Can't wrap my head around this concept

Hi,

I've been spending some time recently parousing around David Hobby's incredible Strobist blog. No matter how many times I read this page:

http://strobist.blogsp​ot.com …osition-distance.html?m=1

I can't seem to wrap my head around this concept. How is it that the closer you bring the light source in to your subject, the darker the background becomes? I would think it would be the opposite because the further back the light is the more falloff there would be and the light wouldn't reach back that far.

Does someone have a good explanation for me to make it click? The way I learn best is that I need to understand the "why" in order to maximize potential for me.

Thanks,
Randy

Inverse square law.
With the light farther away from the subject the light is less far from the background relative to the subject. For example; If subject is 5 feet from background and light is 5 feet from subject, the background is twice as far away from the light. Moving the light back so its 20 feet from the subject, its now 25 feet from the background but the background is only 1/5th farther away from the light.

So, if the light is sufficient to provide proper exposure on the subject in both cases the background is much more under exposed in the first example than in the second. Because light falls off at the square of the distance. i.e. if twice as far away only 1/4 the light. While one quarter farther away means only 1/8th less light.

Its basically about how the light falls off, using the law of inverse square. I could write it up poorly, but watch this first.

https://www.youtube.co​m/watch?v=nk9cTa3UthM

Think if it as two exposures in one. One exposure for the subject and one for the background. As they are different distances from the light they need different exposures. But you can only use one set of settings, obviously. So the closer to the light obviously has more light falling on the subject. The background is farther away so less light is hitting it. Since you can only set the camera to one setting.... if you want them lit equal they need to be the same distance, as the same amount of light will fall on each. At you move the subject closer to the light it will receive more light. But as that changes the required exposure settings the background, as it does not move, will become underexposed. OR you can turn down the light as the subject gets closer, but again the background gets less light.

Lighting is all about ratios. As the subject moves closer or farther between the light and the background the ratio changes

This video might help.

http://www.youtube.com​/watch?v=nk9cTa3UthM.

DunnoWhen wrote in post #16963122
This video might help.

http://www.youtube.com​/watch?v=nk9cTa3UthM.

Gee why didnt I think of that.

DunnoWhen wrote in post #16963122
This video might help.

http://www.youtube.com​/watch?v=nk9cTa3UthM.

Great video - much clearer now.

Thank you

gonzogolf wrote in post #16963127
Gee why didnt I think of that.

Bu**er, see what happens when you answer the phone whilst composing a response.

RandMan wrote in post #16963134
Great video - much clearer now.

Thank you

Now that you have watched this video and understand how the inverse square law works, watch this video also by the wonderful Mark Wallace to see how you can use it to light a group of people with one light -

http://youtu.be/L0pBaP​RRg_o