The Inverse Square Law (part 2) - in plain English

Having had a few days to mull over the concepts I introduced in the last post, now - I want to explain the practical application of this principle.  In place of the first beer bottle, lets use a model and instead of the second bottle, we'll use the wall of my imaginary studio as a backdrop...


The back wall of my studio is 6 metres behind the model, and the model is 1 metre from the softbox.  If you note the exposure for the model, and then do the same for the back wall, you should find that there is a little bit over 5 stops difference between them.  So if you take the picture exposed for the model, the back wall will only be receiving about 1/36th of the light, and will look pretty dark.

Now- leave the light where it is (and the back wall - obviously... I don't want the roof to collapse), but move the model back, so that he/she is just 1 metre from the wall - and therefore 6 metres from the softbox.  NOTE - our distance multiple unit is now 6 metres, and therefore the wall is now only 1/6th of a unit behind the subject.  In our examples above, we lost 2 stops of light in the first movement of 1 distance unit, so we should lose considerably less than that in just 1/6th of a unit - maybe even less than half a stop.  This means that if you expose the picture perfectly for the model, the back wall will only be half a stop darker, rather than the 5 stops we saw before - hardly even noticeable.

Now a final experiment - bring the softbox back up close to the model again - to 1 metre.  You now have 1 metre from the light to the model and 1 metre from the model to the back wall.  So bringing the light that much closer - surely, it will make the wall brighter, won't it?  Let's see - adjust your exposure for the model and, uh-oh... the wall just got darker again!  Why?  Remember in our first experiment, we lost 2 stops of light when we moved the subject back 1 distance unit, and this should be exactly what we see here - the model will be perfectly exposed, and the wall will be 2 stops darker, as the wall is 1 distance unit behind the model.

And here are a few test shots I did, just to prove the theory...

So, the upshot of all this is 3 simple rules...

• If you want your subject against a really dark background, have the light source close to the subject, and the subject far from the background.  
• Alternatively, if you want the subject and background with similar exposure, then have them relatively close together, and the light source some distance away (maybe 5 to 10 times the distance between the subject and background).  
• Finally, if you want the background just a little darker than the subject, then have the background a similar distance behind the subject as the light source is in front.  

Sounds simple when you say it like that - so simple in fact, maybe I'll start the post off with these rules.

Happy snappin'
Grum

PS - That's it, you can leave my studio and go back home now... dream's over, go on, go home!