Post & Photo by Joe Farace Follow Joe on Twitter
Light, as a wise photographer once told me, is light. The most important characteristics of any lighting system are the quality and the quantity of the output. The kind of hardware used will have an impact on both aspects and in general you’ll find that studio lighting equipment is divided into two basic categories: continuous or electronic flash. 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 figure out that this can have an impact on the quality of light too.
The above photograph was made in my basement using three Flashpoint II monolights and a Canon EOS 40D with a 50mm f/1.8 lens that I bought on eBay for fifty bucks. Exposure was 1/60 sec at f/10 and ISO 100.
Continuous lighting is so named because it is ON continuously, much like a light bulb or the sun, enabling you to use your in-camera light meter to measure the light falling on your subject. Continuous lighting equipment lets you see how the light falls on your subject and because it can be inexpensive makes a good starting point for anyone on a budget. Electronic flash is more familiar because every point and shoot camera has one. The light from electronic flash is almost instantaneous so you can’t directly see the effect of the light on your subject, which is why studio lights usually have a MODELING light to show an approximation of what the lighting will look on the final image.
Much as a digital camera¹s resolution is measured in megapixels, flash output is often measured in Watt-Seconds, a unit of electrical energy equal to the work done when a current of one ampere passes through a resistance of one ohm for one second. Sometimes also called a Joule it’s basically a way to measure the power and discharge capacity of an electronic flash’s power supply. Think automobile horsepower but because Watt-Seconds doesn’t consider reflector design it’s not a perfect indication of the total amount of light that can be produced. That’s why sometimes you’ll see Effective Watt Seconds used to give you some idea of what to expect from the flash unit.
Another method of measurement is the lumensecond. The amount of light when a flash is fired can be specified in lumenseconds. A Lumen is a unit of measurement of light intensity falling on a surface. A lumensecond refers to a light of one Lumen for one second or the equivalent, such as two Lumens for half a second. The number of lumenseconds produced by a particular flash system depends on how effectively the flash turns electrical energy into light energy or Watt Seconds into lumenseconds.
Some people prefer to use Guide Number (GN) as a measurement of flash output because it, more of less, considers the entire lighting package. Guide Numbers are quoted in feet or meters (depending on where you live in the world) and are valid for a given ISO setting. 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 – not the camera-to the subject.