bdombrowski
Contributor
I'm reposting some info from another thread. This is about using a strobe's guide numbers to get proper exposures when shooting in manual mode. Pretty much a requirement for balanced-light wide-angle shots.
Guide numbers are not that hard to use. You can even make a table on a slate so you have zero math to do in your head while diving.
First, you need to know the underwater guide number for your strobe. I know Ikelite has these on their web site for all their strobes. Note that this number assumes a specific ISO film speed, usually ISO 100. You can adjust by a stop for different film speeds, but we'll just keep it simple for now.
You use the guide number to compute what the proper strobe to subject distance needs to be for a proper exposure as a function of aperture setting. This is the formula....
stobe to subject distance in feet = Guide Number / f-stop-number
Here an example...
I'm shooting with my shutter fixed at 1/125 and my stobe's guide number for 100 ISO film is 28. I'm shooting a balance light composition, meaning I want blue water in the background and a foreground subject illuminated with the strobe. So I aim the meter at the blue water and adjust the aperture to zero the meter (assuming I have an SLR with a reflective light meter built-in.) Let say this happens at f/8. So my strobe to subject distance needs to be 28 divided by 8, which is 3.5 feet.
Easy thing to do is put a table on a slate with your pre-computed strobe-to-subject distances for all f-stops for a given guide number.
Couple of details to mention....
1) Remember the strobe-to-subject distance is not the same thing as lens to subject distance.
2) Water magnifys and thus distorts your normal perception of distance. Aim the strobe a bit behind your subject and use the distance from your shoulder to your finger tip as an estimate of 3 ft when extended straight.
3) You may need to drop to the next farther distance (reduce light by 1-stop) when the subject is highly reflective to prevent over exposing it. Barracudas are a prime example.
4) Know that multiple power settings on the strobe can be used to reduce the light power in increments of 1 f-stop. Let's say in the above example, 3.5 ft is too far for the composition I want. If I reduce the strobe power from full to 1/2, the strobe to subject distance becomes 28/11 or 2.5 feet.
5) If you are using dual stobes you only need to be concerned with the guide number of the more powerful strobe, the one with the higher guide number. Stobes are not additive. They only increase coverage.
Brian
Guide numbers are not that hard to use. You can even make a table on a slate so you have zero math to do in your head while diving.
First, you need to know the underwater guide number for your strobe. I know Ikelite has these on their web site for all their strobes. Note that this number assumes a specific ISO film speed, usually ISO 100. You can adjust by a stop for different film speeds, but we'll just keep it simple for now.
You use the guide number to compute what the proper strobe to subject distance needs to be for a proper exposure as a function of aperture setting. This is the formula....
stobe to subject distance in feet = Guide Number / f-stop-number
Here an example...
I'm shooting with my shutter fixed at 1/125 and my stobe's guide number for 100 ISO film is 28. I'm shooting a balance light composition, meaning I want blue water in the background and a foreground subject illuminated with the strobe. So I aim the meter at the blue water and adjust the aperture to zero the meter (assuming I have an SLR with a reflective light meter built-in.) Let say this happens at f/8. So my strobe to subject distance needs to be 28 divided by 8, which is 3.5 feet.
Easy thing to do is put a table on a slate with your pre-computed strobe-to-subject distances for all f-stops for a given guide number.
Couple of details to mention....
1) Remember the strobe-to-subject distance is not the same thing as lens to subject distance.
2) Water magnifys and thus distorts your normal perception of distance. Aim the strobe a bit behind your subject and use the distance from your shoulder to your finger tip as an estimate of 3 ft when extended straight.
3) You may need to drop to the next farther distance (reduce light by 1-stop) when the subject is highly reflective to prevent over exposing it. Barracudas are a prime example.
4) Know that multiple power settings on the strobe can be used to reduce the light power in increments of 1 f-stop. Let's say in the above example, 3.5 ft is too far for the composition I want. If I reduce the strobe power from full to 1/2, the strobe to subject distance becomes 28/11 or 2.5 feet.
5) If you are using dual stobes you only need to be concerned with the guide number of the more powerful strobe, the one with the higher guide number. Stobes are not additive. They only increase coverage.
Brian
