Struggling to understand why this is happening

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Robdoc

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Struggling to understand why this is happening

PostWed Jun 16, 2021 11:17 pm

Hi everyone,

I was hoping that someone can help me figure this out. I have been trying to see when I need lighting in a shot and when I don't. I recently did an interview with someone where the false color showed green and gray but then the video came out grainy.

Here is another example below. I am using the lens I use for interviews (Rokinon 85mm T1.5 Cine DS Lens) on the BMPCC 6k.

The first picture below shows with the false color on: (With the green an grays I would think that shot would be ok). The second shot shows what the view from behind the camera looks like. When I bring this is resolve the shot is noisy even in the parts that are green/gray. Does anyone know why this is happening and how I can avoid it?


Image

Image
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rick.lang

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Struggling to understand why this is happening

PostThu Jun 17, 2021 12:14 am

Your exposure is possibly three stops under. That light wall should be light grey (or even the beginning of yellow) in false colour. You have lots of blue which means it is in danger of clipping. The purple is clipping the shadows (i.e. black). You might be able to save the shot in Resolve but it might not come out looking good without adding some noise reduction.

It does look like it would be properly exposed in the first image. Is there any chance the iris is stopped down when you’re recording the false colour and wide open when you’re judging the shot in true colour?

Or are you applying a custom LUT when recording?
Rick Lang
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Peter J. DeCrescenzo

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Re: Struggling to understand why this is happening

PostThu Jun 17, 2021 12:34 am

The Pocket cameras' native ISOs are 400 and 3200. ISO 400 records less noise than ISO 3200.

Refer to pages 46 & 53 in the camera's user manual. As a starting point, if your goal is to record a relatively low-noise image with relatively high dynamic range, set the camera to ISO 400. Turn on False Color. Adjust aperture, or Shutter Angle/Shutter Speed, or lighting so human skin displays between green & pink in False Color.

Another good starting point is ISO 1250 because it's less noisy than ISO 3200 (but of course is more noisy than ISO 400).
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Robert Niessner

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Re: Struggling to understand why this is happening

PostThu Jun 17, 2021 8:24 am

You have to understand what native ISO means.
As Peter wrote, the camera does have two native ISO stages. Stage 1 is rated as ISO 400. If you select a lower ISO the camera just uses a different gamma curve onto the signal to darken the image. If you select a higher ISO up to 1000 the camera uses a gamma curve to brighten up the image.
This raises the signal and of course also the visible noise. If there is enough light in your scene then you won't see noise.

It is like recording a voice from a distance in a noisy ambiance. If you raise the gain of the voice you will also raise the noise level. To get a better separation you will need to get your mic closer to the speaker.

When you switch the camera into gain stage 2 - starting with ISO 1250 - the camera will use a different gain circuit. It will have a lower noise floor at ISO 1250 then ISO 1000, but at the expense of less dynamic range in the highlights. It still needs some light - just cranking up the ISO increases the noise level again.

Now when you are using false color for checking your exposure and then just change your ISO to make it green/gray this means you are not increasing the light hitting the sensor but raising the signal through processing. And when you do that you will also raise the noise floor.

Blackmagic cameras do not have a strong post denoising stage in the signal chain like for example Sony Alphas. You can do that in post in Resolve if you want - best starting point is using the temporal denoiser. By doing this in post you have the best control over the outcome.
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rick.lang

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Re: Struggling to understand why this is happening

PostThu Jun 17, 2021 10:56 am

To summarize, it appears your first shot looks properly exposed but it has been raised three stops in camera by the selection of using ISO 3200 versus ISO 400. However the histogram, when the false colour is displayed while recording, is showing all your true light levels are approximately in the first quartile which indicates under exposure of your interior scene.

