# slr 100mm to 300mm vs 10x 20x [duplicate]

SLR zoom capacity is described in term of mm instead of `x` like `4x`, `8x` etc. What is the difference? How can we convert mm into `x`.

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## marked as duplicate by mattdm, Itai, ahockley, jwenting, MikeW♦Feb 7 '13 at 17:41

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.

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the number can NEVER be converted to focal lengths. 10x could be 10-100, 20-200, 100-1000, etc. etc. for example. IOW the indicator is utterly without meaning unless the lower and/or upper end of the range indicated is known as well. – jwenting Feb 7 '13 at 17:06

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.

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