At the moment there are four iPhone models for sale: the iPhone 6, 6 Plus, 5S and 5C. On paper they all have similar technical specifications, but the newer models have better image. All four iPhones have an 8 megapixel camera, but the newer models have better image quality because they have larger sensors. The cameras all have a fixed aperture 4mm lens; f/2.4 in the 5C and f/2.2 in the 5S, 6 and 6 Plus. Given the crop factor, the lens is equivalent to a 30 to 35 mm lens on a full-frame camera.
The iPhone has a shutter-speed range of 1/2000 of a second to 1⁄2 a second. The ISO range is 34 to 2000 in the 5S and 5C and a slightly wider range of 32 to 2000 in the 6 and 6 Plus.
It's interesting to know the photographic abilities -- and limitations -- of the iPhone 6. Not that much different than a Nikon F2.