The usual way to write an f-number, such as f/2.8. The f-number is the focal length divided by the diameter of the aperture. A lens name often shows its maximum aperture, for example 50mm f/1.8.
Many zoom lenses show two values, such as 18-55mm f/3.5-5.6. This means the maximum aperture is f/3.5 at 18mm and becomes smaller, f/5.6, at 55mm. On the lens barrel, the maximum aperture is often written as a ratio, for example 1:3.5-5.6.
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