Strobes watts/sec...What does that mean in output?

[musiphotog]

Hey all,

I was going to start a new thread, but felt that this may be more applicable: How would one correlate strobe lighting output to continuous lighting output? What is needed in general situations (closed studio, overcast, sunny 16, night scene)?

The reasoning behind the question: I've been lighting television sets professionally for the past 4 years... therefore, all my experience is in video, tungsten (occasionally HMI and LED). In other words - if someone told me to light a subject properly outside for a nice sunny day, I'm gonna reach for a 4k and up! Yet, I know that's not what most still photographers are doing!

So... any help for a half-loaded crossover?

 

[Big Mike]

Are you talking about the color temp of the lights or the power/light output?

If it's color...most (practically all) flash/strobe lighting is made to balance with daylight...5000k to 5600k. To change that, you can use gels.

If you are talking lighting power...it's not a simple comparison. With continuous lighting, your exposure is based on the aperture, ISO and shutter speed...but with flash, the burst is much faster than the shutter speed, so the shutter speed isn't a factor in the exposure (but is is for any ambient exposure).

As for what power is needed for certain situations...that really depends on several factors.

 

[Derrel]

Guide Number is the accepted method for relating/comparing flash output across brands.OF course, the Guide NUmber, often abbreviated GN, has to be compared at the same ISO value, and in the same distance units, either feet or meters. Typically US-made or marketed in the USA strobes will have GN spec'd in Feet, at ISO 100; European units typically in meters. GN varies with reflector size and efficiency, which correlates mostly to the angle the reflector produces. A 50 degree reflector will produce a higher GN than a 100 degree reflector, with the same exact number of watt-seconds being pumped through the same flash tube. The same type of GN variation also applies to speedlight flashes; the trend in speedlight GN has been to report the power level at a narrow-angle beam spread, typically the most-concentrate or "most-telephoto" setting on the zoom head if the flash has a zoom head. On average, a 400 watt-second flash unit will produce a GN at ISO 100 in Feet of around 210 with a 65 degree beam spread, which is 210 / 10 feet equals f/21, for those not familiar with GN.

 

[musiphotog]

Mike: Ok, that makes some sense. I was talking power - I suppose I'll just have to find a starter unit and experiment!

Thanks!