Nikon D90 Autofocus System
The Nikon D90 uses the same Multi-CAM 1000 autofocus sensor as its predecessor the D80, but incorporates some of the advanced AE/AF integration first seen in the D3 and D300 professional models. It also offers 3D focus tracking for more accurate focus on moving subjects, the same technology found in its big brothers. In the paragraphs below, we'll look first at the general characteristics of the camera's AF system, and then talk a bit about the advances carried over from the D3 and D300.
The D90 inherits the same Multi-CAM 1000 AF sensor module used in the D80, so many of its basic specs mirror those of the D80 quite closely. As we noted in the review section covering the viewfinder, the D90 has a total of 11 autofocus regions, arranged with 9 in an almost square matrix in the central area of the frame, with two additional ones positioned to the right and left of the central array. The central point of the array is a selectable normal/wide point, optionally able to look across a wider area in determining focus. Each of the 11 focus areas can be used individually, the center sensor can be switched to wide-frame operation for broader coverage, and an Auto-area AF mode measures all 11 focus areas, automatically determines which of them are on the primary subject and activates only those areas. As with the D80, only the center sensor is cross-type, able to respond to subject detail oriented either horizontally or vertically.
The D90 lets you choose either auto or manual focus via a small dial on the front of the camera, next to the lens. Setting the switch to "M" puts the camera into manual focus mode, "AF" places it in autofocus mode. Single AF (AF-S), Continuous AF (AF-C), and Auto-switching AF (AF-A) options can be selected by pressing the AF button on the right side of the camera's top panel repeatedly, to cycle through the choices. Single AF simply means that the camera sets focus only once, when the Shutter button is first pressed halfway, and is best for stationary objects. Continuous AF means that the camera continuously adjusts the focus, as long as the Shutter button is halfway pressed, and is best for moving objects. In AF Auto mode, the camera begins focus operations in single-focus mode, but switches to Continuous Dynamic AF if it detects motion within the active AF area.
There's an important difference between Single and Continuous Servo modes: In Single Servo mode, the shutter won't release unless the lens is focused, or the lens itself is set to manual focus. (Focus Priority) In Continuous Servo mode however, the camera will fire regardless of the state of focus. (Release Priority) If you want to be sure that the camera is focused when you snap the picture, use Single Servo mode. Use Continuous Servo for moving subjects, and/or times when the instant of shutter release is more important to you than sharp focus.
The AF Area Mode option (a1) on the Custom Settings Menu lets you select between Single Area, Dynamic Area, and Auto AF modes, and adds a new 3D Tracking mode, brought over from Nikon's recent pro-level SLRs. Single Area AF simply means that the camera judges focus based on one part of the subject, and the user can manually select the AF point by pressing the arrow keys. Dynamic Area AF employs all of the autofocus points, though you can still manually select the main point. When Dynamic Area focusing is enabled, the camera first focuses on the subject in the primary focus area. When the subject moves to a different AF area, the camera shifts the focus to "follow" the subject. This is great for irregularly moving subjects. (Sports and kids come to mind.) The Auto-area AF option means that the camera first focuses on the subject in whatever single AF area is selected, but will switch to Dynamic Area AF if it detects subject motion. The new 3D Tracking mode uses the 420-element RGB exposure/white balance sensor to aid in subject tracking. (See the section below on AE/AF integration for more details on this.)
In any of the AF modes, you can change the primary focus area by unlocking the focus area selector (the Four-Way Arrow pad on the back panel) and then shifting the focus area using the up, down, right, or left arrow directions on the control rocker. You can lock the focus area selection by turning the switch back to the lock position. By default, the D90 does not "wrap" the focus area selector as you scroll between focus areas. Custom Settings menu option a5 lets you opt for a "Wrap" function. What this means is that if you press the right arrow key again, after the right focus area is already selected, the selection will immediately jump to the left focus area. The same thing happens when moving the focus area selection vertically as well.
The center focus area of the D90 can be set to either a normal or wide zone, through Custom Settings Menu option a2. The wider zone could be better for initially acquiring a moving subject, particularly when combined with the Dynamic Focus option.
There are three methods by which you can lock focus on the D90. The first is to half-press the Shutter button to lock the focus, placing your subject in the selected focus area, halfway pressing the Shutter button, then realigning the composition and firing the shutter. (This is the default behavior of the Shutter button, but if the AE-L/AF-L button is set to AF Lock, it will override the shutter button as long as it's held down.) Alternatively, when using Single Servo AF, you can press the AF-L/AE-L button to lock focus (and exposure, unless that button is set for focus only in Custom Settings menu f4). Keeping this button pressed will lock focus and/or exposure, even if the Shutter button is released. This lets you recompose the photograph without keeping your finger on the Shutter button, but on the AE-L/AF-L button instead. (Reducing the chance that you'll accidentally trip the shutter when you don't intend to.) The front-panel FUNC button can also be set (via Custom Settings Menu option f3) to set focus temporarily.
