When you fire the test button, the amount of power it will draw will depend on the mode and settings of the flash.
What mode are you using (manual or E-TTL) and if manual, what power level have you set (e.g. 1/1 = full, but there's also 1/2, 1/4, 1/8, etc. )
If you give it a full-power burst in manual mode you have to wait for the flash to recycle the capacitors before it's ready to fire again. Capacitors are a bit like rechargeable batteries except the rate at which you can drain a battery is somewhat limited and a capacitor can dump its energy incredibly fast. So the way the flash actually works is to have the batteries charge up the capacitors until it has enough power to fire, and then the capacitors power the flash (because they can send a surge of power to the strobe.)
Most people tend to use these in either E-TTL/E-TTL II mode -or- they use them in manual (M) mode. But there are other modes.
If you want to manually control the flash for your situation, then press the 'Mode' button repeatedly until you see the "M" displayed in the upper-left corner of the LCD panel (now you're in "Manual" mode and the flash will always fire at the same power level... in E-TTL mode it will try to auto-detect how much power it uses and a lot of things can give you variable results because, frankly, you asked the flash to auto-detect and vary the power level.).
Being in manual mode is only half the fun.... now press the soft button below the LCD panel near the "+/-" and this allows you to control the power level. By default it's going to fire at full power (1/1). But you can now rotate the dial to reduce the power level (and you probably want to reduce the power level because 1/1 is a lot of power.) For example... maybe 1/8 power would be more reasonable... or maybe 1/4 power.
But the other reason to use manual mode is once you set the power level, it will ALWAYS fire with that same amount of power (until you change it.)
Keep in mind that when using manual flash, the flash has a "fall off" effect that causes the light to appear less intense if the flash is farther or nearer to the subject. This is called the "inverse-square rule" and it's a law of physics (no getting it around it - sorry.) That means if you want your subject to get consistent illumination then not only do you need to use "manual" mode and set it to a specific power level for all shots... but the flash itself needs to be at the same distance from the subject (if the subject moves... then you need to move the flash to keep that distance consistent for each shot.)
Tim Campbell
5D III, 5D IV, 60Da
