What does the camera lens do?

The camera lens plays a fundamental role in photography by allowing light to enter the camera body. It gathers light rays from the scene being photographed and focuses them onto the camera's sensor or film. This process is crucial because, without the lens allowing light in, there would be no image captured. The quality and characteristics of the lens, such as its focal length and aperture, also influence how light interacts with the sensor to create a well-exposed and clear photograph.

In this context, while the other options refer to effects on light or image quality, they don't capture the primary function of the lens. Keeping light out is contrary to the lens's purpose, while the concepts of sharpening or dulling images are outcomes of lens performance and quality but are not its primary function. The essence of a lens is its ability to control and direct light into the camera.