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

Previous comments
Alex Coleman's picture

Manual focus is a great option and mirrorless makes accurate MF even easier via the emphasis on "live view" inherent to the camera.

One prominent landscape example is that classic foreground flowers with a mountain background - that shot is made easier by the wider spread of AF points across the frame.

If you're guessing and are stopped down enough that everything will be within your DOF (or diffraction area), then AF at all doesn't provide a benefit. What's going to most help you here will be an accurate focus scale, on the lens, or in the camera.

Where AF at the sensor makes a big difference is in short DOF scenarios, where microfocus adjustment would be an issue. Even in landscape shooting, not every shot will have infinite DOF. Sometimes you'll want to isolate a subject.

Agreed but, even in those situations where I use a shallow DOF, I've not had any problems with DSLRs. I'm sure there are occasions that ML is better but I can't see them improving my keeper rate which is mostly hampered by my questionable composition skills. ;-) Even for narrow DOF portraiture, I don't have any problems, but that may be due to subconsciously adapting my technique to my camera's limitations.

For near DSLR levels of performance, read 'exceeds DSLR levels of performance like for like in most cases when we'll implememted.

If I understand what you're trying to say, I disagree, but I'm not sure I understand.

Alex Coleman's picture

For accuracy, mirrorless has won in my experience. But for tracking or low light with assist, DSLRs still win. That's really the basis behind this article: for certain applications, one type of camera provides better results.

I was insanely tired when I wrote that, sorry. But I think the two first lines for each system are still a little skewed.

FOR DSLRs you've got said they're still best at high frame rates, which I don't really agree with. Sonny's Alpha range for example, especially with the new firmware, is insanely accurate at high frame rates, especially compared with DSLR counterparts, who can't even hit the same heights of speed. Fuji's Sports Finder and new PDAF system are very good too, even in the X-T30.

You then say that CSCs offer near DSLR quality focus M, even in bright light. I'd argue that, although not quite there in low-light (and I mostly agree with that), in bright light the benefits elevate a good mirrorless AF system beyond DSLRs in almost every other area. They're often quicker, just as if not more accurate and things like Eye AF are built natively into the system.

Even the EOS R, which has problems, says it focuses to - 6EV (with a lens caveat). And on static subjects (wouldn't dare try it in low-light), I found that was pretty accurate.

I just don't think it's as black and white as it seems.

Alex Coleman's picture

It isn't black and white, and a single article over the two main camera systems can not cover every minor detail. This is more a roundup of potential pros and cons of the two systems.

Each of these are generalizations. While some mirrorless cameras are in the top group for continuous/tracking, DSLRs typically beat their comparable mirrorless, excluding something like the A9. Haven't tried Fuji's sports finder yet, so I can't speak to it.

I distinguish between performance and accuracy for mirrorless, based on my experience with them, so for your last two major paragraphs, I think we agree. Mirrorless in good light is the best AF experience for most subjects, especially at high MP.

I think for me, the biggest pro has to be adapting the entire frame into AF points. It's eradicated the focus and recompile issue that leaves people wondering why they're OOF below F4. Game changer. The acquisition is generally better in my experience, and usually because you're not confined to central points.

The Sports Finder is okay, Fuji has come a long way, even since the X-T2, and it works well with the limited testing I did. Obviously sky you've still got some contrast detect systems that struggle in low light, and others are just below the pay grade of pro cameras. The Nikon Z's struggled at launch for low-light, but they've been given an extra EV rating. Haven't tried the Z6 yet, but getting the Z7 in for work tomorrow, so will see how it handles compared to launch.

Generally, I agree with low-light, though don't think that'll be an issue for long. And in the grand scheme of things, I'd have to give like for like at the top end a mirrorless win for benefits compared to equivalent DSLRs. I know there will be a few A9s at the Olympics for sure.

Jacques Cornell's picture

Next article: "Why Reflex Autofocus Involves Tradeoffs". Front/back-focus, dontcha know.

