I just learned some new things about flashes that I thought I'd share. You probably already know about this but anyway: The SB900, until today, Nikon's flagship flash is notorious for it's thermal cut-out after a period of prolonged use. This is caused by the BATTERIES overheating. It is aggravated by harder use such as rechargeable batteries, longer distance to subject, bounce flash, increased flash value, diffusers, or anything that makes the flash work harder. This also causes the recycle time between flashes to increase. Shooting at a higher ISO or wider aperture helps, but there are other solutions that take care of all of these.
- The obvious and most expensive solution is to have a backup flash to spell the main flash while it is cooling down (sometimes 15 minutes or more)
- (Bad solution warning!) Shut off the thermal cut-out resulting in the risk of cooking your flash.
- Special rechargeable batteries replacing current rechargeable batteries or not using rechargeables in the first place.
- Compromise your shooting techniques... why even mention this one? Only if you're at the end of a shoot and don't have other options would you remove your diffuser or stop bouncing etc.
- Here's the biggie: A Battery pack not only practically eliminates thermal cut-out, it has the huge added benefit of decreasing flash recycle time!
This is why many wedding photographers use these things. The downside of battery packs is that they add weight/bulk, some require a pocket or a belt, some require a cord, and added expense. The upside is virtual elimination of thermal cut-out and faster flash recycle time.
You ought to check out these Youtube videos if not just for the great soundtrack:
