SLR zoom capacity is described in term of mm instead of
8x etc. What is the difference? How can we convert mm into
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 100mm - 300mm zoom would be described as a 3x zoom, i.e. 300 / 100 = 3.
A 50mm - 500mm zoom would be described as a 10x zoom, i.e 500 / 50 = 10.
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.