The Pentax 645D features Pentax's SAFOX IX+ autofocus system, which is an evolution of the SAFOX VIII system found on earlier DSLRs. This same autofocus was also deployed on the Pentax K-5, where it saw a great deal of praise. It has since then been succeeded by SAFOX X, which works in even lower light (down to -3EV according to the specifications). SAFOX IX+ features 11 AF points, 9 of which are cross-type (acting as both horizontal and vertical sensors). It can be configured to use all 11 points or only a single point (user-selectable) in both single and continuous servo mode. The 645D's AF points are rated for lenses up to F5.6 (there are no 645 lenses slower than this) and the operating range is -1 to +18 EV.
Below is a diagram showing the the 645D's viewfinder. Cross-type AF points are shown in red. The AF points cover such a small area because SAFOX IX+ was designed to be compatible with APS-C DSLRs.
The Nikon D800E features Nikon's flagship Multi-CAM 3500FX AF module, which has also been deployed in the Nikon D4. It has a total of 51 AF points (15 cross-type), all compatible with F5.6 lenses. 15 of the points support lenses slower than F5.6 but faster than F8, and 11 of the points support F8 lenses (more details on this found on Nikon's web site). The D800E's AF works from -2 EV to +19 EV.
Below is a diagram showing the the D800E's viewfinder. Cross-type AF points are shown in red:
The grid can be enabled or disabled via the menu on the D800E. Also note the presence of a 2-axis horizontal level in the viewfinder; this feature can be mapped to one of the customizable buttons.
The D800E's AF system can be set to utilize all 51 or just 11 of the AF points. It can be used in both single and continuous/servo mode, where it can be configured to use single-point, dynamic-area, 3D-tracking or auto-area AF. The 3D tracking mode works very well to track moving subjects; we tested it by trying to keep the AF locked on a person's face while swinging on a swing, and it was able to consistently maintain proper focus.
Both cameras perform adequately-well for landscape and studio shooting. Pentax's new SDM 645 lenses focus quickly and silently, but not as quickly as Nikon's SWM lenses. The D800E has a clear advantage for action photography, and its AF system does an excellent job of tracking moving subjects. The fact that the D800E has such an advanced AF system makes it much more versatile than the Pentax, though an in-depth comparison of the exact AF response times would be outside of the scope of this review. For what the 645D is designed to shoot, its AF system is more than sufficient, though the AF viewfinder coverage certainly seems a bit small!