Lens, optical system, projector, and imaging device

A lens comprising:

• a first surface;
• a second surface opposed to the first surface;
• a first outer circumferential part disposed at an outer circumferential side of the first surface;
• a second outer circumferential part disposed at an outer circumferential side of the second surface;
• a plurality of, three or more, first reference parts provided to the first outer circumferential part;
• a plurality of, three or more, second reference parts provided to the second outer circumferential part;
• and
• a first reference surface disposed at an outer circumferential side of the first outer circumferential part and the second outer circumferential part, wherein
• the first surface and the second surface have a common design axis,
• a first imaginary plane including the plurality of first reference parts and a second imaginary plane including the plurality of second reference parts are perpendicular to the design axis respectively, and
• the first reference surface extends in parallel to the design axis.