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Tuesday, April 12, 2011

The Truth About HD DSLRS

The latest pro DSLRs boast incredible photo and HD video capabilities. But are they really the best choice for both still and motion?


What Do Pro DSLRs Offer On The Video Front?
With the video advantage tilting toward most advanced DSLR models, are there still reasons why a pro-level DSLR may be the best tool for your video projects? Perhaps.

Most pro-level DSLRs are more durable and weather-resistant, feature higher-capacity rechargeable batteries and include advanced image-processing circuits to reduce noise caused by heat or shooting at higher ISOs. That’s the case whether the pro camera uses an APS-C sensor, a Four Thirds sensor or a full-frame sensor. In this last category, only two full-frame pro DSLR models from Canon can output up to 1080p video—the EOS 5D Mark II and the EOS-1D Mark IV. These deliver an advantageous full field of view from an ultrawide-angle lens, plus an increased depth-of-field “separation” at large apertures compared to smaller APS-C or Four Thirds sensors. According to extensive RAW data comparisons found at www.DxOMark.com, the larger pixels found on full-frame sensors contribute to lower-noise images and increased shadow details (especially at higher ISO settings), and it’s likely these benefits are passed on to the video frames.

Other still photo advantages of full-frame sensors, such as typically higher resolution and improved color and tonal gradations, unfortunately may be lost in translation when recording video. The reason is that for full HD 1080p resolution, only a fraction of the pixels found on a typical DSLR sensor (approximately 2 megapixels) are needed for each video frame, so a full-frame, 21-megapixel sensor may not produce video that’s any sharper than a 12-megapixel APS-C or Four Thirds sensor (assuming all use a high-quality lens and are focused accurately). In addition, as video is being recorded and saved, the white balance and color bit depth get locked in, removing useful tonal data. What you wind up with is closer to 24 or 30 JPEGs per second recorded to video instead of RAW files containing additional tonal data. Someday, HD DSLRs may include a RAW video recording mode, but for now the volume of data produced by capturing 30 RAW frames per second at 1080p resolution (approximately 1440 Mb/s, or 11 GB a minute!) is too much for even the fastest memory cards and would bring most computers to a standstill in postprocessing.

Shrinking the pixel data captured by the entire sensor in a DSLR down into individual 16:9 aspect ratio frames requires a mixture of in-camera cropping and pixel-sampling techniques. On most DSLRs, pixel sampling merges the exposure and color data from multiple pixels into each pixel used in a video frame. This results in improved light sensitivity, lower image noise at high ISO settings, greater shadow and highlight details, and truer colors. Performing this magic at a constant 24 or 30 fps, however, requires a super-fast video-processing engine, which produces heat as a by-product. Unless that heat is removed effectively—which is more likely in a pro DSLR body containing a magnesium-alloy frame—the image quality of the video will begin to suffer. Some DSLRs, such as the Pentax K-5, have heat-warning indicators and will shut down automatically at a threshold temperature in order to protect sensitive circuits.

The bottom line: For the price, full-frame DSLRs and pro-level APS-C DSLRs may not provide significant HD video quality advantages compared to semipro and advanced DSLRs, but they’re still a better choice for their rugged construction and reliability.

 

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