Autofocus (Subject recognition & tracking)

By Rishi Sanyal

As we alluded to on the features page, the D750's AF system works with the D750's metering sensor to provide advanced subject recognition and tracking, including face detection, during through-the-viewfinder shooting. As a refresher, take a look at what we believe the D750's metering sensor 'sees' when evaluating a scene. Given the high resolution of the metering sensor it's able to pick out not just faces, but also fine details like eyes, which extends to tracking movement across the frame.

As we showed on the previous page, this is a representation of how we believe the 91k pixel metering system 'views' a scene. We took a full-size image and scaled it down to 213 x 142 pixels, then blew it back up for ease of visualization, then overlaid the D750's AF grid. 213 x 142 x 3 (for R, G, B) = 90,738, or approx. 91k-pixels. The image above is downsized so click to see the full-size version.

Below is an example of D750's automatic face detection when shooting through the optical viewfinder. The camera automatically focuses on faces when 'AF area' is set to 'Auto', in both AF-S and AF-C modes. Even if there's an object closer to the camera, the D750 will stay locked on a face.

In most situations the D750's face detection system (which can be used when shooting with the optical viewfinder) works as advertised. Cameras without this feature tend to focus on the closest object which isn't always desirable.

Perhaps one of the most compelling uses for the RGB metering sensor is in 3D focus tracking. All AF-continuous modes engage tracking along the depth axis; 3D focus tracking mode also tracks a subject around the frame, taking it up to three dimensions. The fine resolution of the D750's metering sensor, combined with tuned AF algorithms, enables recognition of detailed patterns to help track subjects accurately - by automatically selecting the appropriate AF point to stay locked on the initial subject.

The D750 stays locked on the subject's eyes despite the camera moving side-to-side and forward and backward. That is, the camera tracks the subject in three dimensions, hence the term '3D tracking'.

In the video above you can see how the camera stays locked on the eye of the subject as the camera is moved in three dimensions. As the enlarged photos show, the D750 is spot on nearly every time. 3D focus tracking works well enough to 'trust' the camera's focusing system to follow the subject while the photographer concentrates on more important things, like composition. Essentially, you choose the subject you want in focus by initiating focus on it, and the camera then worries about focus - leaving you free to recompose, or the subject to move, without ever having to worry about losing focus on the intended target.

Mothers shooting their erratically moving babies and wedding photographers shooting running brides down the aisle: rejoice. You can shoot your fast primes wide open and trust that the camera will stay on your subject, so you're not forced to select an AF point manually (which you often don't have the time to do) or constrain your composition just to make sure your selected AF point still covers your target. And avoid the plane shift when focusing-and-recomposing: the camera automatically refocuses for you after you've recomposed.

Below is another example, taken in continuous shooting mode. This is quite stressful to the AF system, as we're shooting with the Nikkor 35mm lens at f/1.8, where depth-of-field is measured in centimeters for the subject distances chosen. Further complicating things is the fact that the camera is in 6.5fps continuous shooting mode, with the mirror flapping upward 6.5 times a second, essentially 'blacking out' the AF system between each shot. Still, the camera is able to stay on the subject in three dimensions - both selecting the appropriate AF point to stay on the eye given the recomposition, as well as refocus along the depth axis to keep the eye in focus.

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Even when shooting continuously at a relatively fast frame rate, focus tracking still works remarkably well. Consider us very impressed.

This capability isn't actually new to the D750. Nikons have had some form of subject recognition and 3D tracking for many generations now, even utilizing a high-resolution 91k-pixel RGB metering sensor since the D4 and D800. The thing is, it's been iteratively getting better, with higher accuracy when Nikon stepped up to the higher resolution metering sensor, and now simply class-leading speed and overall results given the increased processing power and clever algorithms.