Basic lens specifications are inscribed on the interchangeable lenses. The numbers and letters will provide the focal length, maximum aperture (f-stop setting), filter size, and other information related to the lens.
For example, on the front of the SEL16F28 lens the inscription reads: E 2.8/16 and 0.24m/0.8ft and Ø49. The E prefix indicates that it is an E-mount style lens. The 2.8 number indicates the maximum aperture (f-stop setting). The 16 is the focal length of the lens. The 0.24/0.8ft designation indicates the minimum focusing distance of the lens in meters and feet. The Ø49 indicates the size (in millimeters) of filter you can attach to the lens.