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SLR zoom capacity is described in term of mm instead of
8x etc. What is the difference? How can we convert mm into
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When zoom is described in "x" (times), it's a comparison between the shortest and longest focal length.
Converting from millimeters to "x" is done by dividing the longest focal length by the shortest focal length. For example:
A desciption like 4x can however not be converted to millimeters, because both a 10mm - 40mm zoom and a 50mm - 200mm zoom would have that description.
You can but it would be nearly worthless to do so. The X is just a ratio, it does not tell you what you would see through the lens.
If you take the longest focal-length and divide it by the shortest, you get the number of X. As mentioned in my answer to your previous question, 18-55 is about 3X because 55 / 18 is almost 3. The same is true of 70-200mm because 70 / 200 is also almost 3. While both these examples have 3X zoom, the former starts at wide-angle to normal while the latter starts a bit beyond normal and does into telephoto.