Why use a light meter at all? When photographing with modern digital cameras today, you rarely need such a device if you are not using professional lighting. You simply shoot in RAW and expose to the right, i.e. to the highlights (ETTR), and do the rest on the computer. With film, the whole thing is not so simple: if you were to expose every shot in this way, you would have a massive jump in light with every cut, even in a simple shot-counter-shot situation. Okay, you can hold the shot with the most intense light just before clipping the highlights and shoot the others with the same values - the rest will have to be dealt with in post-production. But if light is to be set or even just a few reflectors are to be used, then professional light measurement becomes very useful. It is just as helpful to be able to use such a device to judge what you need to bring along for good lighting when scouting for locations. Why only iPhone and iPad here? Light meter apps for Apple devices have been ...