Does flash illumination vary?

I was watching a professional photographer capture a long series of images of a set of museum specimens a couple of years ago. I asked him why he was using a pair of incandescent-filament light boxes as illumination, rather than a studio flash setup. He said that he found the light output of flash too variable from one shot to the next; I think he said that there was about 20% variability.

Most of us would probably not be bothered by less than half a stop inconsistency, but to a pro it mattered.

I can see that if successive shots are taken at varying intervals, in which the flash capacitor might not reach "full charge" (being exponential, 100% is impossible to achieve) the quantity of light might vary a bit from one exposure to the next. Are there any other technical factors that might cause flash output to be inconsistent?

As an amateur I like flash (bounced or diffused) for recording objects because the millisecond exposure will negate minor camera movements that can happen even on a tripod with 2 second mirror lockup.

Martin

Yes, flash varies. I know this from my stacking work in which I regularly do sequences of 500 pops. If you're operating at lower outputs (ie 1/32) the output tends to be bluer, and isn't actually at 1/32 unless you wait 10-30s after the ready light comes on. If you just go by the ready light then as many as 10% of the pops will be off compared to the others, depending on model, up to a stop. Metz 58 > Pentax 540 > Pentax 360 for consistency. For most consistemcy it helps to pop off 10 shots to 'warm' flashes up.

Modern Bowens flash heads do not vary in output or colour balance, but you get what you pay for

I am afraid that Bowens DO vary in output and colour balance, even new ones. I know to my cost yesterday in the studio with new 500watt monoblocs.
It only really happens with heavy use during the day, as they struggle with recycling constantly.

According to one professional photographer I know, (as it happens my uncle) who for years who did studio, commercial and video work. The only strobe monoblocs that could be relied on to be constant were the very expensive Broncolours.
Apparently the ProFoto monoblocs are as good.

Tungsten lighting were preferred as they were constant, and with control you knew exactly what you got. Being tungsten you either put colour balance gels on them, put a filter on the front on the lens, or used tungsten balanced film.

Today, it is a different matter, as the colour balance issue can be corrected in cameraRAW or PS, so one shot will look like another, colour balance wise.

Personally I would not worry. Shoot in RAW and let the PP sort the colour balance out. The most important thing is to play with the lighting set up in the first place to get what you want. Then Post process accordingly.

Pentax lenses don't focus properly under tungsten and
the lights usually get too hot for gels and they cost
about £6 each. It's also hot for the model.

Flash is a better bet, check the exposure regularly
with a cordless flash meter.

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