How do I compare the output of an LED panel with that of a flash?

Question

How do I compare the output of an LED panel with that of a conventional hotshoe flash?

I haven't been paying attention to the LED panels, but they've been on the market long enough so that I'm assuming they're stable and understood.

From what I've read, most of the smaller ones are really only effective from a few feet away. The larger ones are probably more money than I'd care to spend. I'm happy with my current flash setup (Canon; two 580ex and one 430ex), and I'm trying to figure out what I'd have to get to duplicate this setup using LED continuous lighting in terms of output.

Answer

It's very difficult to compare directly to flash of any sort, since flash (speedlight or studio flash) takes the shutter speed component of the exposure out of consideration. A more useful comparison is to traditional tungsten studio lighting -- quartz/tungsten/iodide lighting rather than photofloods, since that was more-or-less the industry standard before cool lighting alternatives (HMI, fluorescent, LED) came along.

A typical studio set-up for hot light portraiture would have consisted of a 650-1000W key light (the Ianiro Red Head and Lowel DP Light are good representatives of the type), with secondary lights of various wattages for background (usually a wide-throw light, such as the Lowel Tota), hairlights, kickers and accents. That is, as you might have guessed, a hell of a lot of light, but it was what was required to get a sufficiently high shutter speed to mostly eliminate subject motion -- with a posed subject -- at an ASA of 100 or so. I'm taking about 1/30 to 1/125 with typical portraiture apertures of f/5.6-11, depending on how far the light was from the subject and the kind of modifier you may have been using.

Now, in this day and age, we don't have to worry quite so much about sticking to low ISO settings, since there are few small-format cameras (APS-C or 35mm-format) that can't turn in a respectable performance at ISO 800, and many are excellent a couple of stops faster than that. That's both at least three stops of light you don't have to pay for and three stops of light your subjects don't have to endure (and if you've ever been shot on film under hot lights, you'd really appreciate that). So something that gives a 250-watt tungsten equivalent would be adequate for use in a small studio. You won't be able to turn night into day the way you can with 20KW HMI spots, but it's certainly good enough for posed portraits and large product shots, if you're willing to forget about very fast shutter speeds. When you can move the lighting in tighter, you can go faster. But if you're working hand-held or the subject is moving, you still need a metric crapload of light to make it work since you become dependent on the shutter speed.

That said, continuous lighting and flash are very different, and only overlap in use occasionally. Continuous lighting can be a lot easier to work with for still life, product and (fully-styled) food shots, where you can afford to take forever going over all of the placement details, reflectors, gobos and flags to achieve perfection and aren't too very worried about the shutter speed. With people, it pretty much means posing. LED lighting will probably get more powerful in the next little while, but there is also a limit to what people are going to put up with in terms of brightness. You can buy an Arri HMI spot right now that will work on household power and get you a fast shutter speed, but it's like staring at the sun. If people is your game, then studio flashes would be a better step up from speedlights than continuous lighting. The modeling lights will let you see shadows and highlights well enough to direct your subject, and it's a lot more comfortable for them to sit under while you're working.

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