What element measures the amount of light passing through the lens?

The aperture opening is a critical element in photography that directly measures and controls the amount of light entering the camera through the lens. It is defined by its f-stop value, which is a ratio of the focal length of the lens to the diameter of the aperture opening. A wider aperture (represented by a lower f-stop number) allows more light to pass through, while a narrower aperture (higher f-stop number) restricts the amount of light. This control over light is essential for achieving proper exposure and can also influence depth of field, allowing for creative effects in photography.

The other elements mentioned in the question serve different purposes; for instance, shutter speed determines the duration of the exposure, film sensitivity (or ISO) refers to the film’s or sensor’s sensitivity to light, and focal length pertains to the lens's ability to magnify or widen the view of the subject. Each of these aspects contributes to the overall exposure and effect in a photograph, but it is the aperture that specifically measures and controls the light passing through the lens at any given moment.