Projector lenses on Digital Cameras
Camera and projector lenses work on essentially the same principle, just in reverse. They both collect light from one side and project it out of the other. For camera lenses, they collect light from the world and projected it onto your small sensor. For projector lenses, they collect a concentrated amount of light from a film or sensor and project it out into the world. People have been projecting images on walls and screen like surfaces for centuries. References go back to the 5th century in China for pinhole image formation, and the Greeks were aware of the phenomenon by the 4th century.
While the primary functions are opposite, both types of lens can theoretically work in either direction.
The only major differences is that camera lenses offer more control over features like aperture and focus distance. Projector lenses are focused on the projector screen by moving the lens toward or away from the light source.
But this is not essential as most creative photography is all about using lenses wide open to create bokeh, a shallow depth of field suits the technique and in a projector lens the plane of focus for a projected image is very shallow (perhaps less than an a couple of centimetres) so as to be essentially two-dimensional. As a result, projection lenses typically have low f-stop numbers, providing a larger aperture and, consequently, brighter images on the screen.