I asked Juan Martinez, of Sony at Park Ridge, New Jersey, what was up with far red contamination and the Sony EX cameras. Here’s what he had to say:
“Yes, the prism on the Ex has broad spectrum dichroic filters. As you suspect, the Ex IR cut is 700nm, but it’s not steep, to preserve far red response. This issue occurs mainly with black polyester fabrics which reflect most of the IR while absorbing the visible, never with natural fibers or substances.”
Here’s my take:
The color red has always been the bastard stepchild of the video world. The rule of thumb for wardrobe and NTSC was “No white, no black, no fine patterns, and no bright reds!” White and black were contrast issues (we could hold one or the other but not both), and fine patterns had to do with moire and interlaced video. Red had the least bandwidth of any of the broadcast channels and strong saturated reds had a tendency to smear.
The first time I tested a Sony F35, at Videofax in San Francisco, video engineer Jim Rolin gasped as we aimed the camera at a Persian carpet. “I’ve never seen so many hues of red before!” With the advent of digital television and digital cinema, there’s no need to cripple equipment such that it works best in the limited world of NTSC video. Today’s cameras need to capture color in a way that their predecessors never did, because HD broadcast and digital cinema have much wider color gamuts.
Sony’s on-sensor hot mirror on the EX and F35 cameras starts cutting at 700nm, right at the edge of the visible red spectrum, so the issue isn’t really poor filtering on Sony’s part. Sony is trying to keep as much of that red spectrum in the image as possible in order to open up the possibilities for more subtle hues and shades of red. The problem is that synthetic fabrics tend to reflect a lot of far red/near infrared, and you can’t cut that part of the spectrum without crippling some of the enhanced red response of the camera.
The good news is that we have the option of enhanced reds if we want them, and if we don’t we’ll shortly have a filter that gives us that choice.
It’s not a bug, it’s a feature. It has its drawbacks at times, but I’d rather have the option than not!
Art Adams is a DP who lives on the edge of the visible spectrum. His web site is at www.artadams.net.