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I saw a brilliant thread talking about why the colour red is noisy in post:

Why does the color red always appear pixelated on TV and videos on the PC?

I found this incredibly helpful in understanding why red appears that way. What I want to know is what can you do when filming dark red to avoid high levels of noise?

I was recently filming a museum installation and all the walls were dark red brick. This museum was inside so the natural light was very limited, so naturally I shot at my highest aperture (at the time was 3.5) and set my ISO as low as possible. My camera hasn't got the best sensor but I just wondered if there was an explanation or other tips as to what to do when you know you're in a location with lots of dark red.

Cheers

Lav

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    Can you say more about what kind of camera you are shooting with? What codec? Can you post a clip of what you consider to be unacceptable quality? – Michael Tiemann Feb 16 '17 at 15:31
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Filming any light that is strongly a single color is likely to produce a lot of noise because it's absorbing more light. (In your case, the red brick is absorbing green and blue, leaving only the red.)

Furthermore, if you're filming with a Bayered CMOS sensor like the ones found in most phone cameras and DSLRs, the sensor captures twice as much green light as it does red or blue light. The sensor sites are generally set up in a red-green-blue-green order.

So you're starting with less light reflected from the material and your collecting less light at the sensor. Any noise is going to be magnified because the resolution of the red channel is half that of the green channel, too. So how do you avoid it? You have a few options:

  1. Supply more light to the scene
  2. Use a camera with a sensor that has good low light response
  3. Apply noise reduction in post
  4. If you can change out the background (either by replacing it or filming from a different angle), that might be better.
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Red is the (in most cases/sensors/cameras) the noisiest channel in the RGB encoding scheme.

The reason is simple, it's the least sensitive to light, so if you think of Red Green and Blue, in Additive terms (light, not paint/ink), and compare it to a 50% grey card... and if you were to make a 50% IRE Red, Green and Blue card, Green would appear the brightest, then blue, then red.

The fact that Red is (on average) about 1 stop less than green in terms of brightness - when measured next to G or B under identical light, inherently is more noisy, as it's 1 stop under exposed compared to green.

This is why shooting green screen is easier than blue - it's brighter and needs less light to hit the target IRE levels.

In closing, any channel will be noisy when underexposed, depending on the camera of course - even when shooting 444. You'll expose correctly, but RED will always be roughly 1 stop under.

  • I think you've swapped red and blue in your description. The eye is least sensitive to blue, and it is weighted the least in, for example, luminance calculations. – user1118321 Feb 18 '17 at 16:15
  • I stand corrected, for whatever reason I had them swapped. R comes in higher in terms of IRE than blue, and green stands out highest. I'm not sure if it's the eye... human that is, that's least sensitive to blue. Blue comes in with a lower luminance than red on scopes, I just checked, so I think under normal lighting, you're right, Blue is darkest... now put them under extreme Kelvin temperatures... I'm not certain... perhaps Red "becomes" less bright than blue in terms of image acquisition - because we typically light under 5700 K. Which is blue... making red Purple. ??? – McFlySoHigh Feb 19 '17 at 13:45
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In a Museum using a Flash or 'constantly on external Light' is not permitted, so adding light is not an option in that case; more light would equal less noise.

Similarly adding a Filter would enhance the red's representation compared to blue and green, but that's a better option for a sunny day and not in limited light.

The final choice being White Balance; with a Cellphone tap on the Brick (on the Screen) and try to ensure the result is not causing too pronounced reds. Tap a few times to find the best result.

Both Cameras and Cellphones have settings for the type of light used; sometimes choosing Fluorescent or Daylight can push it far enough to get a better Shot.

For more Info:

http://www.graphics.com/article-old/photography-fundamentals-white-balance

http://www.red.com/learn/red-101/color-cast-tutorial

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