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Nikon D5100 Live View

The Nikon D5100 includes live view capability with one-touch activation, via a dedicated Live View switch on its top panel, in very convenient reach of your index finger. Flick it at any time, and the mirror flips up, with the camera immediately entering Live view mode. A button located adjacent to the shutter button is used to start and stop movie recording when live view is active.

Live View Mode Displays

Shooting mode
Focus point (face detect in this case)
"No movie" icon
Audio recording indicator
Time remaining (live view mode)
Time remaining (move mode)
Live view autofocus mode

A particularly nice feature of the Nikon D5100's live view mode is its (optionally available) full information display. The live view 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 image preview area. Information displayed includes current metering mode, shutter speed, 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. There's also an optional grid overlay mode, as well as a reduced size preview with menu options to the right and below. One thing we miss in the Nikon D5100, though, is the live histogram display that's an option on the Nikon D3 and on some competing SLRs, including those from Nikon's arch-rival Canon.

Like the D5000, the Nikon D5100 has only one type of autofocus method in Live view, and it's not the traditional phase-detect AF. The D5100's only option for autofocus in Live view mode is contrast detection. This is a fairly radical departure compared to other camera makers.

Contrast-detect vs. Phase-detect Autofocus

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 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-capable 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 D5100 has a shutter lag of only 0.273 second when using the optical viewfinder and a single AF point. This increases to an average of 1.276 seconds in Live view mode via contrast-detect, 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. The Nikon D300, for instance, can focus and capture a shot in 0.465 second in Live view mode using phase-detect AF.

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, and that they've carried that limitation forward on the Nikon D5100. The Nikon D5100's contrast-detect AF in Live view mode clearly isn't something you're going to be using for sports shooting.

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. The Nikon D5100 lets you put the focus box anywhere in the frame when in Live view mode.

Click to view movie. MPEG-4 player required.

Face detection. Here's a look at the D90's face detection working with a live subject, which operates the same way on the Nikon D5100. Click to view/download 1.7MB MPEG-4 file.

Besides the ability to position the AF area wherever you like it, the Nikon D5100's contrast-detect AF in Live view mode also offers face detection. Up to 35 faces can be detected in an image in Face Priority mode, and the AF areas adjust to match the size of each face detected. 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. While the Nikon D5100 can detect many more faces simultaneously, its face detection autofocus mode is otherwise very similar.

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.)

Other Nikon D5100 Live view autofocus modes are: Wide Area, designed for hand-held shots of landscapes and other non-portrait subjects; Normal Area, designed for pin-point focusing using a tripod; and Subject Tracking. This mode is designed to track just about any selected subject as it moves through the frame -- it need not have a face.

Manual Focusing in Live view Mode

During manual focusing, the Live view display on the Nikon D5100 can be magnified quite considerably, but it seems it's not 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 7.7x. 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.


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