Relationship between light output and contrast in a room.

[Ocon]

Hi Roger and fellow cinematographers,

I’ve always been curious about whether the contrast in a scene changes if you adjust both the output of your lamps and the t-stop on your lens by the same proportion in a controlled lighting environment (no natural light etc.).

For example: If you’re filming a scene at night in an apartment room, using only a few practicals and a film lamp outside the frame, and you change your t-stop from 2 to 2.8 while doubling the output of all practicals and film lamps, will the contrast of the image remain the same? Do the room’s bounce light and the light from the lamps change proportionally?

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[gabj3]

In our world, there are too many variables to give this a definitive answer; however, if one were to view it from a mathematical perspective, technically, everything should stay the same.

Why – everything is relative.

E tan i + E tan R = E tan T

Fresnels Laws –

How much light is reflected?

E (r) = ( n1 + n2 / n1 + n2 ) * E (i)

How much light is transmitted?

E (t) = ( 2*n1 / n1 + n2 ) * E (i)

Notice how it’s proportional to the amount of energy as a linear multiple.

Now, it’s important to note that everything is relative; if you have daylight spill coming into the room and doing the same, you will have a contrastier image. But principally, no nothing changes.

 

 

Infinityvision.tv
Gabriel Devereux - Engineer

[dmullenasc]

In theory in a controlled environment with no practical or natural lighting of its own illumination level, then doubling the light level should give you the same contrast, the brighter lamp bounces back brighter off of surfaces, etc.

However, visually when you shoot at wide apertures and the background goes soft, it can feel lower in contrast because the highlights and shadows in the distance blur over each other — you can see that on a waveform monitor just pointed at a 11-step grey card, if you throw the image out of focus, most of the waveform signal piles into the middle grey zone even though the signal edges still hit the far targets.  The in-focus subject doesn’t change, contrast-wise, but with the blurry background softening contrast back there, the contrast can feel less harsh overall.

[simon m]

David, that is a great bit of hands-on information. Thanks so much for that! cheers.

[Author]

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