On many cameras, subject tracking (choosing your subject and letting the camera track as you hold your composition) has gotten very, very good.
Nikon AF-S 24mm F1.8 @ F2.8 | 1/8000 | ISO 320.
Photo by Rishi Sanyal

Digital cameras, and for that matter, film cameras, have offered autofocus for a number of decades now. It's evolved from just one point to many hundreds of points over the years, allowing for varying degrees of control. You can leave the whole focusing process up to the camera and let it choose what it thinks is your subject; you can just use a single point of your choosing; or you can dance in the middle-ground using a zone or group of points that you select and keep over your subject, while the camera attempts to compensate if your subject veers toward the outside of that zone.

Outright subject tracking, though, is something else. You select the subject you want, usually with a single point or a single zone, initiate focus, and the camera does the rest. It will attempt to identify the size, color and distance of your chosen subject and do its best to track that subject around the frame, whether your framing changes or your subject moves.

This isn't a particularly challenging use case, but it's a good demo of how subject tracking works on Nikon's D5 in 3D Tracking mode. Note focus is initiated at the beginning of the run, and the camera automatically keeps an AF point over Richard while constantly refocusing; even when shooting at 12fps.

Many people don't have trust their cameras to do this, and until the last few generations of digital cameras, we wouldn't have recommended it; but manufacturers continue to invest in pushing this technology forward. Established professionals in particular are highly unlikely to shoot this way, because once you've worked one way and can reliably get results you're happy with, why would you change?

But believe us; good subject tracking is really something special, and it's worth your time to give it a go. Frankly, it has the potential to forever change the way you shoot, for the better.

Why does it even matter anyway?

There are cases both for and against using subject tracking. In high speed, peak action sports, an experienced photographer would likely do a fine job (or better job) by using a cluster, group or zone of autofocus points and follow the action his or herself. But for those who are less experienced, or when shooting at longer focal lengths where following the subject can be more difficult, or when just shooting really erratic and unpredictable motion, subject tracking can be a tremendous help. It got me a number of keepers at a rugby match on a Panasonic camera even though it was the first time I'd ever photographed rugby, and the Nikon D5 was great for low-light soccer.

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In the above example from a Nikon D5, our tech editor Rishi Sanyal initiated focus on the kayaker, fired a short burst, kept tracking him with the shutter half-pressed, and then fired another burst that kept accurate focus despite the kayaker basically disappearing underwater for a moment. This gave Rishi an abundance of options for editing, allowing him to get just the moment he was after with that may not have been possible without the use of 3D Tracking. Click here to see the final edited photograph.

It's true that most manufacturers, despite constantly improving their tracking algorithms with newer models, somehow still don't recommend subject tracking for these sorts of situations; but in our experience, it still seems to work most of the time anyway.

What other sorts of situations could benefit from using subject tracking? Turns out, a lot.

Weddings are one situation in which I find subject tracking invaluable. For this shot, you could frame up your composition, initiate autofocus on the bride at the beginning of her walk down the aisle, and fire away as she moves through the scene with the camera constantly focusing. Ironically, I didn't use tracking here because on this camera it's a menu-dive to access that setting, and single AF is super fast; but the catch is I ended up with fewer options this way. 
Leica Q | ISO 2500 | 1/125 sec | F2
Photo by Carey Rose

Events and weddings are great use-cases for subject tracking. You can initiate focus on the bride (or groom) in a scene, and simply keep continuously focusing on them as they move around, dance, interact with guests, and so on. You don't have to take the time to move your focus point around, which could results in missed shots, and you don't have to focus and recompose, which can result in missed focus when shooting at really wide apertures. You can end up with a greater variety of images and more options to choose from when it comes time to edit.

Another use case is candid portraiture. When you can reliably lock focus on a subject's face or eye and are able to move the camera around while it continues to track focus, that allows you to sample multiple compositions really quickly. It also allows you that much more creative freedom to focus on those compositions in the first place instead of constantly having to move your focus point to catch up to what you're seeing in your head. Autofocus point placement becomes just one less thing you have to think about.

The elephant in the article - just give it a try

Now, you may have noticed that most of the examples and references in this article are from high-end Nikon cameras, and the reason is not a personal bias; we've consistently called out this feature for a while now on mid-to-high end Nikons because we find it to be industry-leading.

But if you're not a Nikonian, don't fret! Almost every major consumer camera manufacturer has subject tracking in some form. Panasonic's tracking system works reliably well, in both rugby and in social situations. Sony's Eye AF feature is truly amazing. Olympus' C-AF + Tracking is fairly robust, and Canon's Dual Pixel AF is probably the best face detection and recognition system out there.

In short, experiment a little, give it a try and happy shooting!