I have noticed there are very good deals for used name brand (e.g. Canon) speed lights. My opinion is that used speed lights are worth considering, because the only components that could realistically fail are:
- The capacitor, and if the flash is charging and firing, it shouldn't be an issue (although you will never know if it fails after a month of buying it). Also, non-use could be a bigger issue for capacitors than use.
- The locking mechanism, if the flash has been subjected to continuous removal and attachment, could degrade, but if you have the possibility to check it, you should be able to notice a faulty locking mechanism
- Catastrophic failure
- Easy to check if it has already failed
- Gradual failure
- Sputtering: this only decreases the light output
- Ablation: gives the glass a frosted appearance which should be easy to check
Is there something I'm missing? How long is the lifespan of a speed light? From Wikipedia, I can see that
When operated below 30% of the explosion energy, flashtube lifetime is generally between a few million to tens of millions of flashes.
I can also see that:
At 60% of the explosion energy, the lamp will usually fail in less than a hundred.
...and I assume most flashes are operated below 30% of the explosion energy most of the time (2 stops less than 100% of explosion energy, 1 stop less than 60% of explosion energy, and I believe name brand flashes have the full output below the level that causes it to fail in less than a hundred cycles, so 30% of explosion energy would be less than 1 stop below full output), so I don't see how a flash could easily fail, especially in a manner that would be impossible to detect.
Also, if something fails in a speed light, what would be the component most likely to fail?