How far will my Better Beamer reach?

As I tell my workshop participants:

"It will reach to the moon! (But the light level will be very weak ... )"

So what about back here on Earth?

In practical terms, the maximum distance depends on your flash’s power, the ISO speed for your film or digital camera, the maximum lens aperture, and whether you are using fill flash and therefore reducing the amount of flash in the exposure.

You can do a quick estimate if you know your flash’s Guide Number (GN), usually given by the manufacturer. The formula is

Distance  =  GN  /  f-number

Be sure to check whether the Guide Number is quoted in feet or meters, because that’s how the distance is determined. Suppose your GN at ISO 100 is 140 feet with the flash zoomed to 50 mm (e.g. Canon Speedlite 580EX). The Flash Extender should give you at least 2 f-stops more light, which is equivalent to doubling the Guide Number to 280 feet. If you’re using an f/4 lens wide open, that gives

Distance  =  280 feet  /  4 = 70 feet

That means you could achieve full flash exposure (flash compensation = 0) at 70 feet.

If you are only using fill flash, the GN increases considerably. For every f-stop of negative flash compensation you dial in, the GN and distance will increase by a factor of 1.4 (square root of 2). If the flash compensation is –2, the distance increases by a factor of 2.

Finally, every time you double the ISO speed, the GN and distance also increases by a factor of 1.4. Changing from ISO 100 to ISO 400 will double the GN.

So, with an ISO speed of 400 and a flash compensation of –2, your effective distance would quadruple to 280 feet. (A flash compensation of –2 is somewhat weak. A better starting point for natural looking fill flash is –1.7, which will give you slightly less reach.)