In My Home Studio: Measuring Flash Output

Today’s Post by Joe Farace


With more and more photographers, switching to mirrorless cameras, it’s not surprising that many of them are being used in studio portraiture, including boudoir photography as this image was made using an Olympus Micro Four-thirds camera. Today begins a new series that will run twice a month (or more often if there are requests or high page view numbers) and is about measuring studio flash output.

The output of studio electronic flash units is usually measured in Watt-seconds (Ws,) a unit of electrical energy equal to the work done when one amp passes through a resistance of one ohm for one second. Sometimes you’ll hear it called a Joule but it’s just a way to measure the power of an electronic flash’s power supply.


Another measurement method is lumensecond that refers to the light of one Lumen for a one second or the equivalent, such as two Lumens for half a second. Most electronic flash units produce between 15 to 50 lumenseconds per Watt-second, so sometimes an efficient 300 Watt-second system can produce as much light as an inefficient system rated at 1000 Watt-seconds.

Guide Number is another measurement of flash output that considers the entire package. The higher the guide number, the greater the output. Guide Numbers are quoted in feet or meters and are valid for a given ISO setting.

The above image was made with an Olympus E-M5 Mark I and the wonderful Lumix G Macro 30mm f/2.8 lens with an exposure of 1/125 sec at f/20 and ISO 200 measured with a Gossen Luna Star F. Main light was a 320 Ws Buff DigiBee DB800 with an Alien Bee B800 as side light. Image converted to monochrome with Silver Efex Pro and finished with Color Efex Pro.

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If you’re interested in shooting portraits and how I use cameras, lenses and lighting, in my in-home studio, please pick up a copy of Studio Lighting Anywhere” which is available from, with new copies selling for $17.50 and used copies selling for less than five bucks, as I write this, a bargain no matter how you look at it.