All quotes here from link above.
"...four AF-area modes for use during movie capture: Face-priority AF, Wide-area AF, Normal-area AF, and the aforementioned Subject-Tracking AF."
"As we've noted in other SLR reviews, the good news with focusing for video is that you can get surprisingly good depth of field in video mode by stopping the lens down, thanks to the relatively low resolution of the video image. With a pixel resolution of only 2.1 megapixels in the Nikon D5100's highest-resolution 1,080p "Full HD" mode, 0.9 megapixels in 720p HD mode, and just 0.3 megapixels in the standard-definition mode, images that would be unacceptably blurred as 16 megapixel still shots look perfectly fine as video frames. This not only provides greater depth of field at any given aperture, but is also more forgiving of diffraction limiting at very small lens apertures. Diffraction at small apertures means you'd usually want to avoid f/16 or f/22 for still images, but again, the results generally look perfectly fine at video resolutions. Bottom line, with the Nikon D5100's lens set to f/16 or f/22 (assuming you're shooting under fairly bright conditions), you'll be surprised by how little focus adjustment is needed during a typical video recording."
- "With the Mode dial set to any position except A or M, the main exposure variables -- shutter speed, aperture, and ISO sensitivity -- all remain under automatic control when recording movies with the Nikon D5100. "
- "in the A or M positions, however, you can shoot movies in Aperture-priority mode, with the proviso that you must dial in your desired aperture prior to entering Live View mode, unless you're shooting with a PC-E lens. "
- "Regardless of the metering mode selected, movies will always be recorded using Matrix metering, with the Center-weighted and Spot options available only for still image capture."
- "You can, however, adjust exposure compensation, and use the Autoexposure Lock button to lock the metered exposure, even after movie capture has already started. "
- "You can apply Nikon's Picture Controls as well as specify the color space for movies, as long as they are selected before recording begins. This is useful if you'd like to for instance record in monochrome, or with more saturated colors, etc."
- "Like most interchangeable lens cameras capable of AF during video, the D5100 has a tendency to pick up autofocus drive noise quite clearly in the audio track of its movie clips. This can at least be mitigated somewhat by use of an external microphone, and by adjusting the D5100's three-step manual gain control, however. "
- "...three sensitivity levels to choose from (Low, Medium and High), along with Auto gain, and Off settings. The settings apply to the built-in microphone as well as to an external mic. "