Can You Accurately Determine Lighting Ratios from Film Stills?

[Diego Benz Tejada]

I've recently been exploring the idea of analyzing still frames from films to determine the contrast ratios cinematographers use to achieve a specific look and feeling. This is something I've seen discussed quite often online. However, if I understand the technical aspects correctly, it doesn’t seem possible to do this in a truly reliable and accurate way because:

• The LUT and post-production workflow can alter the contrast ratios. For example, even if a shot appears to have a 2:1 key-to-fill ratio in the final film, the cinematographer might have originally lit it at 1.5:1 on set, with the LUT increasing the contrast.
• It's impossible to determine the exact stops used, as you can't extract light levels in stops based solely on an image. While you can see where something falls in terms of IRE, IRE values are linear, not logarithmic, meaning there’s no straightforward way to convert them into stops.

Is my understanding correct, or am I missing something?
I’d be curious to hear about your approaches to determining the lighting ratios on set —especially in relation to a show LUT.

Edited March 26 by Diego Benz Tejada

[Nicolas POISSON]

You are correct: there is no way to determine the lighting ratios because you have no idea of the curve used in post. It is even possible that "power windows" were used: some objects with the same reflectance could end up at different IRE levels, and some objects with different reflectance could end up at the same IRE.

However, if you have access to log footage, the formula of the curve is often documented and you can determine the scene reflectance from IRE values. You can do that easily in DaVinci Resolve using a CST from the log  to linear space.

By the way, this is exactly the principle of EL Zone system.

 

Edited March 26 by Nicolas POISSON

[Albion Hockney]

you can get a rough idea though and studying frames this way is a great way to learn.

[Stephen Baldassarre]

Check out some of the old Kodak film brochures.  Many of them have screen shots from actual theatrical films and what the relative exposure is for different parts of the frame.  That'll help you develop a baseline for looking at other images.  I seem to remember an engineer from Kodak sending me a book with a handful of images and relative exposures notated in them.  I think I lost it years ago though.  Kodak also has a great deal of material on YouTube.  Check out the following video, around 11:08

 

Edited March 27 by Stephen Baldassarre

[Steven West]

I built a 5 stop False Color Powergrade for Resolve that essentially tells you the contrast ratios in a still frame (within 5 stops)

Like you said, it’s impossible to know the ratios set on set for a few reasons, but what you can take away from back-engineering shots this way is if you find a look you like or are aiming for, you can check the ratios in the Powergrade and apply them on set yourself.

The Powergrade I made is limited to REC 709 also, so it’s really only useful for looking at end-result stills.