15x optical zoom is a comparison between the shortest and longest focal length of the lens. It gives you no real idea what the capabilities the lens is. 15x could be any of the following (theoretical) lens combos. Theoretical lens combos could be 1-15mm, 5-75mm, 10-150mm, 15-225mm, 20-300mm, 25-375mm or 25-450mm. The number (15x) is nearly useless without actual specs.”]
Source: https://gizmodo.com/sonys-big-zoomin-h9-cyber-shot-does-15x-optical-240030