Understanding Light Falloff

Lets say I set up a shot with one light at 45 degrees to a model and a reflector up against her other side. Lets also say the correct exposure is f/8 at 1/200s and ISO 100.

As I move the light farther away from the model (adjusting output to maintain the same exposure) does the reflected light become more prominent?

Why not do the experiment yourself and post the results?

No.

Yes, it does if my mind is still working correctly this late . It all depends on the light source distance to the subject, and the reflectors distance between the two of them.

Ie, lets say the light source, subject, and reflector are all 5' from each other in a triangle. The light to the subject is 5', but the light source bouncing off the reflector is 10' away since it has to travel 5' to the reflector, then 5' to the light source. The light will be reduced because of the longer distance. Its twice the distance of the light source to the subject.

Now, if you move the light source back 50' lets say. The light source to the subject is 50', and the light source reflected off the reflector is now 55' (50' to the reflector and another 5' to the subject). Its barely longer then the light source to the subject, so the intensity will be about the same.

In the first example, the difference the light has to travel is double, and the light intensity will fall about a full stop.

In the second example, the distance is only 10% longer, and intensity will be very close between the two. So the effect from the reflector will be more prominent than in the first example.

sfaust wrote in post #6083883
Yes, it does if my mind is still working correctly this late . It all depends on the light source distance to the subject, and the reflectors distance between the two of them.

Ie, lets say the light source, subject, and reflector are all 5' from each other in a triangle. The light to the subject is 5', but the light source bouncing off the reflector is 10' away since it has to travel 5' to the reflector, then 5' to the light source. The light will be reduce because of the longer distance. Its twice the distance of the light source to the subject.

Now, if you move the light source back 50' lets say. The light source to the subject is 50', and the light source reflected off the reflector is now 55'. Its barely longer then the light source to the subject, and the intensity will be about the same.

In the first example, the difference the light has to travel is double, and the light intensity will fall about a full stop.

In the second example, the distance is only 10% longer, and intensity will be very close between the two. So the effect from the reflector will be more prominent than in the first example.

tim wrote in post #6083701
No.

bobbyz wrote in post #6083631
Why not do the experiment yourself and post the results?

Good idea, intuitively I think that fixed reflector fill is proportional to the distance of the light source from the subject as sfaust has described.

I will have to do this and post the results; however, wanted a deffinitive answer for tonight. I'm going to go w/ my intuition.

Tim, I respect your comments from my previous post and really hope you are correct.

I would have thought that the fill is proportional. The fill is based on how much light is reflecting off it, which when you move the light further from the reflector then there is less light reflecting. I would think (now I am no lighting genius, just thinking) that the only way to change the proportion is to move the reflector closer to the subject.

Think about the angles, I suspect the light from the reflector will be proportionally less than the main light on the subject as the light source gets further away. It depends which way the light source goes to some extent.

Stephen is right.

Mmm, inverse square law plus trigonometry. Beautiful stuff

Curtis N wrote in post #6084071
Stephen is right.

Curtis is right about Stephen being right.

Leo is right about Curtis being right about Stephen being right.

TMR Design wrote in post #6085392
Leo is right about Curtis being right about Stephen being right.

Cool, that means my intuition was right too.