Works best for small objects

Truely a useful idea!
Concerning precision its simple as that: The larger the credit card in the photo, the easier it is to allign it precisely, the better the measurment.
Ben Kamens could easily impove the usefulness of this app also for larger objects by giving the option to align to a sheet of copier paper (US legal as well as A4 please). Alternatively allow manual entry of known lengths (e.g. height of a person in a photo).

Wintanclan about RulerPhone - Photo Measuring