What does a lens focal length in millimeters mean?
Asked 4/17/2013
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When a lens is labeled 100mm or a zoom is labeled 50–250mm, what does that number actually describe? Is it the physical length of the lens, or something else? How does focal length relate to how “zoomed in” the image looks, and why can a lens be shorter or longer than its stated focal length?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
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The number of millimeters is the focal-length. This is not the size of the lens but it is loosely related. If lenses where made of a single glass element, it would be the much closer to the lens length plus flange distance of the mount. Modern lenses are made of considerably more elements so there is no simple relation.
The most important thing about focal-length is that, when combined with sensor-size, it defines the angle-of-view. Lenses which have shorter focal-lengths have a wider angle of view. Lenses with longer focal-lengths have a narrow angle-of-view. This is why a longer focal-length is seen as more zoomed in on a given camera.
Zoom lenses are defined by a range of millimeters, say 50-200mm, which means the focal-length can be adjusted anywhere from 50 to 200mm.
Originally by user1620. Source · Licensed CC BY-SA 4.0
user1620
13y ago
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Generated from our catalog & community — verify before relying on it.
The millimeter number is the lens’s focal length, not its physical length. In simple terms, focal length describes the optical geometry of the lens: for a simple lens focused at infinity, parallel light rays come to focus that many millimeters behind the lens.
What matters most in practice is that focal length, together with sensor size, determines angle of view:
- shorter focal lengths = wider view
- longer focal lengths = narrower view
That narrower angle of view is why a longer focal length looks more “zoomed in.”
So a 100mm lens is not necessarily 100mm long. With modern lenses, multiple elements can make the barrel shorter or longer than the stated focal length. The quoted focal length is an optical property, not a measurement of the outside of the lens.
For a zoom such as 50–250mm, the lens can vary its focal length across that range. At 50mm it shows a wider scene; at 250mm it shows a much narrower portion of the scene, making distant subjects appear larger in the frame.
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