Strobe vs Continuous Light Output

[Lee Saxon]

I'm pretty experienced with strobes (still photographer) so I'm trying to use that as a point of reference for how much continuous lighting I'm going to want getting into motion work.

I use Alien Bee B800s which are rated 320 wattseconds / 14,000 lumenseconds. Is there an easy way to compare that to lux or other ratings used by continuous lights?

 

[j1clark@ucsd.edu]

I'm pretty experienced with strobes (still photographer) so I'm trying to use that as a point of reference for how much continuous lighting I'm going to want getting into motion work.

I use Alien Bee B800s which are rated 320 wattseconds / 14,000 lumenseconds. Is there an easy way to compare that to lux or other ratings used by continuous lights?

Not really an 'easy' way. While strobes are rated in terms of 'watt-seconds', the actual duration of the flash is much less than a 'second', and independent of the shutter speed you select as well...

The relationship between 'watts' and 'lumens/lux/footcandles' in continuous lighting, is further obscured because different bulb technology produces different relationships...

About the only thing you can sort of get an idea about, is take the f-stop you typically use with your strobe set up, then find out how much light you need for a 1/60 (or 1/50, depending) shutterspeed.

 

[photonashville]

Here's some basic info that helps with the whole continuous vs strobe:

http://www.scantips.com/lights/flashbasics2.html

Think of this way. A 1000w strobe at a 1/60 of a second is about 6 stops more real world light output than a 1K Tungsten. Because the tungsten light is based on a 1000w at 1 second exposure. As a photographer I have gone through this as well to get a correlation of what I need for video. Higher ISO and wider apertures are just the expected compensation for video lighting.

 

[nejuicer]

I use Alien Bee B800s which are rated 320 wattseconds / 14,000 lumenseconds. Is there an easy way to compare that to lux or other ratings used by continuous lights?

The way to compare strobes to continuous light sources is through the resulting exposure. According to the Alien Bees website, the output of a B800 measured with a Gossen Ultra Pro flashmeter set for ISO 100 at 10-feet from the flashtube and with a direct 11” reflector (no soft box or bounce) provides an exposure of F32. The website doesn’t specify the shutter speed for that reading, but it is probably safe to assume that it was a 1/60th of a second. To translate that to Footcandles all you need to know is the it takes 100FC to get an exposure of 2.8 with an ISO 100 film with a 180 degree shutter at 24FPS (1/50th of a second shutter speed.) Ignoring the slight difference in shutter speed, the 7 stop difference in exposure would mean that the Alien Bees B800 puts out 12800 FC at 10’. An Arri 4k HMI Par with super wide lens puts out 3577 FC at 10’. An Arri 1k Fresnel at full flood puts out 183 FC at 10’ (55 FC after correction with full CTB to 5000K.) A Diva 200 puts out 14 FC at 10’. A Parabeam 400 puts out 63 FC at 10’. Welcome to our world.

For more detailed information on the use of continuous light sources, I would suggest you read the article I wrote for our company newsletter .

This article is cited in the just released 4th Edition of Harry Box's "Set Lighting Technician's Handbook" and featured on the companion website "Box Book Extras." Of the article Harry Box exclaims:

"Great work!... this is the kind of thing I think very few technician's ever get to see, and as a result many people have absolutely no idea why things stop working."

"Following the prescriptions contained in this article enables the operation of bigger lights, or more smaller lights, on portable generators than has ever been possible before."​

The article is available online at http://www.screenlightandgrip.com/html/emailnewsletter_generators.html.

Guy Holt, Gaffer, ScreenLight & Grip, Lightng & Grip Rental in Boston