The Right 8 Bits

For DSLR shooters, the cool current "must-have" feature is Clean HDMI. When used in conjunction with a digital recorder, cameras like the Nikon D4 and D800, as well as the Panasonic GH2 and a few others, can capture high-quality ProRes files via the cameras’ HDMI output. ProRes is a lossy intermediate codec that gives you a much higher bit rate and less compression artifacts than your DSLRs’ H.264 or AVCHD file formats. One of the most popular recorders, the Atomos Ninja 2, can record 10-bit ProRes or Avid DNxHD files and has really swayed a lot of DSLR shooters to the Nikon D800 over the Canon 5D Mark III, which doesn’t offer Clean HDMI.

I recently shot some footage with the Nikon D800 capturing ProRes files to the Ninja, as well as capturing H.264 (Nikon uses B-frame data compression at 24 Mb/s) files to CompactFlash cards. On paper and in theory, I was expecting to see a huge difference between the two, but when I sat down and compared the 1920×1080 shots on my 15-inch MacBook Pro—I have to be honest—I couldn’t tell the difference between the two shots. (We should acknowledge that the Nikons still capture only 8-bit instead of 10-bit ProRes 422 files when used with the Ninja).

I was also pretty surprised at how poorly I faired in selecting the best cameras (i.e., most expensive) from the Zacuto web series Revenge Of The Great Camera Shootout 2012. In the episodes, viewers are asked to distinguish between a full range of digital camera systems, including the 16-bit Sony F65 to the iPhone 4S. (I’m the editor of HDVideoPro. I should know these things.)

I recently spoke with Neil Smith, who is the Managing Director and DI Supervisor of Hollywood DI, about color perception and sensor resolution (aka bit depth), in particular the difference between 8-bit and 10-bit. The boutique post house has finished numerous independently financed films that have made it to the big screen, including Like Crazy, the 2011 Sundance Grand Jury Prize Winner that was shot with a Canon 7D. According to Smith, there’s no straightforward answer. "If you have a choice, 10-bit is the way to go over 8-bit, and you’ll see differences with scenes that have a lot of fine gradations," he explains. "You probably would not want to shoot a sunset scene at the Grand Canyon in 8-bit."

If you didn’t know, bit depth measures the number of bits to ID the color of one pixel in your video image. There are 256 levels of gray for 8-bit color, and 10-bit has 1,024 levels, which is four times as much color information in terms of gray-scale information. Sounds like a lot, right?

But having said that, Smith also believes that if you find the right sweet spot in a shot, and there’s not a lot of gradation with bad moire-type effects, even the experts won’t usually see a difference. Smith worked with cinematographer Rodney Charters in the early days of the 5D Mark II, and even though the images were shot by an ASC cinematographer, Smith was still shocked at how good the 8-bit images looked. According to Smith, he asked Canon engineers on how they could get such a great-looking image from 8-bit, and their response was, "As long as you have the right 8-bit, you’re fine." DSLRs like the 5D Mark III have 14-bit sensors, but for video capture, they’re converted down to 8-bit. It’s this 8-bit "secret sauce" that the experienced engineers are able to pull off. "If the engineers are getting the right 8-bit in that final codec, it’s visibly lossless," says Smith. "Now if you’re doing vfx pulls, do you want to be working in 8-bit? I don’t think so, but if you’re shooting normal video like close-ups, skin tones, medium shots, you should be fine. Besides, what’s two bits between friends?

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