What is primarily used to control the amount of light coming into a camera?

The aperture is the primary control for the amount of light entering a camera. It refers to the opening in the lens through which light passes. The size of this opening can be adjusted, allowing photographers to regulate the light that reaches the camera's sensor. A wider aperture (represented by a smaller f-stop number) allows more light to enter, which is beneficial in low-light situations, while a narrower aperture (larger f-stop number) reduces the amount of light, which can be advantageous in bright conditions.

In addition to controlling light, aperture also influences depth of field, which affects how much of the image is in focus. A larger opening results in a shallower depth of field, creating a blurred background, while a smaller opening increases the depth of field, keeping more of the scene in focus.

While shutter speed affects how long light is allowed in, and ISO adjusts the camera's sensitivity to light, aperture is the main variable directly responsible for controlling incoming light at the lens level. Focal length, however, pertains to the distance from the lens to the sensor and mainly affects the framing and perspective of the image, rather than the amount of light let in.