A zoom ratio is a description of a lens’ ability to zoom. Technically, a lens’ zoom ratio is the relationship between a zoom lens’ longest focal length and its shortest focal length and is typically expressed in rounded integers:
For example, a 70-200 mm zoom (200/70=2,86) would typically be described either as having a ‘3x zoom ratio’ or ‘3:1 zoom ratio’.
While it is today – thanks to computer-aided design and advanced materials science – somewhat feasible to produce high-quality zoom lenses with significant (>3) zoom ratios, in the era when zooms were first marketed, any zoom ratio above 2 was considered adventurous (even by those who did not decry zooms in every instance).
Even so, a lens with a high zoom ratio is always more difficult to design than a lens with a lower zoom ratio.
Zoom ratios are used instead of focal ranges for marketing purposes especially with consumer-oriented compacts, but mere zoom ratios are problematic unless they also define the range of angles-of-view supplied by the lens&sensor combination (Both a 18–200 mm zoom and a 50–500 mm zoom are 10x zooms, but are usable for very different use cases).« Back to Glossary Index