Reducing noise

[Andrew Somers]

A) In theory when you are correctly white balanced, a green element that is sufficiently narrow should (in theory) occupy the green channel with little cross talk in the red and blue.

B) When you are working in AE in LINEAR color space, then a stop is just like in the real work – one half or double – i.e. 0.5 is one stop less than 1.0, and 0.25 is one stop less than 0.5, or two stops less than 1.0.

C) As for AT METER, when measuring REFLECTANCE using a SPOT METER on the surface, “at meter” implies that you are setting exposure for that value. If we are saying that you are setting exposure using the reflectance of an 18% card (putting that card in “Zone 5”, then your green screen can be as much as two stops darker than *that* level.

Of course, there are issues relating to the angle of reflectance, etc etc et that I am not going go into deeply here (for instance, in reality the meter inside dSLRs is calibrated to be “at meter” based on a 12% card, and this is a subject I am not going to get into at the moment, but here are two links with opposing viewpoints on the subject:

https://www.bythom.com/graycards.htm

https://dpanswers.com/content/tech_kfactor.php

But in terms of YOUR situation: If using a spot meter on an 18% grey card at your SUBJECT’s location you get a specific reading, and then point that spot meter at the green screen at roughly the same angle, the green screen should be NO BRIGHTER than the grey card, and can in fact be as much as 2 stops darker than the grey card. I normally aim for 1 stop darker than the grey card.

This assumes that you are shooting your subject “at meter” that is, at the settings as indicated by a light meter.

As the brightness of the green screen increases past “meter”, you will get into a region where it becomes clipped and/or washed out – washed out means that you are adding red and blue to the image which is exactly what you DON’T want.

[Oliver De morassé]

Thanks for the tutorial Andrew – very informative ;O)

I would be interested in what you think of the sample videos I posted:

1. Is the white balance now better?
2. Which picture style do you think is best for keying?

Thanks again.

[Oliver De morassé]

Have I understood correctly, based on the results I posted, the picture styles have the following approximate stop differences between the green & blue channels:

Cinestyle: 0.5
Faithful: 1.5
Landscape: 1.5
Marvels Adv: <1
Marvels Cine: <1
Neutral: <1
Portrait: >3
Standard: <1

Based on these findings, then either Faithful, Landscape or Portrait would give the best keying results?

[Oliver De morassé]

Hi Andrew,

Have you managed to have the time to check out my demo files? If you have the time, I would really appreciate your comments on which picture styles you find best and if the white balance is OK.

Many thanks ;O)

[Oliver De morassé]

Hi Andrew, any chance of your comments to my final question. Many thanks in advanced.

[Oliver De morassé]

Hi Andrew,

I’ve got a recording coming up tomorrow – I would really appreciate your thoughts on which picture style best suits keying.

thanks again for your tremendous input.