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MillimeterKnow Your Formats, Part 1
Apr 14, 2008 12:00 PM, By Jan Ozer
Figure 2. Formats and their aspect ratios.
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Aspect Ratio
By the term "aspect ratio," I mean how the video is stretched during display. Figure 2, a shot of Adobe Premiere Pro’s Interpret Footage screen, shows the aspect ratios of many common formats—including DV, HD Anamorphic (which is the same aspect ratio of HDV), and DVCPRO HD, which has a pixel aspect ratio of 1.5. Let’s walk through what this means and you’ll see why it’s important.
Figure 3. Although it's displayed at 1920x1080, DVCPRO HD/100 video is actually composed of 1280x1080 pixels.
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Figure 3 shows the preview window in Premiere Pro’s Project Pane, illustrating the properties of a video of the front wheel of my trail bicycle shot with the Panasonic AG-HVX200. As you can see in the small preview window, the wheel looks round, despite being composed of only 1280 horizontal pixels. That’s because during display, Premiere Pro scales the horizontal pixels by 150 percent, essentially the same as you could do in Adobe Photoshop or another image editor. Again, that’s the significance of the 1.5 shown in parenthesis after the 1280x1080 resolution.
Figure 4. The oval bicycle wheel at its native resolution.
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Want proof? Figure 4 shows a video frame exported from Premiere Pro at its native resolution of 1280x1080. As you can see, the wheel is oval. These are the pixels actually captured by the Panasonic.
In Figure 5, I’ve expanded the frame by 150 percent to its intended display resolution of 1920x1080, and the wheel is round. This is the image you would see in your video editor or on your HDTV, which both know to expand DVCPRO HD video out to 150 percent of horizontal resolution.
Figure 5. The round wheel expanded horizontally by 150 percent.
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Without question, the HVX200 takes absolutely stunning pictures, and Figure 5 looks grand, even after the stretching. Yet intuitively we know that any time scaling occurs, detail is lost. All things being equal, a camera with an aspect ratio of 1:1 that captures the 1920x1080 image pixel for pixel should reproduce more accurate detail.
Why does the HVX200 capture 1280x1080 rather than 1920x1080? Probably because Panasonic couldn’t source imaging chips that would capture the native image, pixel for pixel, at the necessary price or size. As you can see in Figure 1, however, other formats—including XDCAM HD, AVCHD, and AVC-Intra—do or will support true pixel-for-pixel capture and storage. Again, all other things being equal, these cameras/formats should deliver better detail and resolution than DVCPRO HD, or HDV for that matter.
