We wouldn't normally devote two pages to discussing a DSLR's AF performance, partly because AF - and especially AF tracking - is difficult (to say the least) to test in a consistent and clear way. Naturally, after analyzing the many hundreds (if not thousands) of frames that we might shoot during a typical DSLR test we're confident that we're in a good position to comment on any obvious strengths or failings. However, ultimately, when it comes to discussing the AF systems of a camera as complex as the Nikon D3S, we know we don't have the same authority as a professional sports photographer, for example, with years of experience.

With that obvious caveat in mind, we do make every effort to test particular DSLRs in the environments for which they are designed, where applicable. The Nikon D3S is intended as a multi-role camera, and AF speed and versatility are its key selling points. We therefore went out of our way during the shooting for this review to test AF performance as thoroughly as possible, in a range of different lighting conditions. As you'll see in our upcoming in-depth review of the Canon EOS 1D Mark IV, we shot the same subjects with Canon's new flagship sports camera as well, under the same lighting, and with as near-parity in lenses as we could manage.

What follows is our take on the efficacy of the D3S's AF system, and although it shouldn't be considered definitive, we hope that our notes - and the images shown here and in the main image gallery - will serve as a decent summary of the D3S's AF capabilities. A full test of its rival, the Canon EOS-1D Mark IV, will follow this one online very soon.

Key AF specifications

  • 51 point Phase-detection AF with 15 cross-type sensors
  • 11-point AF pattern mimics D2XS AF array (when DX crop mode is selected)
  • 3D AF tracking uses dedicated 1005-pixel CCD sensor to incorporate color/contrast information when tracking moving subjects
  • Virtually 100% compatibility with all AF Nikkor lenses (excluding those designed for the F3-AF)
  • Electronic rangefinder and focus indicator for manual focus with AF or MF lenses (excluding Pre-AI lenses, many of which will jam when mounted on the D3S.)

The D3S features the same 51 point auto focus sensors the D3/D3X. The center fifteen (3x5 grid) are cross point sensors. There are two AF point selection modes (CSM a8), and you can opt to be able to select from all 51 points or 11 points in a similar layout as the D2XS. The D3S's larger sensor area means that the AF points don't occupy as much of the frame as the D300S, but obviously, if you switch to DX cropped mode the focus points cover virtually the entire frame.

51 point AF selection mode 11 point AF selection mode

Single AF

With the main AF mode switch set to AF-S, the D3S acquires focus quickly and positively in almost all situations, and is impressively resistant to flare caused by sidelight. Although it struggles with silhouetted subjects, when presented with a convenient edge, the D3S can almost always achieve accurate AF lock. It is reassuring, too, to see that even in light so low that ISO 102,400 might conceivably be the only option for exposure, the D3S can usually deliver accurate focus given a little bit of contrast to work with.

In general use, the major limiting factor to AF speed is the type of lens used. Most older AF-D lenses are barely any slower than Nikon's most recent large-aperture AF-S offerings (the AF-D 80-200mm f/2.8 for example acquires focus almost as quickly as the new AF-S 70-200mm f/2.8 II) but ironically, some current AF-S lenses really slow the system down. The worst offenders are Nikon's smallest, most compact AF-S optics, including all 'kit-type' DX designs and (more annoyingly for prospective D3S owners) the new 50mm f/1.4.

The Nikon D3S offers virtually 100% compatibility with all AF lenses created for the F system. There are a couple of quirks, however, which users of older lenses should be aware of. The first is that on rare occasions when using AF-D lenses, the D3S might decide that accurate focus has not been achieved, even when it has. The camera's default behavior in this situation is to flash a warning in the viewfinder to let you know that AF cannot be acquired.

This happens very rarely in our experience, but if you notice it, the issue can be solved easily by changing AF-S priority selection from 'Focus' to 'Release' via csm a2. We have noticed the same glitch on other Nikon DSLRs, including the D3 and D300s, and the focus priority selection fix does the trick in all cases.

However, regardless of whether you use AF-D or AF-S lenses, should you for whatever reason feel the need to set csm a2 to 'release', you should be prepared for something rather odd to happen when the cameras is set to continuous shooting mode (either L or H). For some reason, when this custom function is set and drive mode is switched to 'CL or CH', the D3S attempts to reacquire focus after every exposure, regardless of the position of the main AF-S/AF-C' switch on the front of the camera body.

If your preferred method of shooting off-center subjects is to focus with the center point and then recompose, this means that potentially, if you're in continuous drive mode, only the first frame in your sequence of images will be in focus. The fact that the D3S exhibits this odd behavior even during a bracketed burst - where as far as we're concerned it is wholly undesirable - is surprising. We have voiced these concerns to Nikon, but at time of publication, we had yet to receive an official response. At such time as we received detailed feedback we will update this review accordingly.

Continuous AF

The continuous autofocus system of the Nikon D3S is essentially unchanged from that featured in the D3 and D3X. As such it represents a huge leap forwards in terms of complexity and versatility compared to the 11-point AF system in the previous generation D2-series. The AF array has 51 points, selectable automatically or manually, of which 15 are of the cross type (and work with lenses with maximum apertures of f/5.6 or faster). There are two 'master' AF modes, - Single and Continuous, which are set using a physical switch on the front of the camera. When set to Continuous AF, the camera can track moving subjects in four modes - single point, dynamic-area, 3D tracking AF, and Area AF.

3D AF Tracking debuted in the D3, in 2007, and is a sub-option of dynamic area AF. When 3D tracking is enabled, all 51 AF points are called into service, and the D3S aims to track a moving subject based on its color/contrast, and distance from the camera. To do this, it uses information from the same dedicated 1005-pixel CCD sensor that provides data to the camera's metering and white balance systems.

Despite this complexity, the AF system of the Nikon D3S is remarkably easy to get to grips with. The key settings (AF/AF area modes) are adjusted using physical switches that can be accessed without removing your eye from the viewfinder, and few of the 10 custom functions that relate to AF performance relate to aspects of the camera's performance that need to be changed frequently. You'll find a more detailed account of the various settings, and how they perform, on the next page.