D-SLRs + HD Video
New cameras from Canon and Nikon are the first SLRs to include video
Digital video has been a feature of compact digital cameras for years, and recently HD capture has become more common. This feature has now made its way to interchangeable-lens cameras.
Though not a replacement for your dedicated HD camcorder, this opens up the possibility to make video part of your usual photography experience, with the added benefit and control of using the optics of your choice.
At the time of this writing, there are only two SLR models that can shoot HD video: the Canon EOS 5D Mark II and the Nikon D90. The popularly priced Nikon D90 was the world’s first digital SLR with HD video, and is capable of recording 720 (1280 x 720) resolution video, while the pro-level Canon EOS 5D Mark II records in full HD 1080 (1920 x 1080).
Digital SLRs work differently than video cameras, and until now, they’ve been dedicated to stills only. The reason that we’re finally seeing video in the Canon and Nikon models today is based loosely on Live View technology, which was introduced a few years ago by Olympus but now is an increasingly common feature in cameras from many of the major brands.
Live View basically moves the mirror so that the image sensor is completely exposed to the focal plane. This mirror is a major component in both digital and film D-SLRs, because it allows you to see through the viewfinder before you press the shutter, and enables autofocusing mechanisms. Once we press the shutter, this mirror moves itself out of the way, and light is then reflected on the sensor or film plane, giving us a still image.
So for a D-SLR to capture video, the Live View principle is used; the mirror moves out of the focal plane, letting light continuously fall on the sensor. Instead of capturing a single frame, the sensor continues recording. Digital camera sensors are much higher in resolution than is needed for HD video, so some down-sampling is required. The exact methods for going from still camera sensor to HD-video output are proprietary, but we do know the camera’s processor is heavily involved with converting the stream of data into full-motion video.