Metering studio lighting is done in the same way as it when shooting with natural light: By using a light meter. But you’re going to need a handheld light meter that also reads flash output. In most cases, the aperture that the flash meter provides will be close enough for your first test shots but you should take the time to look at the test image’s histogram and refine the exposure though repeated test shots until you have the exposure you want. Then you can concentrate on working with the subject to produce the kind of pose and expression that you want.
Exposure on the above image was 1/200 sec at f/2.5 and ISO 400. Camera was Canon EOS 50D with EF 85mm f/1.8 lens, my favorite portrait lens for DSLRs.
If you are not familiar with the histogram function of your digital SLR, take some time to read your camera’s User’s Guide about how it works or read my post ‘Hysterical Over Histograms.’ Knowing how to interpret these histogram readings will help improve the exposures all of your images—flash or not.
Studio flash users are almost always going to need a meter that reads flash but when shooting with continuous light sources, you can get by with an in-camera meter. When pointed at a subject, reflected light meters, whether in camera or hand held, are calibrated to give an accurate exposure with reflectivity somewhere around 18% grey; the exact value varies and the details are complex with some meters measuring 12% (the most common) and others at 14%.
Before the model or subject arrives for a photo session I always make some test shots to determine the proper exposure. Sometimes I use an assistant or my wife Mary, or when I used to shoot in my friend Jack Dean’s studio, we used his mannequin Anna.
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 Amazon.com, where your purchase helps this blog.