Alex Coleman's picture

Mentioned that as part of alignment issues :)

That was actually one of my biggest issues before I switched to mirrorless, since a number of my lenses needed AF calibration.

Jacques Cornell's picture

So why the headline singling out mirrorless as somehow hobbled? Happens a lot around here, and it's annoying.

Alex Coleman's picture

This article evolved out of a discussion among writers regarding AF assist lights on mirrorless. It's positioned as looking at mirrorless since more photographers will already understand the experience with DSLR AF systems, which can provide a frame of reference.

Jacques Cornell's picture

My issue is with the headline and its singling out of mirrorless as having AF issues. The implication is that mirrorless is somehow more, or even uniquely, compromised. A layman who reads only the headline will conclude that mirrorless is to be avoided.

To Alex Coleman , I have a few basic questions:
1) What is the purpose of AF, that you're looking for ? Are you looking for speed ? Are you looking for accuracy or are you looking for low-light ?
-Answering this would define what you're inherently going to give up.
2) How would you be using this AF for ? I.e. in what condition and environment would u be mainly using AF for ? If you're a landscape photographer, AF pretty much isn't even a main or crucial "thing" for your camera body that you would considered to be important.
3) Have you use a A9 before ?
4) Have you done extensive review between the A9 vs. D5 vs. 1D ?
5) Have you use and done reviews on Panasonic S1R or the Fujifilm GFX100R? These are mirrorless too which uses mirrorless AF as u spoke, but it seems that you haven't thought much about them. They should be just as relevant talking point about ur post title.
6) Z7 is still new, and it's probably too early to judge or generalized all other mirrorless AF inferior to that of traditional DSLRs. Perhaps you're one of the many disgruntled Nikon users? (No offense) Then try switching to others.

Remember, photography is an art, camera and lens are TOOLs. If it's preventing you from creating your art. Then switch it, it's just a tool after all.

In conclusion, technology advancement would nearly always be an improvement over older tech. I.e. driving a stick shift vs. fully automatic vs. paddleshift drivetrain cars. Are there any trade-offs? Certainly. But the gains outweighs them; that overtime it'll be a non-issue.

Rod Kestel's picture

A fairly complicated article which I only partly understood, but thanks for the explanation.

The main thing that strikes me is the technical differences may or may not be meaningful. The real test is...a real test putting cameras side by side to see how they perform in the real world. What does all this mean in practice?

Alex Coleman's picture

Hi Rod- I'm happy you found the article useful.

The big two differences I think you'll see in the real world have to do with the AF assist light and the number/spread of AF points. Both of those will be clear even without side by side comparisons.

I'm sure you are aware of this but both of the Olympus OMD EM1 cameras have both phase and contrast autofocus and with the latest firmware both of these cameras autofocus to -6 EV. It is a delight to use, fast, accurate, includes continuous with tracking and continuous during video.

Alex Coleman's picture

That hybrid process seems like the best approach. I'm seeing it implemented in more mirrorless cameras, to good results.

I noticed that the Canon DPAF was purposely ignored that on the R can AF just fine in -6 EV.
Why was this ignored?

Alex Coleman's picture

Hi Lawrence- DPAF is on-sensor phase detect, which was the main focus of the article.

In really low light conditions, there are more impactful considerations than the EV spec of the AF system, such as the contrast in the subject. Top cameras can do -4, -5 or -6 EV, but the inability to use IR AF assist lamps is a more significant differentiation.

Jacques Cornell's picture

Why do you and other writers here insist that AF aid lights must be IR? The IR grid lights are visible. Red-eye-reduction and TTL preflashes are plenty distracting. And, at a wedding, the video light is way more distracting than an AF aid light from a mirrorless camera. Setting aside the question of whether IR grids are more effective than visible-light spots, I just don't see a big "distraction" issue with the built-in lights on my mirrorless cameras in my low-light corporate event work.

