Nidelva river through Trondheim Norway

Nidelva river through Trondheim Norway
by Saaru Lindestokke                

Submit your Photo
Hall of Fame

Please participate in Meta
and help us grow.
Photography Stack Exchange is a question and answer site for professional, enthusiast and amateur photographers. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

This question already has an answer here:

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.

share|improve this question

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

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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.

share|improve this answer
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.

share|improve this answer

Not the answer you're looking for? Browse other questions tagged or ask your own question.