I hear this question often enough to devote a post to it.
“How far will my small on-camera mounted speedlight reach”?
So I thought perhaps I would try to answer that for you.
Distance = Guide Number / Aperture
The flash’s guide number, in metres at 100 ISO, can be found in its manual. The Canon 580 EX has a guide number (GN) of 58, the 430 EX has a GN of 43. So at f/5.6 the 580EX will reach 10 metres roughly, at full power.
Things to know:
- Smaller f-number (larger lens aperture) = more range.
- Each time I double the ISO, I get 40% more range (square root of 2)
- You can zoom the flash head in more to get slightly more range; wide angles give you less range.
- Modifiers reduce the range.
- An advanced flash will tell you the range, but only if you aim it straight ahead.
- To test, set your flash to manual, full power.
Know your flash’s guide number at least for a rough idea!