Alex Coleman's picture

I'm not insisting that they "must" be IR.

Most assist lamps are primarily IR, while I even mention by name the one that is less skewed towards IR.

A visible light assist lamp faces two issues:
1) for the same AF-apparent brightness as IR, it will be much more visible to people
2) depending on how the on sensor phase detect pixels are setup, they may be under the Bayer array, which will only let particular colors through and therefore require a further brightness increase

Note that there's an asterisk that should be added to the EOS R's -6 EV, in that it's only possible with the 50/1.2. A 2.8 lens will bring that to -3.5 EV. Not sure exactly what the standard F-stop is for measuring AF sensitivity, but it's not 1.2, as not all systems have one of those lenses, nor have they really been considered good at actually AFing (speed and/or accuracy) until maybe the RF one (in accuracy, at least, being mirrorless).

Alex Coleman's picture

Exactly. More is better, but low light AF involves way too many factors to say that -6 makes the EOS R the best for low light. For some photographers, missing AF assist is way more important. Goes back to the tradeoffs inherent to mirrorless vs DSLR AF :)

Not bad. A few other differences to consider, though:

1. Off-sensor PDAF normally has at least one, and usually several, cross-type sensels, that are either vertical and horizontal, or two diagonals. Some PDAF arrays include single diagonals as well. Mirrorless PDAF implementations pretty much all use only horizontal sensels, which can result in difficulty in acquiring focus on certain types of patterns, like horizontal lines.

2. Mirrorless cameras typically autofocus at taking aperture. This is not a requirement, but it is the way these systems usually work. This can result in less light for focusing in dim conditions, but it has the benefit of avoiding focus shift that can occur when stopping down from wide open to take a shot.

3. In the case of infrared conversions, mirrorless cameras gain a clear advantage over DSLRs. In DSLRs, an infrared converted camera has to be adjusted, normally being matched to a single lens, since IR and visible light focus in different planes. However, for a mirrorless camera, which focuses at the sensor, focusing IR is the same as focusing visible light, and no adjustment is needed.

4. Mirror shock and the need for mirror lockup, and the (admittedly small) amount of time it takes the mirror to flip and return, are all not issues with mirrorless. This is why the Sony A9 is able to achieve higher framerates than the legacy "gold standards" of sports shooting, continuously tracking at 20 FPS, and without intraframe blackout.

Alex Coleman's picture

Hi Paul. Interesting points.

1) The newest implementations of mirrorless will supplement with a contrast detect step for these subjects.

2) This behavior varies based on camera, making it tough to generalize, but only reiterates the disadvantage in fast moving, low light situations.

3) I haven't shot an IR converted mirrorless, but I can definitely see how that would come in handy. Interesting to think about!

4) Good to point out! Tried to keep this general for mirrorless, as the A9 is in a pretty unique spot, but good to mention.

Jacques Cornell's picture

Regarding point 2, as far as I can tell, this is not true of my Panasonic cameras. Folks around here seem to overgeneralize about mirrorless cameras as if they're all the same. They're not.

I struggled for a long time with Sony AF in dark event photography situations. For a while I just struggled. Then I used a small LED on a bracket to light up the room some more for AF.

And then I just used 1.4 lenses. What a revelation that was. Cameras that struggled at 2.8 focused flawlessly at 1.4. Sure, you get the occasional out of focus shot because of the kind of aperture you're working at, but focus was usually consistent enough to get nice shots out of it.

The new Godox V1 actually has a modeling lamp that you can switch on if you need more light, and I might try that if I need to shoot at smaller apertures in the dark again.

The flip side to this is the Sony AF works flawlessly in any light that's not complete darkness. With the a9 in particular, its just perfect with any kind of action or sports. Everything is in sharp focus all the time, no matter what you're shooting, it's crazy.

Alex Coleman's picture

That LED is a good call. Sometimes just a little extra light from f/1.4 or an external source makes all the difference.

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