1 List of United States cities by population. ^ 2  Population projection is that of the entity known since 2016 as Ciudad de MÃ©xico and previously as the Distrito Federal. ^  Population estimate is that of the Toronto census division, which consists solely of the City of Toronto.
Lens (optics) For other uses, see Lens. A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (elements), usually arranged along a common axis.
1 The iris of the eye functions like the diaphragm of a camera, controlling the amount of light reaching the back of the eye by automatically adjusting the size of the pupil (aperture).  The eye's crystalline lens is located directly behind the pupil and further focuses light. Through a process called accommodation, this lens helps the eye automatically focus on near and approaching objects, like an autofocus camera lens.