Inverse Square Law

[Glen]

On my course on Wednesday we learnt about the Inverse Square Law, below is a snippet describing how it relates to photography.....


As photographers and therefore visual artists, many people find themselves shying away from the more technical side of the discipline.
While there are large numbers of camera enthusiasts who take great pride in their knowledge of the ins and outs of every conceivable piece of photographic equipment, there are far more photographers who prefer simply to get on and take some photos. In order to become an expert in what you do, however, it is sadly necessary to have some grip on the ‘rules’ of photography.
Studio and flash lighting can be very difficult for the first-time experimenter. If you are used to shooting in natural light and making do with what is available then the new options that will open up to you in the studio may seem daunting. However, they are there to be harnessed and used for your benefit. In order to help you do this successfully, it is useful to know about the ‘inverse square law’.

Practicalities

The inverse square is commonplace in physics and mathematics; this article, of course, will concentrate not on the technicalities but on its practical application in photography. In very basic terms, when applied to lighting the inverse square law states that an object twice the distance from a light source will receive only a quarter of the light. Thus, if you had two objects, one of which is six feet from a light source and the other of which is twelve feet from it, the object twelve feet away will receive only a quarter of the light being received by the object six feet away.


It is a common misconception that moving an object twice as far from the light source will halve the available light. Instead, the inverse square law explains why light ‘falloff’ occurs so rapidly. This can be seen very dramatically with on-camera flash; if you are using a non-directional light source then you will notice that objects in the foreground of your images will be much more highly exposed than those further back. This is because the light spreads and diffuses in three dimensions, rather than simply in a straight line away from the source.

F-stops

In the studio, the inverse square law can be very easily applied. Most studio lights have adjustable power settings, usually found on a panel on the back of the light. These settings tend to be expressed as fractions (one quarter, one half etc) which match f-stops on your camera. Thus, if you need to move your subject further back from the light source, you can increase the power of the light by adjusting the settings.

Alternatively, it is also possible to compensate on your camera. Just as the light source can be adjusted in terms of f-stops, so too can the exposure being made. Widening the aperture by two f-stops will have the same effect as increasing the power from the light source by one quarter, and should therefore give you the same exposure as would be made if your subject was half the distance away from the light.

 

[DiddyDave]

Snag is - in the studio we don't use naked light sources nor can we easily double/quadruple the subject to light source distance, and walls/floors/ceilings all reflect too - hence although this is true, it matters not one jot in most studios and why a separate meter (or some serious chimping) is required instead

Glad you learning the techie stuff though, just now need to learn it in relation to practical application too



DD

 

[dazzajl]

Tis well worth knowing about these things, as you say.

It's actually one of those things that seems obvious once you get it but can be hard to explain. This is quite a good diagram.

 

[EdBray]

The important bit you have left out is 'from a point source' the effect is different from a diffuse source until you get far enough away for it to become a point source. It works for any electromagnetic radiation (sound, light, x-rays, gamma rays etc)

The sun is a point source, but as it is so far away, you cannot get far enough away from it for the inverse square law to work.

 

[ranarama]

It's a very technical explanation for a very simple practical rule and is best learnt through practice. Try moving a flash closer to and further away from a fixed subject in your living room while maintaing shutter speed but changing aperture to keep exposure on the subject the same.

Have a look at this post on strobist. David Hobby explains it so much better.

EdBray, just to be a pedant the inverse square law still applies most definitely with the sun but we can only move such a short comparable (and that's the important bit) distance from the sun that the difference is unnoticable. Physics doesn't stop because we don't notice it.

 

[Matt]

tis a cornerstone of acoustics too. inverse square law to define sound power level at a set point away from a point source

 

[EdBray]

EdBray, just to be a pedant the inverse square law still applies most definitely with the sun but we can only move such a short comparable (and that's the important bit) distance from the sun that the difference is unnoticable. Physics doesn't stop because we don't notice it.

I thought I said that in my post, what bit of "you can't get far enough away for the inverse square law to work" is different to your statement.

My background was in Nuclear Physics (hence my quote about all electromagnetic radiation) and I am fully aware of how the inverse square law works thanks!

 

[ranarama]

I thought I said that in my post, what bit of "you can't get far enough away for the inverse square law to work" is different to your statement.

The bit where you say it doesn't work. I said it does but it's too small to notice.

My background was in Nuclear Physics (hence my quote about all electromagnetic radiation) and I am fully aware of how the inverse square law works thanks!

You're welcome

 

[Glen]

oops can of worms

If any of the above text of mine is incorrect, far enough, it was only a copy and paste of the first bit of info I came accross on the net

I was never that good at physics...