A round or adjustable opening in a camera lens that controls the amount of light reaching the film or digital sensor. By changing the aperture size, photographers can regulate exposure and influence depth of field, affecting how much of the scene appears in focus. Larger apertures (smaller f-numbers) allow more light and create a shallow depth of field, while smaller apertures (larger f-numbers) reduce light and increase the area in sharp focus. Understanding aperture is essential for achieving correct exposure, creative focus effects, and optimal image quality.