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(Page 1 of 3 pages for this article 1 2 3 >) Filed under: Cameras •Production •Tips •Training • Tiffen tests new no-ND IR filter for EX1/EX3/F35Crisp clean blacks, and only a half-stop light loss, make this new filter a winner
The broadband dyes used in the Sony F35, EX1 and EX3 sensors work wonderfully well in most regards. They allow us to capture subtle hues of color that electronic cameras have not been able to capture in the past. Unfortunately this comes at a price: the red dye passes a little too much light at the edge of the visible spectrum, so the cameras see red in some places where our eyes don’t.
I’ve written before about how silicon sensors are most sensitive to infrared (heat) energy. To ensure that the sensor sees the world the way people do, every sensor has a “hot mirror,” or dichroic filter, to reflect and reject the invisible wavelengths of light and leave the visible wavelengths of light free to pass through to the sensor. Additional IR filtration is necessary because the on-sensor hot mirrors don’t always cut enough IR to keep the image free of IR contamination, especially when using neutral density filters. Neutral density filters reduce the amount of visible light hitting the sensor but don’t block invisible light, which then fools the sensor into seeing light that, to our eyes, isn’t there. Some cameras, like the RED, have hot mirrors that work well under normal circumstances but fail to cut enough IR when neutral density is added. Because the problem lies in the realm of invisible light it is necessary to use a hot mirror to eliminate it. Dichroic hot mirrors are very good at removing wavelengths of light that we can’t see. Here’s an illustration of what causes IR contamination in daylight: Under normal circumstances the small amount of IR that reaches the sensor makes no difference, but when the amount of visible light is cut without cutting IR as well then the camera sees red where there shouldn’t be any. Dichroic hot mirrors reflect invisible light to prevent it from reaching the sensor:
The problem with Sony F35, EX1 and EX3 cameras is completely different. They have a very good hot mirror in front of the sensor, or chip block, that rejects all light wavelengths above 700nm. The issue instead seems to be with the broadband dyes that Sony uses on the sensors of their newer Cine Alta cameras. In the past, the red, green and blue sensor dyes passed only narrow wavelengths of light, and this limited the ability of cameras to see secondary colors. Up until only a few years ago it was very difficult to obtain accurate yellows, oranges, cyans and purples because the spectrum passed by the sensor dyes didn’t include any of these colors. At NAB 2007 a friend asked an old-time video engineer, “Why did it take so long for the technology to advance to the point where broadband dyes became possible?” The answer was that the technology has existed for twenty years, but the NTSC spec was so limited that there was no point in trying to capture those colors because they could never be broadcast accurately. With the advent of digital cinema, in which subtlety of color is both possible and desirable, that has completely changed, and Sony has responded admirably. But nothing comes for free. The good news is that the red channel sees more subtle hues of red than ever before, which on the surface is a good thing. The bad news is that this makes the camera more sensitive to “far red,” which is red light at the edge of visible light. Our eyes don’t see far red but the camera sees it very well indeed. Since the goal is to reproduce only the visible spectrum, and far red is invisible to our eyes, it looks a bit odd when it appears in a picture. And it can appear even when no filtration is being used at all. It is possible to make a dichroic filter heavy enough to cut red down to around 680nm, which is where the problem seems to start, but the filter is so heavy that it’s impossible to use it with wide-angle lenses. Dichroic coatings look clear when viewed straight on but change color when viewed at an angle. Wide-angle lenses capture light right to the edges of the filter, and the angle of that light is sharp enough to cause a color shift. Here’s what’s happening:
The green rays show the angles at which light will change color when passed by the filter. Here’s an example of what that looks like:
The filter used here is the Schneider Tru-Cut 680, a filter that (as best I know) has never been publicly released. It’s based on the wildly successful Schneider 486 filter, which was developed for early digital still cameras that were manufactured without a hot mirror. It’s a spectacularly effective filter when it comes to cutting IR and far red—nothing beats it!—but it can’t be used on wide lenses at all. The picture shows what happens if you use this filter on an EX1 zoomed all the way out to 5.8mm. So we know that the issue with the Sony F35, EX1 and EX3 is not IR but far red, and we also know that dichroic filters that cut low enough to block far red are too heavy to use on wide-angle lenses. What’s the solution? More on page two…
(Page 1 of 3 pages for this article 1 2 3 >) 
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