Nikon D90 Live View
The Nikon D90 brings Live View to that company's prosumer DSLR line for the first time (unless you consider the D300 a prosumer model; we class the D300 as a fully professional model). As we'll see though, it goes quite a bit beyond most Live View implementations, and in one respect, is absolutely unique as of the time of its introduction in late August, 2008.
One welcome enhancement to the Nikon D90's Live View mode is its one-touch activation, via the Live View button. On the D300, Live View mode had to be selected via the Drive Mode wheel on the left side of the camera's top panel; a somewhat cumbersome arrangement. By contrast, the D90 sports a LV button on its rear panel, in very convenient reach of your thumb. Press it at any time and the mirror flips up and the camera immediately enters Live View mode. A nice surprise, it does a lot to make Live View a much more fluid part of your photography than it's been in the past.
|Nikon D90 Live View Data Display|
|The Nikon D90's Live View display includes a good bit of relevant shooting data, including shooting mode, autofocus mode, image size, image quality, white balance, audio recording, time remaining, metering mode, shutter speed/aperture, exposure compensation, ISO, and more.|
The second nice surprise in the Nikon D90's Live View
mode was the full information display optionally available while
working in that mode. The Live Mode shooting info display shows you a
lot of what you'd normally see looking through the viewfinder, yet
manages to keep most of the information out of the way of the live
image area. Information displayed includes current metering mode,
shutter time, aperture, exposure compensation, ISO setting, shots
remaining, exposure mode, face detection active/inactive (more on this
in a bit), file mode (RAW or JPEG) and white balance setting. It also
shows microphone status and minutes/seconds of movie recording
available at the current resolution and quality settings. (Wait a
minute: Movie recording? Yes, that's not a typo, more on that
in just a bit as well.) One thing we miss in the D90, though, is the
live histogram display that's an option on the Nikon D3 and on some
competing SLRs from Nikon's arch-rival Canon.
In a new wrinkle, the Nikon D90 has only one autofocus mode in Live View, and it's not the traditional phase-detect AF. The D90's only option for autofocus in Live View mode is contrast detection. This is a fairly radical departure for Nikon (or any camera maker).
By way of explanation, the phase-detect/contrast-detect distinction is one of the fundamental differences that separates digicams from digital SLRs, and is the core reason that SLRs focus so much more quickly. Contrast-detection autofocus involves looking at the image from a camera's main image sensor and evaluating it to see how abruptly brightness values change from one pixel to the next. If an image is soft and fuzzy, brightness changes between adjacent pixels will be relatively slight, but if it's sharply focused, they'll be much greater. The point of ideal focus is found by moving the lens elements back and forth and determining whether the contrast signal gets stronger or weaker. Achieving focus this way necessarily involves some back-and-forth hunting, which can take a while to accomplish. By contrast (no pun intended), phase-detect AF uses a system of prisms, lenses, and a secondary sensor to determine not only whether the image is in focus or not, but by how much it's out of focus and in which direction. The camera can then adjust the focus setting to exactly the position needed in a single step. As a result, phase-detect AF systems are generally much faster than contrast-detect ones.
The catch with phase-detect AF though, is that it requires some of the light passing through the lens to be diverted to the focus sensor. This is fine in an SLR when the mirror is down between exposures, as part of the mirror is typically partially transmissive, with the light passing through it deflected by a secondary mirror down to the AF sensor, usually located in the bottom of the mirror box. When the mirror is raised in Live View mode, though, light from the lens can't get to the separate AF sensor. This accounts for the rather lengthy (and noisy) AF cycles in most Live View SLRs: To focus the camera the mirror has to be dropped, focus determined, and the mirror raised again, adding several tenths of a second to the normal non-Live View shutter lag. For example, the Nikon D700 full-frame SLR has a shutter lag of only 0.197 second when using the optical viewfinder and a single AF point. This increases to 0.573 second in Live View mode, all other settings being the same.
While it takes some time for the mirror to drop and re-open for phase-detect AF in Live View mode, the overall result can still be faster than when relying on contrast-detect autofocus. Taking the case of the D700 again, in contrast-detect AF mode, it takes roughly 1.1 seconds for it to determine that the lens is in focus, even when starting with the lens already focused on the target. (It can easily take several seconds for the camera to slew a lens from infinity to close focusing in this mode.)
With the above as background, you can see why we were surprised to find contrast-detect AF the only autofocus option available on the D90 in Live View mode. In playing with a D90 prototype though, AF in Live View mode felt faster than we'd been expecting. Clearly, not nearly as fast as when using the optical viewfinder and phase-detect AF, but it certainly felt faster than the D700 (and D300 and D3) using their "Tripod Mode" (contrast-detect) AF in Live View. Alas, when we tested a production model D90 in the lab, we found that the overall shutter lag (from pressing the shutter button, through focusing, all the way to capturing the image) was still very much on the long side: The fastest times we measured, with the lens already set at the correct focal distance for the subject, was on the order of 2.3 seconds. So the D90's contrast-detect AF in Live View mode clearly isn't something you're going to be using for sports shooting.
|Nikon D90 Live View Autofocus|
|As shown here, one advantage of contrast-detect AF is that you can put the AF box wherever you like it. (In this case, we've put it top center.) When focus is locked, the focus box changes from red to green.|
One advantage of contrast-detect autofocus though, is that you're
not restricted to focusing only on those areas where you happen to have
an AF point. As the pair of shots above shows, the D90 lets you put the
focus box anywhere in the frame you want to when in Live View mode.
|Nikon D90 Live View Autofocus|
|Contrast-detect AF also opens the possibility of advanced modes like face-detection. Here, we see that the Nikon D90 will change both the location and size of the face-detect box to accommodate changes in subject size and position.|
Besides the ability to position the AF area wherever you like it, the Nikon D90's contrast-detect AF in Live View mode also offers face detection and tracking. Up to 5 faces can be detected in an image, and the AF areas adjust to match the size of each face detected. The shots above show the AF box moving and changing size in response to the changing position and scale of a face in the scene. The face-detect box shows yellow when not in focus, and green when in focus (after half-pressing the shutter button). The video clip at right shows the D90 tracking a live face while zooming in and out and panning around.
Playing with face-detect autofocus in Live View mode, we were
pleased to see that the camera not only tracked the faces, but was also
intelligent enough to set focus based on the eyes of subjects, rather
than on their noses or mouths. (In portrait work, the eyes are the most
critical part of the face to render in sharp focus. Other parts of the
face can be soft but viewers will regard the shot as properly focused
if the eyes are crisp.)
Manual Focusing in Live View Mode
With appropriate magnification, Live View displays can be very useful when manually focusing SLR cameras. During manual focusing, the Live View display on the Nikon D90 can be magnified quite considerably, but it seems it's not quite all that you might expect from the specifications. In Live View mode, pressing the magnifying glass button on the camera's back lets you zoom in up to a maximum of 6.7x. If you do the math, this would correspond to a 1:1 (pixel to pixel) ratio between the pixels of the camera's very high-resolution LCD and the sensor. The viewfinder image at this magnification isn't sharp, though; it seems the camera is actually grabbing fewer pixels than that section of the sensor actually contains and then interpolating them to form the image. (You can check this by snapping a shot and then magnifying the resulting image to the same degree in playback mode. Doing this, you'll find the image is quite a bit sharper than what you saw on the LCD in Live View mode.) While the magnified Live View display is still usable for focusing, it isn't as sharp as it could be, nor as sharp as the Live View display on some of Canon's SLR models sporting this feature.