When mirrorless cameras first became available, the way in which they autofocused was essentially in the same way as a compact camera, a system known as contrast-detect AF. This uses the main image sensor to measure the point at which contrast is highest as a lens moves through its focusing range, and as this is the same sensor that ends up capturing the image, the method is very precise: what you see is what you get.
But it’s also quite inefficient. In effect, the camera has to focus to and fro until it zeros in on the focus distance that gives the sharpest image. Compact cameras have much smaller lenses with lighter internal elements, and these can move efficiently when focusing. Mirrorless cameras have larger sensors and hence larger and heavier lens elements, so this trial-and-error focusing method becomes painfully slow.
DSLRs focus using a different method called phase-detection AF. This is typically much faster than contrast-detect AF because it compares two versions of your subject from two different angles and can quickly determine which way to refocus the lens and by how much. Technically there is the potential for it to be less accurate, as the focusing sensor used to do this is separate from the main image sensor, but it’s certainly fast.
Mirrorless cameras can’t use a separate phase-detection sensor because it would obstruct the light reaching the sensor, so for a long time mirrorless cameras couldn’t really compete with DSLRs for autofocus speed. Soon, though, mirrorless camera makers found the answer – they managed to integrate phase-detection autofocus into the camera sensor itself. The latest hybrid mirrorless autofocus systems use phase-detection AF points for speed and contrast AF for precision, and challenge DSLRs for speed.
At the moment, Canon and Nikon have taken slightly different routes in DSLR vs mirrorless technology. For example, the Nikon D850 DSLR uses phase-detection autofocus for viewfinder shooting but contrast AF for its live view mode, whereas the mirrorless Nikon Z 6 has a hybrid on-sensor phase detection system that handles both viewfinder and live view photography – the mirrorless Nikon has a technical advantage.
But Canon uses its on-sensor Dual Pixel CMOS AF phase detection system on both its DSLRs and its mirrorless cameras, so the EOS 5D Mark IV DSLR offers phase detection AF for both viewfinder shooting (via a dedicated sensor) and for live view (via the main sensor). Canon’s mirrorless EOS R and RP cameras use a more sophisticated version of this Dual Pixel CMOS AF system for both viewfinder and live view shooting, but the technical advantage of its mirrorless models over its DSLRs is smaller.
￼Focusing is only one part of what makes a camera suitable for a particular type of photography, and many sports photographers continue to use DSLRs rather than mirrorless cameras. Even so, mirrorless cameras can now be successfully used for this kind of photography where they would have previously been completely unsuitable.