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This series of zoom lenses was created to give a lightweight lens that could be wide angle or telephoto or in-between. 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 widest angle, the second the maximum focal length. If the second is less than 50mm, then it is a wide-angle zoom. If it goes higher than 50mm, it is a wide-angle to telephoto zoom. 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.