The Lumu ($129 direct) is a compact light meter that works in conjunction with a smartphone. It works with Apple devices running iOS 6 or higher, as well as select Android phones, via a free app that is simple and straightforward to use. If you regularly shoot with older cameras that lack built-in light meters, the Lumu is worth consideration. It's a lot more portable—and less expensive—than some handheld meters, although it lacks advanced capabilities like flash metering.

The device first gained attention as a Kickstarter project last June. It's made its way into the hands of the backers who pre-ordered the device (myself included), and is now available for pre-order for those who missed the Kickstarter. It's essentially a covered dome, no bigger in diameter than a quarter, with a connector that plugs into your phone's headphone jack. It plugs into the phone with ease, but its design may require you to remove your phone's case prior to use. A small leather pouch and a lanyard are included with the Lumu, which itself is available in silver or black.

To take a light reading, simply plug the Lumu into your phone and launch the app. Its measurement screen shows three variables: the f-stop, shutter speed, and ISO. Set the ISO and desired aperture, tap "Measure," and the proper shutter speed will display. From there you can swipe to see the other shutter speed/aperture combinations that will result in a proper exposure. To take another reading, just tap on Measure again.

Using an incident meter is a bit different than working with the reflective meters that are built into modern digital cameras. Instead of measuring the light that is reflected back from your subject through the camera's lens, you'll want to place the Lumu right up against your subject, with the white dome pointed toward the location from which you'll be photographing it. This measures the light that's actually falling on the subject, rather than the subject itself. Because you're measuring the light itself, you won't need to apply exposure compensation for especially dark or light subjects as you will with a reflective meter.

I ran a roll of film through my 1950 Rolleiflex Automat using the Lumu as my sole meter. I opted for Kodak Ektar 100, as it has a fairly narrow exposure latitude for a color negative film. I was working with a lot of snow on the ground, which would have required exposure compensation when using an in-camera meter. The exposures were quite accurate, requiring very little adjustment to brightness when scanning the developed roll. The Lumu app has a neat extra feature that lets you take notes for each exposure, so you can later match up the shutter speed and aperture—and if you're using an interchangeable lens camera you can also add a memo recording lens was used for which shot.

The Lumu works with iOS devices running version 6 or later, but Android compatibility is more limited. You'll need to own a Samsung Galaxy S4, Samsung Galaxy Note 2, or HTC One in order to use the device. This limited compatibility is a shame, as it does call into question the long term viability of the Lumu—will the software be updated to work with the phone you use a few years from now?

Aside from that hypothetical worry, the Lumu is an accurate meter that should appeal to shooters who still use older meterless cameras. At $129 it's not an impulse purchase, but it's less expensive than most incident meters on the market, and its size is quite appealing. Old school shooters may prefer something like the Sekonic L-398A ($218), an analog incident meter that doesn't require batteries to operate. But if you've got a compatible phone, and are happy to save some money and space when compared to a traditional meter, the Lumu is a solid option. The only question mark is future compatibility, and that's more of a concern for Android users—if you've got an iPhone you can expect a bit more continuity in hardware and software.