Thursday, December 18, 2008
D-SLR State Of The Art, Part I
The line between still and video fades out as Live View evolves into HD motion video
HD VIDEO RESOLUTIONS
While digital still images consist of tiny (usually) square “pixels,” digital video images are continuously scanned onto the screen as horizontal lines. Thus, video resolutions are given in lines: A standard SD digital video image consists of 480 horizontal lines from top to bottom; an HD video image consists of either 720 or 1080 horizontal lines.
Since HD digital video images have a specific aspect ratio (width to height) of 16:9, each video line resolution is accompanied by a corresponding number of horizontal pixels: a standard SD digital video frame (image) consists of 480 horizontal lines each 720 pixels wide; a 720-line HD video image consists of 720 lines each 1280 pixels wide; and a 1080-line HD video image consists of 1080 lines each 1920 pixels wide.
Note that video resolution is simply lines (or pixels x lines), not lines per inch or pixels per inch. A 1280x720 image contains 1280 pixels across the screen. If the screen is 10 inches wide, there are 128 pixels per inch. On a 100-inch-wide, giant-screen HDTV set, there would be 12.8 pixels per inch.
The line resolution of a video image is generally followed by the letter i (interlaced) or p (progressive). This refers to how the image is drawn on the screen. Progressive images are quickly drawn across the screen one line at a time, from top to bottom—line 1 followed by line 2 followed by line 3, etc. Interlaced images are drawn in two fields—the first consisting of the odd-numbered lines, followed quickly by the second, consisting of the even-numbered lines. Progressive scanning produces smoother action and slow-motion effects.
Generally, 1920x1080 HD video is interlaced (1920x1080i) while 1280x720 HD video is progressive-scan (1280x720p), but both the Nikon D90 (1280x720p) and Canon EOS 5D Mark II (1920x1080p) use progressive scanning for their HD videos. The EOS 5D Mark II automatically adjusts output to match the resolution of the display device being used (for example, the HDMI port outputs a 1080i signal).
Video image frames can be scanned onto the screen at various rates. The most common are 30 fps (used by the Canon EOS 5D Mark II) and 24 fps (used by the Nikon D90). Since 24 fps is the rate at which film movies are displayed, it’s said to produce a more “film-like” effect, especially when motion is involved, as with moving subjects, pans and zooms.
Page 4 of 4