This is another app on my iphone called Light Meter. It is a free app and it works just like a standard light meter. I use it for analogue photography, I just input my film's ISO, then point the phone camera at what I want to shoot and it tells me the correct shutter speed and aperture. I was a bit reluctant to use it at first because it was free and I didn't know how reliable it was, but I tried it with a roll of film and the film turned out fine.