The Digital Photography Book

Why Some Lenses Have Two F-Stops

When you see a zoom lens that has two different f-stops, what that means is that at the shorter range (let’s say it’s an 18–200mm lens, so we’d be talking about when you’re at 18mm), the f-stop can go as low as f/3.5, but when you zoom it out to 200mm, the f-stop will automatically increase to f/5.6. What this tells you is two things: (1) If you shoot at the wide-angle end (18mm), you’ll be able to shoot in much lower light than you can zoomed in at 200mm (the lower the f-stop of the lens, the darker light you can hand-hold your camera in and still get sharp photos). This also means (2) that this is a less-expensive ...