Does barrel distortion affect a lens’s stated focal length and quoted field of view?
Asked 2/1/2013
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For a full-frame 14mm rectilinear lens, the theoretical angle of view is about 104° horizontal and 81° vertical. But lenses like the Nikon 14mm f/2.8D can show visible barrel distortion toward the edges.
When a lens is labeled 14mm, is that focal length defined independently of distortion, with distortion making the actual field of view a bit wider than the simple rectilinear formula suggests? Or is some distortion effectively used to reach the quoted field of view of a “14mm” rectilinear lens?
More generally: is focal length specified from the lens’s optical design near the image center, or from the overall field of view including distortion?
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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Except in the case of fisheye lenses barrel distortion is an unwanted side effect of wide angle lenses, something lens designers work hard to minimise. Typically the more expensive a lens the lesser the degree of barrel distortion but it's always there, even in small amounts.
There's two important things to bear in mind with regards to focal length as stated in the specification of a lens (e.g. Nikon 14mm-24mm f/2.8). Firstly manufacturers round the figure, generally in their favour, so the actual focal length might be more like 14.8mm, secondly the focal length is measured with the lens focused to infinity, and almost certainly by shining a light through the centre of the lens.
This means you can't use the quoted figure to calculate the exact field of view. It also means you can't say ~16mm Lens + Barrel Distortion = 14mm rectilinear lens. To get technical a lens with barrel distortion doesn't have a single focal length, the focal length will vary across the frame.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
13y ago
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Generated from our catalog & community — verify before relying on it.
Focal length is specified independently of distortion. It’s an optical property of the lens, defined for paraxial rays near the optical axis, not by measuring the full field of view at the frame edges.
So for a non-fisheye lens, barrel distortion is not “counted into” the stated focal length. A 14mm lens is still a 14mm lens even if straight lines bow outward near the edges.
In practice, distortion changes how scene points are mapped across the frame. With barrel distortion, edge regions are stretched in a way that can make the total captured field slightly larger than the simple rectilinear angle-of-view formula would suggest.
Also, the number printed on the lens is usually an approximation, and focal length is conventionally specified at infinity focus, so quoted angle-of-view figures are not exact to the last degree.
In short: focal length is not derived from distortion, and barrel distortion is generally an unwanted side effect in rectilinear wide-angle lenses, not the basis for their stated focal length.
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