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Inconsistency issues with strobes are well known and documented, but less so the inconsistency issues with electronic flashes.

But, flash variability problems do exist, and they are primarily visible in colour temperature and light output.

Light Output Consistency

In all the tests that I have seen recently of manual flash light output consistency the variation is very limited, within +/- 0.1 f/stop. In a test of the Canon EX-540, Nikon SB-800, Vivitar 283, YongNuo YN-460 II, and the YongNuo YN-560, the results were all comparable with the YongNuo YN-560 showing greatest variation and the Vivitar 283 showing the least. The greatest variation was up to 1/3 of a stop which was on relatively low power (1/8). Note that this series of tests was at 1/8 power and above waiting 10s for the flash to recharge.

Colour Temperature Consistency

Studio flashes are well known to change colour temperature with power. For example one flash reviewed in the February 2012 Advanced Photographer was described as having "Colour is good, and measured 5300K at full power, reducing by 400K at minimum.", whilst another had "Colour temperature was 5900K at full power, warming by 650K at lowest output, a noticeable shift."

as you use lower power the colour temperature rises

Consumer flashes are no different and are, if anything, less steady in their outputs. As you use lower power the colour temperature rises, and the colour temperature difference between full power and minimal power can actually be as much as 10%. And at low power, the colour temperature variation between individual flashpops at the same power setting is also greater than at full power. To see for yourself, try shooting 100 shots at the same low power setting, one every few seconds or so. All over the place.

Minimising Flash Inconsistency

One of the variables of inconsistency is recharging time and a note of caution needs to be injected here. In manual mode, battery powered flash units purport to be completely recycled when the light comes on.

But in my own limited testing, waiting a few seconds after the light actually results in as much as 1/2 a stop more light, with the largest difference being at the higher power settings. In fact, the amount of time that you let a flash recharge to make the next pop makes a difference. Wait 60s between shots and the output will be more consistent than if you wait 5s between shots.

popping 10 or so warm-up shots will level out the flash output and make for more consistent results

An obvious cause of inconsistency is if the flash to subject distance changes. When in manual, if the light source moves relative to the subject it necessary to make a corresponding change in exposure setting, either f/stop or ISO.

Lastly, flashes do have a warming up period, and once a flash unit heats up during use, the output stabilises. Regardless of the flash you use, popping 10 or so warm-up shots will level out the flash output and make for more consistent results.

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