Calculating the zoom ratio of a lens requires dividing the longest focal length by the shortest value. For example, the longest focal length in the Canon EF 24-70mm f/2.8L II USM Lens is 70 mm, and the shortest is 24 mm. When you divide 70 by 24, the result is 2.91. So, after rounding the value to the nearest whole number, the zoom value is 3x.