How to Use Your Smartphone as a Light Meter

A smartphone is a powerful tool for any photographer, whether you’re using it to take pictures or make shoots easier. There are countless different apps that can make your phone into almost any photo-gadget imaginable. In this tutorial, you'll learn how to turn your phone into a reliable light meter.
In Jeffrey Opp’s tutorial on using a light meter to get perfectly exposed photographs, he explains what light meters are, why they are important, and how to use one—if you’ve never used one before check out his tutorial before continuing. Unfortunately, a new dedicated light meter can cost a few hundred dollars. If you don’t need all the features of a professional light meter, however, a smartphone with an affordable attachment can do the job admirably.
Luxi For All is a small slip-on attachment for your smartphone. It uses a diffusion dome to turn your phone’s front camera into a incident light meter. The original Luxi only supported iPhones but their latest model, the Luxi For All, has an adjustable clip that fits most modern smartphones with a front-facing camera. It costs $29.
Lumu is an alternative to the Luxi. The Lumu device is a small sensor that plugs into your phone's headphone jack. It is iPhone-only and costs $150.
For this tutorial we'll use the Luxi and the official Luxi app, which is available for iOSand Android. There are also other apps, such as Cine Meter II, which add more advanced features (like colour metering, which is normally a separate and expensive kind of light meter) for cinematographers or professional lighting designers.
You will need a Luxi device and the Luxi app to follow along with this tutorial.
Attach the Luxi For All to your phone by placing the centre of the dome over the front facing camera. As the Luxi For All is compatible with a wide range of devices, it’s unlikely to sit flush to your phone. This means you need to be careful not to knock the attachment off when you’re taking light readings.
To take a reading, first launch the Luxi app with the dome attached to your smart phone. Then set two of parameters of the exposure triangle—FSTOP (aperture),TIME (shutter speed), or ISO. The meter reading will give you the correct setting for the third. I usually set aperture and ISO and let the meter tell me the proper shutter speed.
Tap and hold on any of the settings to dial in a value for it. Tap once on any of them to lock or unlock the value.

luxi ui
The Luxi user interface on iOS.

Once you’ve entered your desired start settings, hold your phone directly in front of the subject you want to meter. The Luxi is an incident meter, so point the dome straight back towards the camera position to measure the light falling on your subject. The app gives meter readings continuously; save a reading by tapping theHold button.
Every smartphone’s camera is subtly different so to ensure accurate readings your Luxi setup needs calibration. Fortunately, you only need to do it once; the Luxi app remembers your settings.
Set your SLR camera to aperture priority mode and select spot metering. While you can set your aperture and ISO to any value, I recommend you have the lens wide open and the ISO at 800. This will guarantee a relatively fast shutter speed in most lighting conditions. Check that you do not have any exposure compensation set. 
Focus your camera on an 18% grey card placed just beyond the lens’ minimum focus distance. If you don’t have a grey card, you can print your own. Take note of the spot meter's settings.
Now attach the Luxi For All to your smartphone and launch the Luxi app. Dial in the same aperture and ISO settings that you’re using on your camera. Hold your phone in the same position as the grey card and compare the shutter speed that Luxi recommends with what your camera recommends. If they’re the same, then your Luxi is properly calibrated. If they give different readings, use the CALIBRATE EVslider to adjust the Luxi’s settings until the two match.

luxi metering
Metering a scene with Luxi.

Let's say, for example, you want to photograph an scene with settings of aperture f/1.8 and ISO 100. You would enter those into the Luxi app and then meter the scene. The app, depending on the light levels, would recommend a shutter speed required to produce medium grey: the appropriate exposure for the lighting conditions. In the image above, that is 1/200 of a second.
As well as recommending camera settings, the Luxi app shows general information about the lighting conditions with two measures: Exposure Value (EV) and Lux Value (LUX).
Exposure value is a number that represents the shutter speed and aperture. All combinations of shutter speed and aperture that create the same exposure have the same exposure value. It is measured in stops of light and is dependant on ISO. Unless otherwise stated, exposure values are assumed to be at ISO 100.
For example, taking an image at f/1.4 for 1/60 of a second and f/2.8 for 1/15 of a second produce a photograph with the same exposure and thus both settings have an exposure value of 7. Light levels, in most situations, are actually fairly predictable, meaning the exposure value required for a properly exposed image can be estimated by eye.

EV example
This image of Ali Clarke was shot at f/2.8 with a shutter speed of 1/60. I could also have shot it at f/1.4 with a shutter speed of 1/250 or at f/8 with a shutter speed of 1/8. All three pairs of settings have an EV of 9.

Understanding the exposure value means that you can quickly calculate and change your camera settings while maintaining the correct exposure for a given scene. This saves you from having to re-metering the scene if you decide to change your aperture or shutter speed. Either perform a quick stop-calculation in your head (if you increase your shutter speed by a stop, you need to close your aperture by a stop and vice versa) or consult an Exposure Value Chart.
While exposure value is dependant on your camera settings, lux is a measure of the overall illumination of a scene. One lux is equal to the intensity of the light hitting a one metre squared object placed one metre away from a candle. The higher the lux value, the greater the intensity of light falling on the scene.
Lux is of more use to cinematographers than to photographers. If you are trying to make sure that a scene is evenly lit, or that when you set up your lights in a different location that the levels are similar, you can use Luxi to measure the lux value at different places and make sure the values are the same.
If you’re a photographer and you’re not using your smartphone on shoots, you’re missing out on an extremely powerful tool. Your iPhone isn’t just a camera, it is a small, portable computer capable of many different things. With a $29 Luxi For All attached, and a free app installed, it can replace a several hundred dollar light meter. While it won’t be as accurate or functional as a dedicated device, for most people the features offered by Luxi are more than enough.


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