Lighting ratio

I am trying to set up a couple of shots of our kids today in our DIY studio. It is the first time that I will be using a two light setup. They will be off camera so everything is manual.

The two flashes have about the same guide number. If I get the key light set at the exposure I want and it is, at say, half power, would I be safe putting the fill at 1/8 power to be two stops underexposed (leaving detail in the shadows)? This would assume that both flashes are about the same distance from the subject.

How does the law of inverse squares (is that the name?) work if the lights are different distances from the subjects?

Thanks in advance.

Setting the main at 1/2 and the fill 1/8 is a good starting point. You can tweak the fill power, if necessary, after taking a couple of test shots.

Every time you double the distance of the light from the subject, you are reducing the light by 1/4 or 2 stops. So, if you have a light placed a certain distance from the subject, you can reduce the lighting by two stops without touching the power level setting by doubling its distance from the subject.

If you only want to reduce the lighting by one stop, you'd have to move the light 1.4 times the original distance.

Thanks PacAce....that is exactly what I was looking for.

Also if the lights are both 4 ft from subject backing the fill up to 5.6 ft will decrease by one stop. To 8 ft two stops. Moving it in from 4 ft to 2.8 ft will give you a stop more. Same as what PacAce said just easier for me because my math skills are lacking.

mwvt9 wrote in post #5168444
I am trying to set up a couple of shots of our kids today in our DIY studio. It is the first time that I will be using a two light setup. They will be off camera so everything is manual.

The two flashes have about the same guide number. If I get the key light set at the exposure I want and it is, at say, half power, would I be safe putting the fill at 1/8 power to be two stops underexposed (leaving detail in the shadows)? This would assume that both flashes are about the same distance from the subject.

How does the law of inverse squares (is that the name?) work if the lights are different distances from the subjects?

Thanks in advance.

Assuming same GN flash units, if fill is at 1/8 power, it is THREE f/stops difference: 1/1, 1/2, 1/4, 1/8

The Inverse Square Law applies to the RELATIVE position of one light vs. the other, using the distance of the first light as the measurement standard. For example if light A = 2.8' away and light B = 5.6' away, the difference in amount of light from B (vs. A) is 1/4 (assuming same power of both. But if if light A = 8' away and light B = 10.8' away, the difference in amount of light from B (vs. A) is only a bit less than 1/2 (assuming same power of both lights) In both examples, there is 2.8' difference in light position, but 1/4 intensity vs. <1/2 intensity, due to the Inverse Square Law. Double the distance = 1/4 the intensity...n:n^2, and if n is doubled n:2n^2 what results is 1:4

Wilt wrote in post #5172488
Assuming same GN flash units, if fill is at 1/8 power, it is THREE f/stops difference: 1/1, 1/2, 1/4, 1/8

The Inverse Square Law applies to the RELATIVE position of one light vs. the other, using the distance of the first light as the measurement standard. For example if light A = 2.8' away and light B = 5.6' away, the difference in amount of light from B (vs. A) is 1/4 (assuming same power of both. But if if light A = 8' away and light B = 10.8' away, the difference in amount of light from B (vs. A) is only a bit less than 1/2 (assuming same power of both lights) In both examples, there is 2.8' difference in light position, but 1/4 intensity vs. <1/2 intensity, due to the Inverse Square Law. Double the distance = 1/4 the intensity...n:n^2, and if n is doubled n:2n^2 what results is 1:4

Wilt, the OP said that he has the main light at half power. You're doing your speed reading thing again.

PacAce wrote in post #5174076
Wilt, the OP said that he has the main light at half power. You're doing your speed reading thing again.

No, Leo, merely answering the explicit question that was asked, "How does the law of inverse squares (is that the name?) work if the lights are different distances from the subjects?"

If he wanted to know the interplay on fractional power and increased distance, he should have asked!

Wilt wrote in post #5175110
No, Leo, merely answering the explicit question that was asked, "How does the law of inverse squares (is that the name?) work if the lights are different distances from the subjects?"

If he wanted to know the interplay on fractional power and increased distance, he should have asked!

I thought your first paragraph was in response to this:

But fair enough, although I fail to see the relevance of your first paragraph to the inverse square law theory, then. But, no matter.

I missed the 'main at half power' remark