F-stop ratios
From The Glossary of Digital Photography
The ratio of f- stops shown on a lens, indicating the maximum aperture available for that lens. For example, a ratio of 1:2.8 means that the lens has a maximum aperture of 2.8 (widest opening, smallest f-stop, or f-number). If there are two numbers printed, such as 1:2.8-3.5, it means that the maximum aperture is variable. The latter is usually seen on zoom lenses. The f/ 2.8 applies at the widest focal length, and the f/ 3.5 applies at the narrowest. In between focal lengths, the maximum f-stop varies between the two values. See f-stop
