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  • Hey all, just changed over the backend after 15 years I figured time to give it a bit of an update, its probably gonna be a bit weird for most of you and i am sure there is a few bugs to work out but it should kinda work the same as before... hopefully :)

noisy footage when using low color temperature

T

totti

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I just came back from a shoot where we set the RED to 2200K to make our scene blue. The scene was low light (T2.8) at 320ASA. We shot 2k / 16:9...

During the shooting we saw a lot of noise but went ahead and shot it anyway. Back in post production we looked at the footage. It is unusable due to the digital noise level. It looks like somebody switched on 18db gain.

We loaded the footage into RED Alert and put the color temperature back to 5600K. The footage looks great, no noise, no nothing, just clear HD image.

Does anybody have an idea why low color temperature adds so much noise to the image? Or am I doing something wrong?

Please let me know
cheers
totti
 
totti said:
It looks like somebody switched on 18db gain.
totti
Click to expand...


You'r right, sensor's best is at about 5500k so exposing @ 2200k is like underexposing your blue channel of 3 to 4 stops = aproximalty 18db.

See you

Pat.

PS: Better practice for you blue effect is to handle camera with 5000k preset and to put filters until your desired blue look (you'll see how much light you will need).
 
Also, shooting in 2K yields the noisiest images (currently).

Shooting in 4K wouldn't help the blue channel, but it would lessen the compression artifacts that are common with 2K acquisition.
 
Guys,

Expose for optimum at 5000 and grade blue in RedCine! Always get the best possible negative and you'll have more elbow room in post to go where ever you want.

Underexposing one channel is bound to give you noise!

Mike C
 
On film I may agree, but when dealing with noise levels associated with colour balance of chips I would always play safe and give the best possible exposure to the chip. In my experience grading blue in RedCine offers a much finer control without any of the downsides of introduction of noise by underexposing a colour channel.

Test, test, test!

Mike C
 
Tresch said:
I'm not sure about that. But this is an old debate about "in camera effects VS post production effect"...
Click to expand...
From what I understand, the white balance in the RED workflow is meta-data. Therefore, white balance on RAW data IS a "post production effect." It is non-destructive, and can be changed in post.

On a RED, the "in camera effect" would be to light the scene with blue gels as opposed to a low temperature source. As most people would agree, the in camera option will produce the most accurate results. That said, now that you've shot the footage, you've got a salvage operation.

White balance is constructed by adjusting the digital gain of the RGB channels. If you shot with low temperature light sources, the B channel is be noisy due to the fact that there is very little blue light in the image. The R and G channels of the image are probably much less noisy than the B.

If you want to tint your image blue, rather than use the white balance, I suggest you export the image using a daylight white balance. Bring the image into an appropriate colour correcting program, and shift non-noisy luma information from the R and G channels to the B channel. This will give your blue tint, but not over-emphasize the B channel noise.

Think of it this way: You shot the image with a red laser, but you wanted a white image. In this case, there would be virtually no information in the G or B channels. The white balance operation would simply multiply the noise in G and B until it was approximately equal to the signal in R.

Instead, if you took the R channel, and mapped that onto the G and B channels, you'd be 'transforming' the excellent R signal into an excellent G and B signal. The result would be a low noise image that is also white.

Of course, this operation will transform other colours in the image too.
 
Spiff said:
From what I understand, the white balance in the RED workflow is meta-data.
Click to expand...

We have a winner.

Yes, he said he shot with RED's color temperature set to 2200k which is simply metadata. So this was never really an in camera effect. No blue filter was added to the lens to give a blue effect. Ultracool lighting was not used.

Shooting 2K is noisier than 4K. So beware if you want the cleanest footage shoot 4K and downsize to 2k in post.

As far as turning the image blue from a clean source (which is what he shot) it should be done in a color correction app for best effect. Spiff has some good thoughts. Another approach is adding a clean blue layer and composting it over the clean image. Then no blue channel noise from the image itself is amplified.
 
joelnet said:
Another approach is adding a clean blue layer and composting it over the clean image. Then no blue channel noise from the image itself is amplified.
Click to expand...

