Sometimes folks have asked why we describe the aperture value in “f/stops” and it seems like weird nomenclature that may not make sense. Actually it makes total sense once you know what it means.
Let’s say you want to know the size of an aperture. We answer with some f/stop number. But exactly how big IS the aperture diameter in millimeters? The f/stop is the answer. “f” is the “focal length”. The aperture size is the focal length divided by the number.
The aperture size (in millimeters) for f/4 is the focal length divided by 4. Let’s say you have a 100mm lens at f/4 What is the actual size of the aperture in millimeters? 100mm/4 = 25mm. How about a 100mm lens at f/2. What is its actual aperture diameter? 100mm/2 =50 mm.
That also explains why longer telephoto lenses that go to lower f/stop values are larger and more expensive. It’s because it takes a larger diameter glass to implement an f/2.8 lens vs an f/4 or f/5.6 lens. If you have a 400mm lens at f/2.8, its aperture diameter would have to be 142.9mm which is quite large (and expensive)! A 50mm lens at f/2.8 would only need to be 17.9mm in diameter.
Now, you know! 🙂
