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

Question

This question already has an answer here:

• How do I convert lens mm to optical zoom times? 5 answers

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.

Answer 1

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.

Answer 2

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.