I agree with Barry S. I just downloaded it to my 3GS (in which it works perfectly) and ran a quick comparison test against a Minolta Auto Meter IIIF (which has 3 modes - reflective, incident, and spot), a Gossen Digisix (which has reflective and incident), and my Mamiya 645 with AE prism (which does average and/or spot readings). All readings were taken for ISO 100 against a red brick wall and also an 18% grey card in bright sunlight over a 10 minute period. There was no more than one stop difference between any of them. Some readings said 1/1000 @ f5.6 and some said 1/500 @ f5.6. The meters with multiple modes varied within themselves between modes, but the iPhone meter was consistently 1/500 @ f5.6 against both surfaces. All this indicates to me that the iPhone meter would be OK most of the time, maybe slightly better when using negative film because I had the feeling that it was consistently reading just slightly over the "real" meters. I have previously downloaded two other light meter apps which seemed to vary by up to three stops and are harder to use, so this one gets my thumbs-up.