There are several options available for the AE-L/AF-L button, which can be set via Custom Settings Menu f4. You can program it to lock either focus or exposure separately, or both together (the default). You can also change its operation so a single press locks and holds the exposure setting. (No need to keep the button pressed down.) Finally, you can set the AE/AF lock button so it alone controls the autofocus system, meaning the autofocus won't actuate when the shutter button is half-pressed, only when the AE/AF lock button is pressed instead.
We don't have any way to measure autofocus tracking performance directly, but on a subjective level, the AF system in the Nikon D80 did seem to be very fast and responsive. We particularly liked the Dynamic AF option, but could see where the Group Dynamic AF option of the D200 would be nice to have in some situations. (Group Dynamic AF handles the common situation where it's difficult to place a single AF point on a very active subject at the start of focus tracking. By having a larger area in which to position the subject, it becomes much easier to initially acquire focus.)
Like past Nikon DSLRs with built-in flashes, the D90's AF-assist light is a bright incandescent bulb that emits from the body near the handgrip. We thought to call particular attention to it, because some DSLRs use the flash as an AF-assist light. This would be fine, but they also require that the flash be used in the exposure, so there's no way to have AF assist for available-light shots. The D90 doesn't have this limitation.
AE/AF Integration - RGB sensor brings extra smarts
When they were announced roughly a year before the Nikon D90, the D3 and D300 brought a new level of sophistication to autofocus algorithms, by taking advantage of information from their unusually high-resolution 1,005 RGB exposure and white balance sensor. For the first time, the D90 brings this advanced AF technology to a SLR aimed at the prosumer market. The technology isn't exactly equivalent to that of the higher-end models though: Where the D3 and D300 have 1,005 pixel RGB sensors, the equivalent chip in the D90 has only 420 pixels. - And, of course, the D90 has only 11 AF areas vs the 51 of the D3 and D300.
While 420 RGB pixels doesn't sound like a lot when compared to the megapixels of the main image sensor, that's a heck of a lot more elements than are found in the 11-point autofocus array. While AF systems have gotten pretty clever at understanding what the subject is doing as it moves across the frame and toward or away from the camera, a view that's limited to just 11 points severely restricts the camera's view of what's happening in terms of subject movement.
The key innovation in the D3/D300 AF system is that data from the RGB sensor is used to provide much higher resolution for keeping track of subject position. The color data also helps identify the subject of interest and separate it from the background or other scene contents. When an AF point initially acquires a lock on a particular subject, the camera immediately notes the pattern of light and color found at the corresponding point on the RGB array. Then, as the subject or camera moves, the camera can track the light/color pattern corresponding to the subject across the face of the RGB array. This provides much finer-grained position information than could ever be obtained from the sparse AF array. The RGB sensor doesn't provide any distance information, but by more precisely tracking subject information, it permits a much surer hand-off of the subject between AF points. Nikon calls this advanced, color-based approach 3D Focus Tracking. As noted above, 3D Tracking focus is one of the options on the a1 Custom Settings Menu screen.
Face Detection in Phase-Detect AF?
Here's a new wrinkle we were frankly a little surprised to hear about: The Nikon D90 can actually use face detection when in normal auto-area phase-detect AF mode. At first, this sounds like an impossibility, and it would be for a purely conventional phase-detect AF system. After all, how could the AF sensors tell whether they were focusing on a face or any other object? All they see is a signal that indicates how far in or out of focus their part of the subject is: There's nothing that would indicate what type of subject they're looking at.
This is where the 420-pixel RGB sensor comes into play again. It obviously doesn't have anywhere near the resolution of the main image sensor, but there's probably enough to give at least some idea of where a face might be in the image. We don't know the details of how the 420 pixels are arranged, but if they were in an array with a 3:2 aspect ratio to match the overall frame dimensions, that would be an array on the order of 25 x 17 pixels in size. Pretty paltry by camera image sensor resolution, but perhaps enough to detect a skin-colored blob against a differently-colored background. In closeup portraits, this is also probably enough resolution to figure out what part of the subject corresponds to an eye vs. a nose.
Of course, as noted before, the RGB sensor itself doesn't collect any distance information, and as such, can't directly control focus operation. In other words, you're still only going to be able to set focus based on areas covered by the AF points themselves. But what the RGB sensor data can do is to help the AF system decide which of the AF points to pay attention to.
Playing with a prototype of the Nikon D90, we found this to work surprisingly well. In auto-area AF mode, if we arranged objects to have several both covered by AF points and at equal distances from the camera, the camera pretty reliably chose the AF point lying over a face. Even more impressive, if we composed a portrait shot with several AF points on the subject's face, but only one over an eye, the camera picked the eye more times than not. (It seemed pretty accurate, but we'll have to spend more time with a production sample before we can give any idea of how reliable this AF system feature is. Nonetheless, what we saw struck us as both impressive and useful in actual shooting situations.