HDSLR Guide Chapter 12: Direct-To-Disk
Many users would benefit tremendously if there was a way to record uncompressed or less compressed video using an external device. While this is possible using the HDMI output from the Canon T2i, 7D and 1D Mark IV (5D Mark II outputs 480p during recording), the resulting image will be less than the full resolution recorded by the camera. Because the outputted image is surrounded by a black frame, the actual resolution of the image will be 1620x910 (instead of 1920x1080). The recorded video will also have a red dot on the corner of the image.
Additionally, the HDMI signal comes out as 1080 60i even though the camera records to 24p. This does not make it a real interlaced image. This is called Progressive Segmented Frames (PSF) which transmits the progressive frames as two fields. As long as the receiving monitor or recording device is set to detect and join the appropriate fields, the result will be 24p.
The color space on the HDMI output is 4:2:2 instead of the 4:2:0 recorded in the camera, and the video can be recorded using a better video codec than H.264, which may result in fewer image artifacts caused by the compression of the camera. For many users, the additional cost, added steps to the workflow and loss of resolution (from 1920 x 1080 to 1620 x 910) will make HDMI an unattractive choice. Some, however, may find the additional colors, lower compression, and edit-friendly codec (which can be edited without transcoding) attractive options.
Judging from the pace of new firmware updates and new models being introduced, it won't be long before a record direct-to-disk option will be simplified and more worthwhile.