Tuesday, February 17, 2009
Straight To Video
The newest evolution in D-SLR technology has finally brought us high-definition video and stills in one camera. Here’s a look behind the scenes.
|HD, SD And Still Capture At The Sensor|
|Still images are composed of pixels, tiny squares measured by microns. Put together, they make up the jigsaw pieces that complete the larger puzzle—the image. Digital video is a series of these images, captured and displayed through continuously scanned horizontal lines. While image resolution is represented by pixel count, lines represent video resolution. Standard definition (SD) consists of 480 horizontal lines from top to bottom. High definition (HD) is available in 720 or 1080 lines, with crisper, sharper detail in the 1080 format. |
Because digital video images have a specific aspect width-to-height ratio—16:9 for HD, 4:3 for SD—each video line resolution is accompanied by a corresponding number of horizontal pixels. An SD video frame consists of 480 horizontal lines by 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.
HD resolution is available in progressive (p) or interlaced (i) display. Progressive provides a smoother picture experience because the image is drawn on the screen one line at a time, from top to bottom. Interlaced video is drawn in two fields, first by odd-numbered lines, followed quickly by even-numbered lines.
In general, 1920x1080 HD video is interlaced (1920x1080i), though 1080p is becoming more and more common, while 1280x720 HD video is progressive-scan (1280x720p). Both the D90 (1280x720p) and EOS 5D Mark II (1920x1080p) use progressive scanning for video capture. With progressive capture, output to any device can be downconverted easily. The EOS 5D Mark II, for instance, adjusts output automatically.
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