Luxi is a small diffusion dome that fits securely over your iPhone’s front-facing camera. When attached and used with a light meter app, Luxi will help you determine the best settings for your DSLR or other camera so that you can take perfectly exposed pictures.
With Luxi, your iPhone becomes a convenient and easy to use incident light meter. Professional light meters with the same features can cost several hundred dollars and require you to carry a whole separate device.
Incident light meters, like Luxi, measure the amount of light falling on the subject you are photographing, not the light reflected by the subject. Incident light meter readings are independent of the subject’s reflectance and cannot be fooled by tricky lighting situations like back-lit subjects (e.g. sunset portraits).
The light meter built into all cameras measures reflected light. A reflected light meter shows an appropriate setting for an average of the whole scene. If you are taking a picture of a scene that does not have an average lighting situation, your camera’s built-in light meter will not be accurate. Usually, that causes an overexposed or underexposed picture.
Luxi is compatible with the iPhone 4/4s and 5/5s and it’s available for $30 here.