How do I convert lens mm to optical zoom times?

Question

What is the rule to convert the mm notationto the optical zoom notation? I searched a bit and found this one:

optical zoom = maximum focal lenght / minimum focal length

For example a 18-55mm lens would have a 3x optical zoom or a 18-200mm lens would have a 11x optical zoom. Is it right?

Answer

The "times zoom" notation is simply the big number divided by the small one, so the examples you give are correct. "3x zoom" simply means the longest focal length is three times the shortest.

This number really isn't very useful, though. On point and shoot cameras, it became popular because the starting focal length was generally about the same — a wide-normal field of view. That makes the times-zoom a reasonable indicator of how far one can zoom in to get a closer view of a distant subject. But with interchangeable lenses, that's less likely to be true, so it's not really so useful on its own. An 18-55mm and a 70-200mm are both about "3x zoom", but a very different range.

On the other hand, the zoom ratio does give you an idea of how much focal length flexibility the lens has, and usually higher numbers are a clue that there will be more compromise on image quality (and/or price, size, and weight).

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