This post is taken from an email I received from the cinematographer mailing list. It is a humourous and well written explanation of the inverse square rule of light.

The Inverse Square Law... yes... it's about calculating how bright a light will be. That's the real world application. By the way it's also know as the Law of Squares.

Let's say you want to put a light on a rooftop across the street from where you are shooting to simulate moonlight.... moonlight comes from above, right?... so you want the light high up. You can't afford a lift that can raise the light high in the air, so you tell your lowly electricians (that's me) to haul the big light up the interior stairs of the building and "just bloody well get it up there". The electricians might all quit because of your nasty way of saying it but they are all poor schmucks who are afraid of big Directors of Photography like you... so... they just do as they are told.

Now, you are not sure what F-stop to shoot at... should you use really fast fast film (which has lots of "bad" film graininess) or the slower stuff that looks better? Hmmm.... well, it would help to know how much light will be falling on your subject... that way you can figure out what f-stop you would be shooting. If you figure out that you are shooting a F 0.7... well, that limits your choices for lenses big time (very few are made to work at F 0.7). So then you would think to yourself... that light I thought would work over on that roof... she is going to have to be bigger! Much bigger!

But back to the inverse square law... if you know (from reading the manufacturer's specs) that a 10,000Watt light will give you X foot candles at a certain distance (foot candles are a measurement of light whichsays that a "standard candle" will put 1 footcandle of light on an object 1 foot away) you can then figure out how many foot candles will be at twice that distance. How? Using the inverse square law of course! The law says that as the distance of the light FROM THE SUBJECT increases... the amount of light reaching the same subject will decrease in proportion to the square of the distance from the subject. That's an annoying way of saying that if you know a 20,000 watt light will give you 200 foot candles at 75 feet (you know this from the manufacturer's specifications which you looked up online) then at 150 feet (which is the distance from the subject to the light across the street on top of the roof) the light will be twice as far and...

150 feet is twice 75 feet... so that's 2x as far... the amount of light will be the square of 2... which is 4...

the number of foot candles reaching our subject will only be 1/4 of 200....

or... the amount of light reaching our subject will be 1/4 of 200 foot candles which is 50 foot candles.

Now to figure out how much that all is in F-stops... F-stops after all are the units of measurement that are written on the lenses and are generally used in those fancy light meters all the Directors of Photography have these days. (If you don't know what an F-stop is... well, I'm not going to get into that here... this would take all week!) We know from... well, who knows how we know it... but somebody knows that if you have 100 foot candles of light hitting a subject and you are using 100ASA film... your light meter will tell you that you will need to shoot at an F 2.8 to expose your subject properly. So if, in our real life shooting situation, we only have 50 foot candles coming from our big 20,000 watt light across the street... then the light meter will tell us we have to shoot at a F 2.0.

Tada! We now know we will be okay to shoot with 100 ASA film... F 2.0 is plenty of light and we are good to go.... lots of lenses are made to shoot at a F 2.0, after all.

PS - I have completely made up the number for how much light a 20,000 watt light will deliver 150 feet away... you will not be able to use 100 ASA film at night with only a 20,000 watt light on the rooftop across the street, let me assure you of that. Okay..... maybe you could, but you'd be crazy! But that was not the point... the point was that to learn about the inverse square law. I hope this helped.

Piotr Jagninski
Gaffer - New York City

There you have it.

Laters

P.S. To Piotr Jagninski I apologise if you see this post and I have not asked your permission, I only do this for the sake of education of my self and fellow filmmakers who may see this blog. If you see it and wish it removed please contact me or leave a comment.

A very good friend of mine recently got "published" online for a film review he wrote for the Hong Kong crime drama Infernal Affairs. It was submitted and accepted to the website www.MontrealFilmCritic.com . The first thing I must says is I am very proud of my friend, this is the first piece of writing I believe he has had published online or otherwise. The second thing is more to the point of this particular post, I guess I felt a pinch of envy for his achievement, but that is only because I have written a tonne of essays and reviews (well alot by my standards anyways) and have never been published anywhere but this blog. But as per usual, the only reason for that is my own lack of confidence in what I do or have done, and thusly don't attempt to even submit such works. But now here is my dilemma; If I were to submit and be published by the same site would I be doing it for my own achievement or the aforemention envy of a friends success.
I guess I should ask my self what would my friend think and my first assertion would he would also be as pleased for my success as I am for his. I realise that few people even know I have this blog let alone read it and so to spread my work may help me be more confident in it. I don't rightly know. Another fact is that I do not wish to be a professional writer I just do it for fun, my friend does want to make a living if not a career out of writing however.

Maybe I could take this as motivation to even attempt to write more, I think my friend would be please if I were to even write more for my blog and would encourage me to do more off blog stuff too. I think the real thing is I miss my friend as he moved away to follow his dreams and I wish I could do the same but as yet can't sensibly or logically justify upping and leaving yet. The lesson learned here is that no matter what other people do, petty envy is nothing more than our own mistrust in ourselves. I am resolute to trust myself and my work no matter what it may be and be pleased for others success especially if they deserve it. Without a doubt my friend deserves every success I can imagine.

As to the point of submitting to the Montreal Film Journal I think I can wait or indeed find an alternate venue. I don't have to use that site, it can be any site really or any medium. If I find an appropriate venue I'll let you know.

Laters taters.