Another consideration is line-skipping. A full HD pro camcorder has a sensor of 1920x1080 pixels. An HDSLR has a sensor with many more pixels than that. To get down to 1920x1080, much of the pixel data must be eliminated. Ideally, this would be done by binning—combining data from several pixels into one. But that would require lots of in-camera processing power—reading a 24-megapixel sensor at 30 fps is a lot of data. DSLRs frequently use "shortcuts." One is line-skipping; they just eliminate, say, two of every three lines to get down to 1920x1080 (2 megapixels) from 22, 24 or whatever is the sensor's actual pixel resolution.
Codecs And CompressionHDSLRs use a variety of formats and compression schemes. The most popular are MPEG-4/H.264 and AVCHD. Canon and Nikon pro DSLRs record in MOV format, using MPEG-4/H.264 compression. Sony's SLT-A99 records in AVCHD Ver. 2.0 (progressive) format, which permits shooting 1080 full HD at 60p; it can also shoot MP4 video. RED cameras use RED's own versatile, high-quality REDCODE RAW format. The newer pro HDSLRs also let you record uncompressed video directly to an external recording device via HDMI.
Some cameras let you choose different compression levels or bit rates. Lower compression and higher bit rates make for better image quality, but larger files.
ErgonomicsDSLRs were designed for still photography, with the camera held up to your eye. To shoot video, the camera has to be in Live View mode, and that shuts off the DSLR's eye-level viewfinder (except with Sony SLT cameras; more on those in a bit). So you can't use the eye-level DSLR finder to shoot video; you have to use the external LCD monitor (or an accessory external monitor). Holding a DSLR at arm's length to use the external monitor isn't conducive to comfortable or steady shooting, so there are lots of accessories on the market to make it easier to use DSLRs for serious video work.
Sony's SLT cameras employ a fixed translucent mirror rather than the typical SLR mirror. In a typical DSLR, when you activate Live View (required for video shooting), the mirror flips up out of the light path so light can reach the image sensor. When this happens, the eye-level viewfinder goes black (because light no longer reaches it), and you lose the camera's normal phase-detection AF (because light no longer reaches the AF sensor), so much slower contrast-based AF off the image sensor is employed for video. With Sony's SLT cameras, the fixed semitranslucent mirror transmits most of the light to the image sensor and directs a small portion up to the phase-detection AF sensor. So you get full-time phase-detection AF, which is much better than contrast-based AF for action subjects—even for video. The SLR's optical eye-level finder is replaced by an eye-level electronic viewfinder, so you also get eye-level viewing for video. That makes the SLT cameras more ergonomic and much better suited for action videos than conventional DSLRs.