Question: how to calculate zoom on lens?
Answer: To determine the overall zoom ratio of the lens, simply divide the longest focal length by the shortest. So, for example, the Sigma 18-35mm F1. 8 DC HSM and the 8-16mm F3. 5-4.5 are both examples of 2x zoom lenses: 35 divided by 16 equals 1.95, and 16 divided by 8 equals 2.