Inverse Square Law

[jromie93]

Okay I am trying to learn how to use this inverse square law—or at least figure it out. I understand most of it except shutter speed and flash power. The formula (f/x)(distance)=Guide Number makes sense where (x) is the f-number, except the shutter speed. If you know the aperture to use, then you clearly should have a preset shutter speed before you determine the aperture. So is there a predetermined shutter speed that you should use when using the flash in this manner?

Also the flash power. Because if I am understanding this correctly, then the guide number is the power of the flash at FULL Power, or 1/1. So how do you apply that to the formula? Just take 1/4 of the guide number (if using 1/4 power)? PLEASE help me out! Thanks!

[Mike367]

Shutter speed has no effect on flash exposure (as long as it's in your "sync speed range"), use it to control the contribution of ambient light. E.G., with a backlit subject and fill flash, a fast shutter = darker background, slow shutter= brighter background. the fill flash is controlled by aperture only. If the shutter speed is faster than your sync speed, then the shutter will only be partially open when the flash fires and part of your frame will be under exposed.

If you reduce your flash power to 1/4, then you have to open your aperture 2 f stops to get the same exposure.

[jromie93]

ahh that actually makes a lot of sense...thank you!

[zona5101]

Take step back and lets look at guide number a bit more... you are missing a couple of pieces: flash to subject distance and iso. The guide number is a rating of the output of a flash. For example "220". Generally it is assumed to be measured at a flash to subject distance of 10 feet and an iso of 100. From the "220" you can determine that at 10 feet your f stop will be f22.

The inverse square law is telling you that as the distance a subject is from the light source doubles, the amount of light received is 1/4.

Back to the guide number of "220" ...at full power a subject at 10 feet will be properly exposed at f22...change that distance to 20 feel and the fstop will need to be f5.6 a difference of 4 stops.

[Mike367]

One mistake:

Originally Posted by zona5101

Back to the guide number of "220" ...at full power a subject at 10 feet will be properly exposed at f22...change that distance to 20 feel and the fstop will need to be f5.6 a difference of 4 stops.

1/4 the light is only 2 f stops.

Guide# = fstop x distance, so Guide# / distance = f stop

220/20 = f 11

[zona5101]

Originally Posted by Mike367

One mistake:

1/4 the light is only 2 f stops.

Guide# = fstop x distance, so Guide# / distance = f stop

220/20 = f 11

right on - thanks for the correction

[ded]

Reversing the Inverse Square Law

[bhursey]

I have purposefully have tried not to learn it I already have enough to think about on shoots.

As David Hobby "Strobist" Says in his lighting seminar "forget the inverse square law" aperture controls flash, shutter speed controls ambient. you move the light source closer its gets bugger and brighter. You move it farther away it gets smaller and dimmer. Thats about it..

The inverse square law comes from a time of film where you did not know what the photo looked like till after it was developed. Now days you can take test pictures to see if it is what you want. That is why you have a histogram. Once you do it a few times you will get to tell what settings you need and setups in certain situations.