You can add light for the sensor (while exposing at ISO 400 for lowest noise or 3200 for brighter image) by opening the iris a couple of stops and using a shutter angle of 360 degrees or by adding additional lighting which would be preferable if feasible. If you add three stops of light and expose at ISO 400, you may still be able to boost the exposure in Resolve and may not need to add some noise reduction. Test and let us know.
Rick Lang
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Robdoc

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Re: Struggling to understand why this is happening

PostThu Jun 17, 2021 2:20 pm

Thank you all for the responses. I plan on following what you said and doing a test shoot Friday night. I will let you know the results. Thanks again!
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Bromine 18

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Re: Struggling to understand why this is happening

PostThu Jun 17, 2021 6:34 pm

Here’s a not widely known fact – even Arri use in-camera temporal noise reduction in all their camera lines: https://www.arri.com/resource/blob/7950 ... r-data.pdf

Although it’s not baked into the footage and stored as metadata for ARRIRAW – but baked in for ProRes, interestingly – I’d be surprised if anyone uses the Alexas with the noise reduction turned off.

Raw video is innately noisy, and by design.

In addition to trying what others have suggested, I’d recommend denoising in the very first node when grading in Resolve. You can use spatial NR with Faster Mode, Small Radius, and uncoupled Luma and Chroma, with Chroma set between 8 and 10. Although keep in mind that the Faster Mode may induce colour artifacts around strong monochromatic lights.

If your editing machine can handle it – or if you’re OK with the rendering time being significantly increased – then use Temporal NR, with 5 frames, uncoupled Luma and Chroma, and Chroma between 8 and 10.
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Robdoc

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Re: Struggling to understand why this is happening

PostSat Jun 19, 2021 12:52 am

Hi everyone.. I just wanted to post an update and get your thoughts.

I shot this short video my wife (using the Rokinon 85mm T1.5 Cine DS Lens) making jewelry. Here is what the false color on the BMPCC6k looked like:
This was using the overhead light and I added one light on the right hand side of the screen.
Image

This is what the raw image looked like in resolve:
Image

Do you think I need to go up more in terms of getting more pink on her face. I thought that was a nice image in false colors. Any input would be appreciated. Thank you!
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rick.lang

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Struggling to understand why this is happening

PostSat Jun 19, 2021 1:41 am

That’s a good exposure for her face which has light grey in the brightest part of her cheek, with most of her face medium grey (between the green and pink), but you could certainly increase the exposure one stop or perhaps more to get some increase in definition in the shadow of her neck which would also show more pink in the false colour and might eliminate the blue area.

In my opinion it’s okay for highlights to touch or approach yellow… get to yellow on her face and then dial the exposure back until yellow just disappears.

However that one stop increase can likely be done in post with good results. As long as you’re not clipping, it’s pretty safe to feed the sensor for a scene that is going to playback as a pleasant scene. Yellow isn’t clipping but you may not want a face to be yellow. The RGB clipping boxes with firmware 7.3 for the Pocket cameras are reliable and invaluable.
Rick Lang
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Dmitry Shijan

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Re: Struggling to understand why this is happening

PostMon Jun 28, 2021 1:52 am

This is your problem.
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javier forza

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Re: Struggling to understand why this is happening

PostMon Jun 28, 2021 2:13 am

Don't confuse the grey around green with the grey over pink.
I think that tonal range in false color should be differenciated clearly.
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BennoZ

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Re: Struggling to understand why this is happening

PostTue Jun 29, 2021 3:53 pm

If you really want noise-free images try the ettr technique. I use it when filming nature and apply the Leeming lut, it’s awesome.
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rick.lang

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Re: Struggling to understand why this is happening

PostTue Jun 29, 2021 11:08 pm

javier forza wrote:Don't confuse the grey around green with the grey over pink...


True. It’s easier at the extreme where very light grey is approaching yellow, but in the image in question, the False Colour scale does show that the grey just above pink is almost the same as grey between green and pink.
Rick Lang
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Rakesh Malik

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Re: Struggling to understand why this is happening

PostWed Jun 30, 2021 4:55 pm

There's an easy way to figure this out: shoot more tests. Shoot the same frame while you adjust the settings to change exposure. E.g. start underexposed, and then open up the iris in one stop increments, snapping a picture of the false color image at each stage, and then pull the footage and put the false color image next to each corresponding image.

Do that and it won't take long to get a good initial sense of how you should expose skin tones to be pleasing to your eye, and then you'll have a good starting point to work with.

Just make sure that the monitor you use to view the test images is calibrated. :)
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