Why doesn’t a lens’s physical length match its focal length?
Asked 5/26/2013
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I’m confused about why a lens’s stated focal length often doesn’t seem to correspond to its actual physical size. For example, a 20mm lens can be physically much longer than 20mm, and the sensor is also set back behind the lens mount. Why isn’t focal length simply the distance from the front of the lens to the sensor? How can some lenses be shorter or longer than their focal length?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
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That is because lenses are made of a complex set of elements that interact.
The focal-length would match to the lens size plus flange distance if it were made of a single optical element. This would make a lens which has strong aberrations which would lead to poor quality.
Telephoto lenses are by definition lenses which are shorter than their focal-length. Wide-angle lenses pose a design problem in that they often need a focal-length shorter than the flange-distance so those, like your 20mm, often use a reverse telephoto design in order to make such a wide-angle possible.
Zooms obviously complicate matters and you have lenses that become shorter as you zoom in or some that do not change length at all.
Originally by user1620. Source · Licensed CC BY-SA 4.0
user1620
13y ago
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Because focal length is not the lens’s physical length. It is the effective optical distance of the whole lens system, measured from the lens’s principal/nodal point to the sensor when focused at infinity.
If a lens were a single thin element, its size would relate more directly to focal length. But real photographic lenses use multiple elements and groups to control aberrations and improve image quality, so the effective focal length can differ a lot from the barrel length.
This also explains special designs:
- telephoto lenses are physically shorter than their focal length because their optics shift the effective nodal point forward.
- wide-angle lenses for SLR/DSLR cameras are often physically longer than their focal length because they use a retrofocus (reverse telephoto) design to leave room for the camera’s flange distance and mirror box.
- zoom lenses are even more complex, and their length may change or stay constant while focal length changes.
So there’s no simple one-to-one relationship between focal length and the outside dimensions of a modern lens.
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