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.

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