Measuring Light

[iND]

There was an earlier poster who wanted to know how one flash would compare to another.


I often wonder how my 580EX at full power (manual) compares to my speed light at full power.


Sure I can talk about stops of light, but I would like to measure the output in lumens or something


and directly compare it.


I have a sekonic light meter but I would like to know my strength of flash in my arsenal as I plan


my shots.


Ie. What manual power on my 580 EX II equals 1/2 power on my speed light.


Ideas? Certainly there is a meter than measures ambient light and could also measure the flash


output.

[Dave Johnston]

I may be completely in the wrong here, but I think the guide number lets you know how "strong it is."


Someone else who knows more should chime in soon.

[Brendan7]

I believe the guide number is a measurement of at what distance the flash is still effective. Don't compare the numbers as distances (i.e. don't plan on using a 580 to light an object 191' away), use them in comparison to other flashes.

[jcrowe87]

Guide numbers work in ratios. You can compare them to distance, but must also take f/stop into effect. For instance, something with a GN of 191' would project light onto something 191' away at an f/stop of 1.0. You can get some idea of the distance a flash will through by working with simple doubling/halving from there.

[peety3]

Guide numbers work in ratios. You can compare them to distance, but must also take f/stop into effect. For instance, something with a GN of 191' would project light onto something 191' away at an f/stop of 1.0. You can get some idea of the distance a flash will through by working with simple doubling/halving from there.
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...would project light onto something 191' away at an f/stop of 1.0 and an ISO of 100...

[jcrowe87]

People shoot at ISO's other than 100? Wild. [:P]