Please note, when we talk about a large or small aperture, we’re not referring to a literal size comparison between different lenses. If this were the case, you’d say, “That NASA telescope has a much bigger aperture than my smartphone camera,” simply because it’s physically larger, which isn’t an accurate definition of aperture in photography. We are referring to the ratio of the focal length of the lens divided by the diameter of the aperture – a mathematical equation that determines the f-stop number.
From an Engadget article about aperture and how it affects your photos.
(Source: Engadget)
