What is the mechanical device in a camera that controls the size of the aperture of the lens?

The diaphragm is the mechanical device in a camera that controls the size of the aperture of the lens. It consists of overlapping blades that can open or close to allow varying amounts of light to enter the camera through the lens. This is crucial for managing exposure and depth of field in photographs.

The size of the aperture affects how much light reaches the sensor and can influence the sharpness and focus of the image. A larger aperture allows more light, which is beneficial in low-light conditions and creates a shallow depth of field, resulting in a blurred background. Conversely, a smaller aperture lets in less light and increases the depth of field, keeping more of the scene in focus.

While the lens is integral to capturing images and the shutter regulates the duration that light hits the sensor, neither of these components directly manages the size of the aperture. The sensor is responsible for capturing the image but does not play a role in controlling light entry through the aperture. Thus, the diaphragm is the correct answer for this question, as it specifically fulfills the function of adjusting the aperture size in a camera.