When the photography world made the switch from film to digital, one casualty of the conversion was the handheld light meter. Those LCDs on the backs of cameras made handheld meters (in the eyes of many, at least) obsolete. But in fact, there’s almost as much reason to use a handheld meter today as there ever was. For exposure accuracy and for fine-tuning lighting, nothing beats the information offered by a handheld light meter.
There are two kinds of light meters: incident and reflective. The reflective kind, often known as spot meters, work much like your in-camera light meter to measure the amount of light reflecting off a subject. Spot meters are part of the reason why greats like Ansel Adams were able to craft such fine black-and-white images: They could target an element in a scene and know how it would render in the final print.
For instance, if a middle gray scene element is reading 1/60th at ƒ/8, when you shoot at 1/60th at ƒ/8, that middle gray subject will, in fact, look middle gray. But, if you meter a black subject under the same lighting, it will read more like 1/60th at ƒ/2.8. Expose that way and you’ll have a wild overexposure. That’s because the incident meter is providing a reading for middle gray, and it’s up to the photographer to calculate the appropriate exposure according to the tone of the subject. It’s very precise, but it’s a bit more complicated than working with a more common handheld meter, the incident meter.
An incident meter measures the amount of light falling on a scene. It doesn’t matter whether you’re looking at a gray scene or a black scene or a white scene, a scene that’s frontlit, backlit or sidelit; as long as you aim the meter at the camera, the amount of light falling on it is measured accurately.
Because in-camera TTL metering systems work the same way as spot meters—by measuring the light reflecting off a subject—they’re easily fooled. Remembering that these meters are, at their core, based on a middle gray reading, pointing your camera at a black cat in a black room is likely to return a reading that would provide an overexposed image because the meter thinks everything is middle gray. With a handheld incident meter, though, the tone of the scene doesn’t matter, only the amount of light falling on it.
Incident meters feature a white dome diffuser, which actually covers what otherwise would be a reflective meter (meaning it’s also possible to use many incident meters for reflective readings, though they’re not as accurate as spot meters, which have eyepieces for precision). The dome shape of the diffuser mimics the roundness of a human face. Just position the meter in front of the subject’s nose and point the diffuser at the lens, then press the button to take a reading.
Some meters have flat or retractable diffusers, which can be useful for determining the amount of light falling on a subject from a single direction. Positioning the meter such that it’s aimed at one source, and only sees light from that source, is good for balancing multiple lights and fine-tuning ratios.
Much like a camera, you can set your light meter to prioritize apertures (with a fixed aperture and a shutter speed that changes) or shutter speeds (the shutter speed stays fixed and the ƒ-stop changes). You can also adjust the ISO to affect shutter and aperture values, and many meters include programmable memories so you can quickly change ISOs or refer back to previous readings in order to gauge lighting changes.
Many handheld meters also offer the ability to measure light from studio strobes or hot-shoe flashes. If you intend to use a strobe with manual output, a flash meter is crucial. It’s possible to simply take a guess and adjust the flash exposure accordingly based on the camera’s LCD, but this method is not only slower, it’s less accurate and can be fooled by inaccurate LCD brightness—just as it can be when using the LCD to check ambient exposure. A calibrated light meter is always the best approach for lighting accuracy, especially with strobes.
To measure strobe output, most modern meters have a few options. First, you can set the meter to wait until it sees a flash of light before measuring, which works fine, but it’s a bit clunky. Better yet is to manually fire strobes via a meter that’s hardwired via PC connection or wireless radio. Either way, simply press the button to take a reading and the strobe will pop. Some strobe meters even feature cumulative measurements, adding up the total exposure from multiple bursts. This lighting technique is useful for maximizing depth of field, and it’s one that’s made easier and more accurate—as so many lighting issues are—thanks to a light meter.