Some speculated the purple fringing seen with the iPhone 5 was caused by either a fault in the camera's design, new lens structure or sapphire lens cover, however it appears the problem lies in one of the most basic shortfalls of camera technology: distortion.
As with all optical elements in a camera's lens array, light is bent at different angles as it passes through the substrate, usually some form of glass or plastic, to converge at a single point on the focal plane. In the case of digital cameras, the focal plane is the unit's imaging sensor which, in the case of the iPhone 5, is of the backside-illuminated CMOS variety.
At issue is a lens array's refractive index which numerically represents the manner in which light, or more specifically wavelengths of light, moves through the optics system. Ideally, a lens will focus all colors, or wavelengths, at a single point on the focal plane, thus creating a near-perfect replication of an image. In practice, however, lenses don't allow for wavelengths to meet at a convergence point, creating what is called chromatic aberration.
Due to a number of factors, including reference tuning, architecture of digital sensors and relatively short focal lengths in smaller camera systems, chromatic aberration usually presents itself in shorter wavelengths like violet.
