August 24, 2015 at 8:15AM, Edited August 24, 8:19AM
HMI vs Tungsten? Solutions for Blue Channel
I was recently on a studio shoot with a red cyc, and lit in tungsten.
I've dealt with a less extreme version of the issue before but this time it was being shot on a 6k Red Epic Dragon and the deliverables needed to be in 5k for a large LED billboard.
I've had a lot of noise and was discussing with the dp and dit of how we can deal with this issue. Since we were on the clock, we gave it a little more power and opened up a half stop and rolled.Since then i've done some research on sensors, and electricity.I think this information can be useful for deciding for lighting and camera scenarios in the future.
>Camera Color Temperature<
---As Camera's have native ISO's, they also have Native WB.
Red is rated at 5600K. As any increase in ISO is just a metadata change the WB is also a metadata change. Shooting in Tungsten automatically increases noise slightly.
---The recommendation i got was to still shoot at 5600K with a lens filter to tungsten just like the film days.
going from 5600k to 3200k by filter you do lose 2/3 stop.
>HMI vs Tungsten<
This was a problem not only in Digital but in Film as well.
Blue (cold) wavelengths are shorter and faster than Red (warm) wavelengths.
The sensitivities of the sensors are the same but the sensor needs a lot more BLUEish tones in a scene to render a clean neutral Blue Channel.
In a LRB Beyer sensor (every camera)
There is no Green channel.
The green is artificially generated by the cameras CPU by scanning and guessing what is missing from the RED and BLUE. Since there is not much blue, the camera is not able to generate a clean Green Channel correctly either. This results in some weird tints in skintones.
---By using Tungsten lighting. You're providing even more information to the RED channel and it further increases Noise in the other two channels.
---By using HMI's you are feeding the blue channel and also naturally adding Cyan to the red channel.
Since there is more information in the Blue channel, the camera can now do a better job at generating the Green Channel. This will reduce overall Noise.
>SET DESIGN FIX<
This is not Ideal, especially for the art director, but with a software like livegrade or resolve live it is very possible to even see it live on a set/client monitor. thru a router and some lut boxes. This will work with either tungsten or HMI or any lightsource besides normal fluorescents.
Instead of RED BG, you go BLUE BG. as long as none of the looks (outfits) are blue hue.
This accomplishes two issues
---Blue is not present in skintones so it will not affect. Any bleeding can be desaturated a lot easier by keying.
---Sensor is fed with enough Blue to feed the Green channel. As a result reducing noise.
---In live grade or Resolve Live you can shift the entire BLUE channel to RED.
this signal can be routed to Client monitors via LUT.
>EMERGENCY SET FIX<
Throw a blue filter on the lens. 1/2 CTB / 205 wil cut 2/3 stop but will significantly reduce noise.
this is isn't the easiest fix but in a channel based node software like nuke or flame, you can feed the clean red channel into the other two channels and match the matte with a curve adjustment to clean the noise. This is much more effective than using noise reduction software. It also isn't overly difficult but it will definitely take long render time.
Please feel free to correct, make suggestions and advise on this topic