The D800 has the same new autofocus sensor as the D4 and, while its headline improvement is that it can now focus in lower light (down to -2EV) , this isn't the only step forward. The sensor module has also been redesigned to enable it to operate with slower lenses (and lens/teleconverter combinations that give slower effective apertures). The D800 features six AF Area modes - single point, 9-point, 21-point, 51-point, 51-point '3D' tracking, and Auto Area mode (shown below). In single-servo autofocus (AF-S) operation only the single and auto modes are available. All other modes require the camera to be set to continuous-servo autofocus (AF-C).
|In single-point AF mode, the D800 simply focusses where you tell it to - you can set your desired AF point using the rear 4-way controller.||In 9-point 'Dynamic Area AF' mode the camera will use the eight AF points surrounding the one you select as 'backup' in case your subject moves.|
|In 21-point Dynamic Area AF mode the surrounding 20 AF points are used...||...and in 51-point Dynamic Area AF mode, the other 50 will also help out.|
|In 'Auto Area' AF mode, all of the D800's 51 AF points are active, and the camera uses its 91k-dot metering sensor to identify your subject automatically. This mode also makes use of automatic face detection.||With AF mode set to 'C' for 'continuous', you can select '3D tracking AF'. In this mode, the focussing system uses color information to track your chosen subject across the AF array.|
With lenses of f/5.6 or faster, 15 of its 51 AF points act as cross-type: sensitive to both horizontal and vertical detail. The 9 central focus points will continue as cross type when used with lenses with a maximum aperture of between f/5.6 and f/8. The D800's central AF point will still operate as a cross-type point with lens or lens/converter combinations with a maximum aperture of f/8. In addition, 10 AF points retain horizontal sensitivity at this aperture. In the diagram above, cross-type sensors are indicated in orange.
|F5.6 or brighter||F5.6-F8||F8|
Autofocus speed / accuracy
While the D800 may lack the fast shooting rate of Nikon's flagship D4, it does inherit that camera's 51-point area AF system, which performs admirably in a range of indoor and outdoor scenes. The D800's low light focusing ability is impressive, among the better performing cameras we've seen. Anyone moving up to the D800 from an entry level DSLR will notice a significant improvement in this regard. During our time spent shooting with the camera we experienced far more hits than misses with regard to focus accuracy.
|This shot was taken at f/4 on the AF-S Nikkor 24-120mm VR, at 1/25sec at ISO 25,600 (equivalent). Even in this extremely low light situation, the D800 locked focus using the central AF point without any trouble at all.||100% crop|
The D800 can be used quite successfully to track focus on moving subjects when you're shooting in a continuous drive mode. We had the most consistent success by using the 3-D tracking AF-area mode. With this setup, the camera utilizes color information from the subject that occupies your chosen AF point at the time you first acquire focus, and attempts to track the subject as it moves within the AF acquisition portion of the frame.
In the four-image sequence shown below, the center point AF was chosen and the camera was set to continuous AF with 3-D tracking as the AF-area mode option.
As you can see, the initial image shows AF acquisition (highlighted in red) on the athlete's jersey. In the following frames the camera locks on to the jersey, shifting the focus point as it detects movement. The 100% crops (inset above) from each frame show that with the athlete's face within the same depth of field as her jersey, the D800 has successfully acquired sharp focus in three of the four frames, with Frame 3 still exhibiting a usable degree of sharpness at magnifications below the pixel level view.
DX mode and AF area
One less than obvious benefit of shooting in DX mode - even with an FX lens attached - centers on the D800's massive 51-point AF array. As you can see in the example below, the AF area encompasses nearly the entire viewfinder when the camera is set to DX mode, allowing you to track subjects nearly to the edge of the frame. And with DX mode still providing a 15.3MP file, this mode essentially transforms the D800 into a D7000 with superior AF performance (albeit a slower maximum frame rate).
|In FX mode, the AF area (outlined by the white frame) occupies the central portion of the frame.||Switch the camera to DX mode, however, and the AF area covers the majority of the frame.|
The D800's fast and accurate phase-detection AF system is not available in live view mode, which of course, includes video recording. In these instances, the camera relies on a contrast-detect AF implementation that is disappointingly slow. And not just in comparison to phase-detection on the D800. We've seen faster AF acquisition with less lens hunting on most of the recent mirrorless cameras we've used. For more information about the D800's performance in live view mode, go to the live view page of this review. You can also watch an example of lens hunting during video recording on our movie mode page.