7.10 Avoiding a Meltdown

It’s an unfortunate fact that most flash units are made of plastic.  Another unfortunate fact is that plastic melts when it gets hot.  In intense shooting situations it’s possible to overheat your flash head and possibly melt one or more parts on the unit.  Very slight melting may only result in decreased light transmission through the bulb cover.  Often, however, a meltdown is disastrous.

Fig. 7.10.1 : Although a flash meltdown isn't quite as bad
as a nuclear meltdown, it can still be disastrous, especially
if you're traveling many miles from home and don't have a
backup unit handy.

The problem is that the gas inside the flash’s bulb heats up during each flash pulse to an astronomically high temperature, and if sufficient time isn’t allowed between shots for the heat from the bulb to dissipate, that heat can damage the unit.  The problem is worse for longer flash durations (i.e. higher
flash power).  Many flash manufacturers recommend that you don’t take a certain number of full-power shots within a certain time interval—say, for example, no more than 20 full-power shots in a minute, or perhaps within 5 minutes.  The exact numbers differ between units, so be sure to read your unit’s user manual.  Note that the limits can apply, to a lesser extent, to even shorter flash pulses, so that, e.g., a certain number of half-power shots within some number of minutes or seconds may also cause a meltdown.
    It’s important to realize that by
full power the manufacturers mean that the flash’s power (duration) is at its maximum, as defined by the highest setting in manual flash mode.  In the autoexposure modes, with TTL flash metering, you generally won’t be able to set the actual power level of the flash itself; instead, you’ll be dialing in a flash exposure compensation (FEC) level, which works essentially like the traditional exposure compensation mechanism for non-flash photos.  The FEC only allows you to modify the flash setting preferred by the camera, by adding or subtracting stops of light.  Thus, it’s generally not possible to know, when you’re shooting, whether the flash level is at full power when using TTL flash.  If the scene you’re shooting is extremely dim, even a negative FEC setting could, conceivably, result in the camera using the flash’s maximum power setting, so that too many consecutive shots could potentially result in a meltdown. 
    For this reason, I recommend using manual flash mode only, and sticking to settings not much greater than 1/4 power.  For challenging lighting situations you can of course turn up the power to 1/2 or 3/4 or even full power, but in these cases you just need to be careful not to take too many high-power shots within a short period of time, as indicated by the specs for your particular make and model of flash unit.  Note that operating the flash unit in manual flash mode doesn’t mean operating the camera in manual mode.  Manual flash mode can be used in any of the camera’s exposure modes, including the autoexposure modes.  The advantage is that in manual flash mode you can set the flash’s power level (duration) explicitly, rather than indirectly via an FEC bias, and this gives you a better chance to avoid a meltdown.  The disadvantage is that as the autoexposure system modifies its exposure parameters it won’t be able to modify the flash output, so your flash ratio (section 7.2) may change unless you (manually) change the flash setting accordingly.  For this reason, it can be simpler to just operate both the camera and flash in manual mode; this is what I do.
    There are a few other things you can do to avoid a meltdown.  Obviously, if you’re taking lots of high-powered flash shots, it’s good to check the flash head to see if it feels hot to the touch.  If so, then you may need to slow down your rate of shooting.  Also, if you have two flash heads (one for your flight rig and one for your tripod-mounted rig), you can occasionally swap them during intense shooting sessions if you’re using one camera more than the other.
    Finally, if you fear that you may be headed toward a meltdown, consider changing your exposure parameters so as to depend less on flash and more on ambient light.  Alternatively, you might consider using a slower shutter speed (in the case of high-speed sync) or a larger aperture (smaller f-number) so that you can turn down the flash
power while still capturing the same amount of flash-emitted light on your sensor.  Note also that some newer flash models (such as Canon's 580EX II) incorporate an auto-shutoff mechanism that can effectively prevent a meltdown by disabling the flash if it reaches a certain temperature.  In some cases, the added cost of buying such a model versus a cheaper model can pay for itself by preventing a flash-induced meltdown.