This series of zoom lenses was created to create a lens that could be various telephoto ranges. The focal length rules still apply to zoom lenses. The focal length is for the format the lens was designed for. This focal length is based on 35mm full-frame cameras or digital full-frame cameras. An SLR that uses a lens such as DX or APS-C most be multiplied by a factor to see the actual focal length that comes from using a sensor that is smaller than 24x36 size. If the lens is used with a full-frame SLR, the first number is the shortest focal length, the second the maximum focal length. If the second. If the base maximum f/stop is a fixed number, the f/stop range will remain constant throughout the zoom range. If this number is two part, such as f/2.8-f/4, then the lens will only be an f/2.8 lens at the smallest millimeter. This will vary throughout the zoom range and will end at the maximum telephoto at the second f/stop. With a camera that meters through the lens, this change will be automatically adjusted. If you are using a flash that is not coupled to the meter, you must adjust the f/stop to compensate for the change. If not a TTL meter, then this must be adjusted in setting the exposure.