Manually adjusting flash intensity/duration ... how?

[Rupert49]

AP 5 May 2012 included a fascinating article about a chap called Tobias Brauning who has produced some amazing shots of coloured water droplets.

Part of his techique is to manually control the intensity/duration of his flash lighting .. he likes to expose his shots with a flash duration of at least 1/15,000sec.

Using a 5D MkII & 580EX II flashgun, the only way I can change the flash intensity/duration is to alter the EV by a maximum factor of +/- 3 stops.

How does one manually set flash intensity/duration with my set up, or is this where my thinking hits a brick wall, because what I really require is some pretty clever electronic stroboscopic equipment?

[PhilW]

Using a 5D MkII & 580EX II flashgun, the only way I can change the flash intensity/duration is to alter the EV by a maximum factor of +/- 3 stops.

In eTTL "auto" mode yes. just switch to manual* and you can set between full power and iirc 1/128th power in stop increments

/duration with my set up, or is this where my thinking hits a brick wall, because what I really require is some pretty clever electronic stroboscopic equipment?

Some googling leads me to believe that the flash duration is determined by the flash power

I was curious as to what I could freeze using my Canon 580Ex flashgun so I had a play in the lab using a photodiode and scope to measure the flash duration of the flashgun (averaged value over 4 flashes) at the various power output settings. The results were:

1/1 power = 1/1000 second
1/2 power = 1/2000
1/4 power = 1/4000
1/8 power = 1/9000
1/16 power = 1/15000
1/32 power = 1/21000
1/64 power = 1/30000
1/128 power = 1/35000

Which surprised me as I always thought it was fixed.

So it would seem that what you need to do is set manual flash at 1/16th, set shutter speed at 1/200th (for max sync) then play with Apeture and ISO till you get a satisfactory exposure. Then shoot away.

*hold down the "mode" button until "eTTL" switches to "M" on the LCD

[Bob Maddison]

Yes, use manual flash. The ultimate flash exposure is determined by the flash intensity which is fixed, and the flash duration; so low power flash means very short duration. Even in auto flash mode, if you use a wide aperture and work close, the TTL control ensuress a short flash duration although you will have better control if you work in manual flash mode.

I don't know about your flash equipment, but my Sony flash allows me to set the maximum flash power in auto mode too. This ensures that when working close, the TTL has less work to do!

[Rupert49]

Thank you PhilW and Bob Maddison .. plenty of scope for experimentation there!

[PhilW]

Yes, use manual flash. The ultimate flash exposure is determined by the flash intensity which is fixed, and the flash duration; so low power flash means very short duration.

Thanks Bob, that makes perfect sense when you put it like that.