Light Fall Off (2 replies)

1 week ago
OMA 1 week ago

Hello Roger and fellow forum members,

Is there a way to figure out how much surface area will fall within a given f/stop?

I did a quick test today to measure the fall off of a spotted 200 Watt Fresnel: (measurements were done along a straight line)

  • f/16 - f/11 covers 24"
  • f/11 - f/8 covers 36"
  • f/8 - f/5.6 covers 48"
  • f/5.6 - f/4 covers 67"
  • f/4 - f/2.8 covers 65"

I guess this is useful to know if you know you will have talents move about a certain area and you want to keep a consistent aperture, but is there a way to calculate the surface area that will fall within a given f/stop/illumination other than by measuring it yourself?

I am assuming that having diffusion or bounce would all significantly alter those results, hence why I am curious if there is a way to figure this out mathematically of some sort.

Thank you for your time!

5 days ago
bk90 5 days ago

Let me preface this by saying my lighting knowledge is still bush league. However I did see ARRI has a photometric calculator on their site which might be hepful»


The Byre
4 days ago
The Byre 4 days ago

We have a saying in German „Wer viel misstmisst Mist“  (He who measures a lot, measures dung.)  and I suppose the same may be said for calculating too much.  You can calculate everything and still be wrong on the day!  (But that link posted by BK90 is interesting.)

Like all things - my approach is suck it and see!  And if you have to move the image from one lighting area to another, try tweaking the aperture, allowing the eye to 'discover' hitherto unseen details or maybe even a person or key object hidden in the darkness.  That can be quite a startling effect and allows the audience to see something that the main protagonist has missed.

Conversely, you can also just bring the lighting up to reveal things hidden in the shadows.

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