How powerful do CFL continuous lights need to be for portrait exposure?
Asked 6/13/2014
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2 answers
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I’m considering a budget continuous-light kit with softboxes or shoot-through umbrellas for portraits, and possibly some video use. I’m used to strobes, but I want to know what kind of continuous-light power is realistic.
What sort of fluorescent/CFL output would be needed to shoot portraits at around:
- f/6.3, 1/80s, ISO 400
- or ideally f/8, 1/125s, ISO 200
Would a head using 5×45W CFL bulbs be enough once it’s going through an umbrella or softbox, or is that too weak?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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You'll be able to make a good guess by testing on a speedlight or a studio strobe. Work in a darkened room so that the entire exposure comes from your lights rather than the ambient, and see what power setting you need for getting a good exposure.
Flashes dump their output in around 1/1,000 of a second. Let's say your 100W/s speedlight gives good exposure on full power, this means you're using the equivalent of a 100,000W bulb for just 1/1,000 of a second, so to get an exposure with equivalent f-stop and ISO in 1/80 (.0125s) you'd need to use 100,000 x .0125 = 1.25KW of lighting.
You'd have to research exactly what power rating your flash was and how quickly it dissipates 100% of its power, but I think 5x45W is not likely to be powerful enough to provide the f/6.3 1/80 ISO400 results you want.
Hell, an even easier way to work out how close 5x45W will get you is to stick a single 100W bulb in a soft box and change your exposure by 1.5 stops - f/6.3 1/80 ISO2,000 and see if you are even close to where you want to be.
Originally by user29742. Source · Licensed CC BY-SA 4.0
user29742
12y ago
0
Generated from our catalog & community — verify before relying on it.
A 5×45W CFL head is likely too weak for those settings, especially once you factor in the light loss from a softbox or umbrella.
A useful way to estimate this is to compare it to flash: a speedlight or strobe releases a lot of energy in a very short burst, effectively acting like an extremely powerful light for about 1/1000s or so. To match that same exposure with continuous light at 1/80s, you typically need a much higher sustained wattage than people expect—often into the kilowatt range.
So if your goal is portraits at f/6.3, 1/80s, ISO 400, inexpensive CFL kits are usually marginal at best, and your more ambitious target of f/8, 1/125s, ISO 200 is even less realistic.
In practice, the easiest check is to test against a flash in a dark room: find the flash power that gives your desired exposure, then estimate how much continuous output would be needed to match it. Based on that comparison, 225W of CFLs is probably not enough for the settings you listed.
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