Saturday, April 30, 2011

The Truth About HD DSLRS

By Michael J. McNamara Published in Digital SLR Cameras
The Truth About HD DSLRS

Where Has All My Memory Gone?

How does your camera compress the video it records, and at what data rate does it store video on your memory card? These questions are important to video shooters as they greatly affect image quality and the length of your videos. To a lesser extent, both may affect the compatibility of your video with editing programs and sharing services.

While nearly every imaging device supports the codecs (compression/decompression algorithms) used in standard JPEG photo files, there isn't a similar standard for video. As a result, a variety of compression algorithms are used to shrink the video data on different DSLRs into a manageable size for storage and playback. The Nikon D90 and many early HD DSLRs used Motion JPEG (MJPEG) compression, which results in a data bit rate of up to 48 Mb/s for 720p format video including sound. (Note: Divide Mbits by 8 to get megabytes/sec). MJPEG-compressed movies can be opened by most video-editing programs, but create fairly large files. Pentax still uses MJPEG compression in its pro 16.3-megapixel K-5 DSLR, averaging 62 Mb/s for 1080p video with stereo sound. To keep video files smaller and reduce bit rates without lowering image quality, manufacturers such as Canon, Nikon, Panasonic and Sony are using the advanced H.264 compression codec in many models. That codec can squeeze 1080p video down to about 17 Mb/s to 44 Mb/s, depending on the camera. The Canon EOS 5D Mark II uses H.264 compression to achieve bit rates of 42 Mb/s in .MOV format. Nikon's D7000 uses H.264 to achieve 44 Mb/s in .MOV format, and Sony's A580 and Panasonic's G-series cameras use H.264 compression within the AVCHD format file, resulting in a bit rate of 17 Mb/s.

Not all manufacturers list the compression format or bit rate in their camera specs. Instead, they list the maximum length of a single video clip or the maximum size in gigabytes of a single clip—which is usually 4 GB due to limitations in the FAT32 file structure used on most memory cards and hard drives. That equates to up to nine minutes of video per clip on the Pentax K-5, up to 30 minutes for the Sony A580 and up to 12 minutes for the Canon EOS 5D Mark II. If you think 4 GB per clip is a limitation, perhaps you're better off with an HD camcorder—since it's more likely your audience will fall asleep watching any continuous clip longer than five minutes.

The Trickle-Up Effect

Someday we may see a liquid-cooled HD DSLR that solves the over-heating problems in video mode, and it's likely that feature will show up in an advanced-amateur-level DSLR first. That has been the trend lately, and examples include the Nikon D7000's active AF capability in Movie mode and the Canon EOS T3i's Movie Digital Zoom feature. Nikon's active AF uses contrast detection and actually can track a slow-moving subject during video recording. Canon's Movie Digital Zoom function magnifies the center of the video frame to the equivalent of a 3x-10x optical zoom while maintaining the 1920x1080 pixels per frame of full HD video, effectively turning a 50mm ƒ/1.4 prime lens into a 50-500mm ƒ/1.4 zoom (with a jump from 50mm to 150mm) for video. While it's possible that optical resolution, light sensitivity and color gradations will change as the magnification progresses due to fewer pixels being sampled, it's still an innovative feature that we bet will trickle up to pro Canon DSLRs soon.

Login to post comments
Subscribe & Save!
International residents, click here.