Happy Leap Day: Part of the confusion about studio lighting equipment is that some photographers think it’s too complicated and too expensive but in reality it can be neither. Part of this misunderstanding is created by seemingly bewildering array of product types and accompanying buzzwords. The purpose of this series of blogs is to take the mystery (and misery) out of studio lighting hardware and help you understand the technology and use these tools to make better pictures
Much as a digital camera’s resolution is measured in megapixels, flash output is often measured in Watt-seconds (Ws,) a unit of electrical energy equal to the work done when a one ampere current passes through a resistance of one ohm for one second. Sometimes called a Joule it’s a way to measure the power of an electronic flash’s power supply but because Ws doesn’t consider the reflector’s design it’s not perfect. That’s why you will occasionally see Effective Watt-seconds used a method of power measurement.
A Lumen is a unit of measurement of light intensity falling on a surface. A lumensecond refers to a light of one Lumen for a one second or the equivalent, such as two Lumens for half a second. The number of lumenseconds produced by a flash depends on how effectively the flash turns electrical energy into Ws into lumenseconds. Most electronic flash units produce between 15 to 50 lumenseconds per Ws and sometimes an efficient 300 Ws system can produce as much light as an inefficient system rated at 1000 Ws.
Some people—especially speedlight users—prefer to use Guide Number (GN) as a way to measure flash output because it considers the entire lighting package. In the USA, Guide Numbers are quoted in feet and are valid for a given ISO setting, usually 100. And it’s a simple measurement: The higher the guide number, the more the light output. Guide numbers can also serve as a way to calculate aperture when shooting without a flash meter. To determine the correct aperture, you divide the guide number by the distance from the flash to the subject.
Finally, keep in mind that all light behaves in accordance with the Inverse Square Rule that states that light’s intensity decreases by the square of the distance from the subject or more simply that light gradually falls off if the light source is far from the subject and rapidly if the light source is close to the subject. You don’t have to be Stephen Hawking to know that this can have an impact on the quality of light too.
For some more tips, tricks and techniques on creating studio lighting effects without spending the big bucks on gear, pick up a copy of my book “Studio Lighting Anywhere” from Amazon or your local camera store.