-time tracking update. Now? It's not even a contest

I admit to never having shot with the a9 or the D4/5 so all I can go on is the reported experiences of others. Some say the DSLRs are superior and others, like you, say the a9 is superior. The majority say it depends on the circumstances. Without calling anyone a liar, which I'm loath to do, I can only assume each person's experiences are based on the circumstances they encounter. My biggest mistake was weighing in on this subject which, to be honest, I don't even care that much about. What can I say? I'm bored. :-(

Colin Johnson's picture

Pat OConnor

Well, I owned a D5 with super fast XQD cards and had one in my D4s.
The 3D tracking for fast moving sports was sometimes hit and miss with the D5.

With the A9, it nailed fast action every time.
I was shooting full contact karate tournaments and the action comes very fast, so you need to be ready and have a camera that will get the shot on focus, otherwise you missed it.
There is no second take!

Years and years ago, I was fought in tournaments but tried to avoid full contact (on myself) as much as possible! ;-) Point taken.

I don't understand how AF on the sensor benefits landscape photography. I either calculate or guesstimate the hyperfocal distance but still focus on one point, almost always within a DSLRs grouping of AF points. Even then, I manually focus more often than not. And given the typical apertures involved, accuracy isn't nearly as important as almost any other genre.

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