Are lens focal lengths exact, or just approximate?
Asked 8/7/2017
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2 answers
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When a lens is labeled 70mm, 200mm, or 24mm, is that exact? For example, could a "70mm" lens actually be 69.5mm, and does the effective focal length change with focusing distance or zooming?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
26
Oh, it is worse than that. Many lenses designed for still photography exhibit focus breathing, where the angle of view varies over the range of distances the lens can focus. Normally this isn't an issue for still photography, but lenses designed for cine use (video) will normally try to minimize focus breathing.
Lenses, at their heart, are mass produced optical/mechanical devices and have manufacturing tolerances. The aperture settings, minimum focusing distances, exact focal length, even things like the alignment of the lens elements and how parallel the lens mount will be to the sensor are all subject to manufacturing tolerances and variability. A good insight into this world is the company blog at lensrentals.com, they tear down lenses and talk about how they are made and what makes individual lenses different (among other things).
I know nothing about AR / VR, but I imagine that fixing things in software would be the easiest path.
Originally by user2228. Source · Licensed CC BY-SA 4.0
user2228
9y ago
0
Generated from our catalog & community — verify before relying on it.
Lens focal length markings are approximate, not exact to a fraction of a millimeter. A lens labeled 70mm or 200mm is generally in that class, with normal manufacturing tolerances. Published focal lengths are usually rounded for practical use.
Also, the marked focal length is typically referenced at infinity focus. In real use, effective focal length can change as you focus closer, especially with internal focusing designs. Some lenses also show focus breathing, where the angle of view changes with focus distance. Zoom lenses, of course, vary focal length across the zoom range, and some lenses may behave unexpectedly up close.
So yes, a "70mm" lens could be slightly different from exactly 70.0mm, and its effective focal length may not stay constant at all focus distances. For still photography this usually doesn’t matter much, but it can matter more in cine/video work, where minimizing focus breathing is desirable.
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