Flash goes off but photo is dark

Question

Hi there!  I'm sure this question must have been asked before, but couldn't find it during a quick search.  I have a T3i that's about 18 months old. Just about a month ago, the flash started doing strange things. One picture would be fine, but the next and subsequent shots are dark, even though the flash DOES flash. An example from tonight is shown below.  I've cleared all the settings, used a new memory card, checked my lens to make sure it's on autofocus.  Originally I blamed the battery because it would no longer hold a charge, so I bought a new one. No change. I'm stumped...  Any ideas?
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Answer 1

Lorilou wrote:
 One picture would be fine, but the next and subsequent shots are dark, even though the flash DOES flash.
That brings up an interesting point, and I'm not sure the answer.  My first thought was that it's not a recycling issue, because you said that it does flash, and it wouldn't do so if it was recycling.  But that said, I'm not sure how the flash functions with eTTL in a low battery situation.  When shooting in eTTL the camera first fires a pre-flash, measure how much light it needs, then the actual flash.  I'm not sure how the camera would react if it doesn't have enough energy for the full flash, but it makes sense that it would fire the pre-flash if it had enough energy, since it won't know how much energy is required for the actual flash until it tests it with a pre-flash.
I'm assuming you're using the flash on the camera, which means it's pulling energy from the same source it uses to power the camera.  So I'm sure there's a large recycle time.  The more I think about it, the more I think the above scenario is exactly what's happening.  Battery wear certainly would affect it, as would the conditions you're shooting in and the camera settings.  The darker the room and/or camera settings (e.g. fash shutter speed, small aperture, low ISO) the more the flash needs to compensate, and thus more power it needs.  I'd guess that the camera battery can't provide very many full power flashes.
If you're frequently using flash for your photography I would highly, highly recommend an external flash unit.  You could pick up a Yongnuo 565EX for under $100.  External units provide you with much more flexibility, better quality light, more power, more flashes, faster recycle time, and it doesn't take energy from your camera battery.  If, eventually, you decide to try "off-camera flash", you would see a significant improvement in the quality of your food photography.