A couple of days ago, a reader asked me this:

I did your flash course about a month ago and enjoyed it so much, I actually bought a flash 😀  I got the 430EX II.  I do have a question about it.  I used it this past weekend doing some Christmas photos and I found that I just ran through batteries REALLY fast.  I’m wondering why.  I thought that the battery life would be longer than an hour or so (continuous shooting but I had it set to ETTL which I thought would adjust for my lighting).  Do I need to set the flash in some way?  Am I firing too high?  I just pulled it out of the box and started using it and made no adjustments and I am wondering if that is my problem.  I used Energizer rechargeable batteries, that were fully charged.  In total I took about 650 photos (was doing shoots for different families) and went through three changes of batteries.

Well… I would say that is roughly about the expected battery life for a shoot where you use the flash at high power. An hour of non-stop shooting takes a lot of power, so you need to carry lots of batteries.

If you had a 580EX, it has twice the power, if you should need it – but it can take an optional battery pack. Whether this warrants the price difference I am not sure: this is a decision only you can make.

You are using your flash right. And the mode you use your flash in makes no difference to battery life: after all, whether you set the flash power level (“M” on the back of the flash) or whether you let the camera do it (“ETTL” on the back of the flash), it’s still going to be the same power level!

So what does affect the power level the flash needs to emit? Factors include:

1. The size and reflectivity of the room, if you are bouncing your flash. A dark high ceiling eats power; a small room with white ceilings and walls reflects more, so needs less flash power.
2. Or if you are shooting direct, which I hope you are not, then the zoom angle of your lens and the proximity to the subject determine power needed.
3. ISO. The higher the ISO, the less power the flash needs. (Set it to at least 400 for indoors flash – often, you may need 800).
4. Aperture – the smaller the “f-number”, the longer your flash will last. Shoot at f/4 if you can, and if your subjects are in one plane).

So you can help the flash along by bouncing wisely, increasing ISO, and decreasing the “F-number”. But in essence, yes, a flash will use batteries – which is why you (a) use rechargeables, and (b) carry many of them.