YES, but you will not achieve the same results as adding gels infront of the lights or on the lens. Objects/colors react diffenrently on light as what you can achieve in post (for ex. red/orange would be near to black on filtered light).

Pat
 
Tresch said:
YES, but you will not achieve the same results as adding gels infront of the lights or on the lens. Objects/colors react diffenrently on light as what you can achieve in post (for ex. red/orange would be near to black on filtered light).

Pat
Click to expand...

But that's not how he shot this so I don't get your point in regards to this thread.

More generally, using secondary color correction technique you can isolate color ranges and desaturate them in different amounts to simulate real life low light or anything else.

I want you to get your shot however you feel is best. On camera filters and gelled lights are great. I'm just trying to clarify the situation in this thread.
 
Tresch said:
You'r right. Post is the only solution for his actual material. I was just giving my opinion on what solutions there are on set or before you have pulled the trigger:biggrin:
Click to expand...

I see, good advice. I guess I did the same thing when I mentioned shooting 4K instead of 2K for reducing noise didn't I? :mellow:
 
Optical: Light entering sensor is desired color. Best to balance to native sensor whitebalance to avoid post processing. Advantage of this is you are willing to overexpose the channels not getting much use (R & G) without clipping in the channels you don't need as much of. No Gain involved.

White Balance: Color channels are manipulated to balance. AKA if you want a cooler scene and reduce the white balance the already noisy over compressed Blue channel is being gained while your good and clean channels are being reduced to create the image.

Hue: A Hue shift will NOT increase noise. The extreme case of this would be a channel swap where you call the green channel blue and the red channel green and the blue channel red. The color will be wildly different. (And if tweaked properly very very blue) but without touching any of the channels' data.

---

The last two are the ones in question here. This is actually a trick compositors can use quite often. If you are given blue screen footage on 4:1:1 (Shudder) and have to extract fine detail an inexperienced compositor will only load up keylight and beat their head against the wall for 10 hours trying to get a good key. Instead you want to look at every channel of data that is available to you (including averaged channels such as an average of RG) you might find that the green channel (say blonde hair on bluescreen) acutally 'glows' pretty nicely in the green channel so instead of trying to use any of the garbage data in the blue channel you just ignore it and fenangle bits and pieces from the full resolution green channel.

This is especially true in a case such as this where you've shot a neutral scene but want it "Blue". You don't actually want to adjust white balance if you can avoid it. White balance will give you "More of the blue that you actually shot". Try to see if you can't use the good data in R and G to create a blue color scheme that looks good. I'll say it right up front you'll lose a lot of your color accuracy. It won't look exactly the same as a cooler color temperature... but it'll be as free of noise as your 5600K balanced footage.

Remember with a bayer sensor not only is the blue channel underexposed relative to R&G but it's also physically less sampled. And noisier footage gets DESTROYED by heavy compression. I posted an example on another topic involving some footage with Nazis and Monsters of some sort a few months back where they had very noisy low light footage and I simply threw away the blue channel completely and rebuilt it from the R&G channels along with some fancy curves. If the channel is just adding noise sometimes it can be done away with completely. Again this isn't perfect in that a black and white negative shot through a green filter is very different from a black and white negative shot through a blue filter, but sometimes the difference isn't very noticeable, especially in extremely desaturated sets.
 
Just making a small related point:

On all electronic/digital cameras blue is the noisiest channel. As far as I am aware this is due to the spectral sensitivity of the imaging chip's semiconductors, not as sensitive to higher frequencies/shorter wavelengths like blue but really sensitive in the reds (esp IR - hence the IR cut-off filters in most digital cameras).

This issue can be compounded by the codec and colour sampling rates.

JohnF
 
Cardmaverick said:
You can make the blue channel of ANY digital camera squeeky clean if you want, it just requires optically pulling down the exposures for Red and Green when under tungsten light... I would know, I've done this successfully many times.
Click to expand...
You have to increase the total amount of light when trying this though, correct?

All that cutting out the red and green does is give you an indication that you need more light to get a good exposure. You can't just filter out red and green and expect blue to get less noisy - you actually have to increase the amount of blue there by adding light wattage. Otherwise, you'll be cranking up your digital gain.
 
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