How do I convert lens mm to optical zoom times?
What is the conversion between focal length in mm (e.g. 50mm) and x zoom (e.g. 20x) commonly used in compact cameras? Is there an equivalence or ar they not able to be compared?
What is the conversion between focal length and times zoom?
There is no "industry standard formal definition" of the term 20x etc BUT
Almost universal usage is that this is the ratio between the maximum and minimum focal lengths available from the lens.
So eg an 18mm - 90mm is 90/18 = 5:1 = 5x.
The extreme available DSLR DX lens so far (as far as I know) is 18mm -300mm = 300/18 = 15.55:1 ~= 16x
"Ultrazoom" compact cameras and video cameras with small sensor areas need correspondingly less glass to achieve a given 35mm-equivalent focal length*, and quality is less to far-less of an issue than with SLRs, so far larger max to min zoom ratios are possible and available with these cameras than with SLRs.
"times zoom" is the ratio between the focal length when zoomed all the way in and the focal length when zoomed all the way out.
So a 17-70mm lens is a x4 lens (70/17 = 4.1) but a 100-400mm is also a x4 lens (400/100 = 4).
So, the "times zoom" number tells us how big the range is for a specific camera/lens but tells us absolutely nothing about the actual focal length (or the 35mm equivalent focal length).