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.

At full power, if your camera is set to 100 IS, your flash will reach:

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!