How far does a flash fire?

[68lbs]

Ok, this is probably a really daft question and I'll have missed something obvious, but how far does a flash fire? As in distance. I understand about the rate the light falls off at, and power must effect things, but if I am using my 580EX II outside for fill, can I use it with say my 70-200 at 200mm on candids?

 

[dazzajl]

I'm guessing you don't mean 200mm, as you'd be blinding people.

Aperture and ISO are the completing sides of the triangle with flash power that allow you to work out the light for the distance, or the distance for the light you have.

 

[68lbs]

I'm guessing you don't mean 200mm, as you'd be blinding people.

No, I meant if I was taking candids (say head n torso) with the 70-200 at the 200mm end... not sure what camera to subject distance would be. :shrug:

 

[JasonRS]

How far?

Until the light output drops to ambient (it's not going to take it belo that) based on the inverse square law, or until it hits an opaque obstruction.

Look into the guide number for the flash to give you some specifics.

http://strobist.blogspot.com/2007/12/guide-number-your-free-flash-meter.html

 

[purpleclouds]

I use my 70 - 200 for candids, usually put it on around 1/200 f4 ISO400, leave the flash on ETTL but dial in 1 stop or so over compensation on the flash and bounce off the ceiling (if it's low enough!)