Inverse Sqaure Law

Hunter Jones · September 26, 2008

I just wanted to see how many have you have used the inverse square law when judging light distances between subjects?

Mitch Lusas · September 26, 2008

While I don't sit down and write out equations, I do run rough 'calculations' (if you can call them that) in my head. Mainly, I know the look I'm going for and am familiar enough with the lights I use that I have a good idea of placement in order to achieve the exposure for the subjects. It may just be me, but I only think of doing serious calculations on distances, subject placements and instrument placements when doing special effects work.

Garrett Shannon · September 26, 2008

While I don't sit down and write out equations, I do run rough 'calculations' (if you can call them that) in my head.

Agreed. Usually when I am using the inverse square law I'm thinking to my self: I can move it back to bring it down which will make it softer or I can keep it where it is and scrim it down and vice versa. Initial placement of a light for proper exposure comes from practice as opposed to sitting down and calculating the FC output of a certain light compared to how far away it needs to be to get a certain stop.

Serge Teulon · September 26, 2008

As I'm terrible at maths I find it very difficult to make calculations in my head. I do resort to a calculator but more often than not I study the photometrics of each light prior to shooting and that gives me a general idea of how far and what light I'm going to use.

Walter Graff · September 26, 2008

I don't know if I have ever sat down with it on paper in lighting scenarios on a set. After years of lighting one learns these things and they just become part of the scenario without having to think about them. For me, I know what the light does (knowing the rule and seeing it occur) so I would say it's not so much using the mathematics of it, but the practicality of it while lighting. Even when one starts out, because we see it as we do it, it becomes quite clear how light falls as you light. Where it does come more into play for me is in lighting designs such as what I do for ESPN. When you see a ESPN show like Around The Horn, or any of the sports commentaries like those of Peter Gammond or Butch Orley, you are seeing permanent remote studios that I design either in their homes or in a newsroom of a newspaper, etc. With such designs as these (I am currently doing five locations around the world for ESPN international) I have the dilemma of having to create a lighting plot to be installed by an electrician. That means I have to give someone who probably knows little about the art of film/television lighting a plot that he has to install without me being there (in some cases) and that means I have to get it right and make it draw by numbers for the installer. That involves me understanding photometric data supplied by the manufacturers well, and translating those photometric to universal measurements that he will use to know where exactly to place the fixtures so they do what I want. And in these cases, it becomes less of knowing that light falls off 4 times at twice the distance between people, and rather what is the minimum I need to make the relationship work.