I know you could just use the TTL (through-the-lens) light meter of your camera, and it would really do a great job much of the time. But I still contend there’s room for the handheld light meter. The former is certainly easier, and requires one less piece of gear. But the latter, the handheld meter, really makes dialing in precise exposures a snap.
First, my case for why handheld meters still matter. Number one: you’d like absolute accuracy in measuring the amount of light falling in a very specific spot. Yes, your TTL meter can still do very well in this situation. But I would argue that you can be more precise more easily with a handheld meter. Consider metering multiple sources, for instance. Handheld meters are ideal for checking ratios with keys, fills and background lights.
Still not convinced you need a meter? Alright then, here’s argument number two: what if you want to use a strobe? TTL meters, no matter how wonderful they are, simply cannot meter strobe output the way handheld flash meters can. Boom. Case closed. Next topic. Meters still matter.
There are two kinds of handheld light meters—incident and reflective. Both allow you to dial in an ISO sensitivity, and a second variable (either shutter speed or aperture) and then the meter will provide the third exposure variable. So, for instance, you dial in 1/250th at ISO 100, and the meter will tell you to use, say, f/11 to complete the exposure triangle.
An incident meter measures the amount of light falling on the meter. When you see a handheld meter with a little white dome on it, that’s an incident meter measuring the amount of light hitting it. Here’s all you need to know to use this kind of meter. First, set it for ambient or strobe, depending on the light source you’re using. Then dial in the appropriate ISO, choose whether you want to dictate the shutter speed or aperture, and then place the meter where the subject is, under the same lighting, and with the dome of the meter (which stands in for the shape of a human subject’s face) pointed at the lens. Then press the metering button. This will tell you the correct exposure, as far as your camera is concerned, based on the amount of light falling on the subject area.
There’s another approach you can take here if you want to meter multiple light sources, and that’s to point the incident meter at each individual source while shielding it from the others. Instead of aiming the dome at the camera, aim it at the key light and use your body to shield the meter from the fill. Or aim it at the hair light and shield the meter from the key. This is a tremendous way to take advantage of the benefits of a handheld meter—measuring different light sources independently to really understand what’s happening in a scene.
A reflective light meter, on the other hand, requires you to point the meter (without a white diffusion dome) directly at an element in a scene—whether that’s a face, a mountain, or whatever you’re basing your exposure on. This is actually how Ansel Adams metered light; pointing a reflective “spot” meter at a specific element in the scene, and taking a meter reading. Based on those readings, Adams knew how to process and print his negatives in order to shift tonalities where he wanted them—making some scenes much more dramatic and dynamic than a straight print might otherwise have been. Your DSLR’s TTL meter, particularly when set to “spot,” functions much like a handheld reflective light meter—except it’s built right into your camera. This, in my opinion, is the best way to use TTL metering to make very accurate exposure readings. If you’re not using strobe, and if you’re not going to use a handheld meter, try this.
Lastly, if you don’t particularly want to invest in a new, somewhat expensive piece of gear, maybe you should consider an alternative: an app for your smartphone that’s kind of a cross between a handheld meter and in-camera meter. Most light meter apps take a picture with the phone’s camera and then base a meter reading off of the resulting shot. Some even allow you to pick specific elements in the scene—just like a very precise spot meter—to determine the exposure. There’s also the Luxi light meter adapter, which is a white diffuser dome that clips on over the smartphone camera and pairs with an app to turn the device into an actual incident meter. Think of it as a starter handheld meter, with a much lower cost barrier for entry.