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Filed under: *VIDEO* •Cameras •Production •Training •

No 85 Necessary on the F35!

One of the first things people seem to notice about the Sony F35 is that it has no filter wheel. “Aha,” many say, “this is the ultimate film-style HD camera. I’ll actually need to use an 85 filter in the matte box when shooting outdoors, just like a film camera. Finally an HD camera I can understand!”

Wrong.

The first time I ran into the “D5600”—or digital 5600k white balance—option on a camera I thought I was being conned. It was on a Sony D50 industrial camera and the filter wheel consisted only of ND’s. Daylight compensation consisted of a single button, labeled “D5600”, so I figured there must be some trick going on that I didn’t quite trust. “All of the other video and HD cameras I use are tungsten balanced because they all have a filter wheel containing a color correction filter,” I surmised, “so Sony must be cutting costs by avoiding the orange filter, boosting the blue channel artificially and compromising the image in some way.” I resisted the idea of simply pushing a button to achieve a daylight color balance, thinking that clearly the proper way to color correct would be to use an optical filter just like every other video or HD camera I’d ever used.

Nope.

When the RED came out there was a lot of buzz about it being a “daylight-balanced” sensor. This is a big deal because cameras are frequently, if not primarily, used in tungsten environments, and who would make a camera that was so obviously biased against tungsten? It turns out that Mother Nature would, because ALL cameras with silicon sensors are biased toward daylight.

Silicon itself doesn’t really “see” colors, at least not in a way that is directly translatable into a usable signal. Red, green and blue colored filters in front of the sensor (or sensors) pass only a limited spectrum of light to underlying photosites, which then create an electrical signal. That signal is passed to a processor that “knows” what color of light is being sensed by that particular filter/photosite combination.

Here’s a graph that shows how silicon “sees”:

(linked from the Delft University of Technology)

Different colors of light cause different reactions in silicon, and that results in varying voltages that are then quantized (or converted into steps of luminance) by the analog/digital converter. Based on the graph above we can see that blue (roughly 400-495nm) is the least “powerful” color in terms of how a blue-filtered photosite will respond. Green (roughly 495-570nm) is next strongest, and red (roughly 620-750nm) is by far the strongest of all. (Silicon sensors are extraordinarily sensitive to IR and “far red” and are heavily filtered as a result.)

As silicon sensors are least sensitive to blue it only makes sense that they are “daylight balanced” because daylight is overwhelmingly blue. The weakness of the blue channel is overcome by the sheer volume of blue in daylight. Tungsten is the weak spot of silicon sensors because tungsten light is very warm, meaning it has very little blue in it. In order to get a proper color balance under 3200k light, gain is added to the blue channel to punch it up a bit. And we all know the result of adding gain: noise!

That’s why blue is always the noisiest channel in any HD or video camera, at least under tungsten light or daylight with a corrective filter. When you add additional gain overall (say, +12db) the resulting noise will contain a lot of blue specks because the blue channel, under tungsten light, has already had gain added to it. It’s being “pushed” farther than the other channels.

See the proof on page 2…

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you mean…
If I work with the F900 , I won’t need to turn the correction filer to the 5600K…but just go with 3200K filter wheel and turn the electronical 5600k mode on in the menu..

Posted by .(JavaScript must be enabled to view this email address)  on  01/23  at  01:16 AM

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