Why is lens focal length quoted without sensor size, and what’s the best way to describe “zoom” across different formats?
Asked 7/28/2010
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I often see lenses and cameras advertised only by focal length, even though different sensor sizes give different fields of view. Since a 50mm lens does not look the same on every camera format, is focal length alone a sensible way to describe how much a lens “zooms”? What terms should photographers use when comparing lenses across full-frame and crop-sensor cameras?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
16y ago
2 Answers
8
Focal length is focal length. Field of view is field of view. Unfortunately, sometimes they're conflated and the former means the latter.
If it's a concern in a particular question, I'd suggest asking that the OP clarify with his/her crop factor.
Originally by user27. Source · Licensed CC BY-SA 4.0
user27
16y ago
0
Generated from our catalog & community — verify before relying on it.
Focal length and field of view are related, but they are not the same thing. Focal length is a fixed optical property of the lens; it does not change when you mount the lens on a different camera. What changes with sensor size is the field of view.
That’s why a lens may be listed simply as 24–70mm: the lens really is 24–70mm on any compatible body. But the scene it captures depends on the camera’s sensor size. A smaller sensor crops the image, giving a narrower field of view.
When photographers compare lenses across formats, the clearer ways to talk are:
- field of view (what the camera actually sees), or
- 35mm/full-frame equivalent focal length, using crop factor.
A common shorthand is: equivalent focal length = actual focal length × crop factor. This gives the focal length on full frame that would produce a similar field of view.
So if the question is about composition or “how zoomed in” a lens looks, sensor size or crop factor should be stated. If the question is about the lens itself, focal length alone is still correct.
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