How can a prime lens focus at different distances if its focal length is fixed?
Asked 5/14/2011
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I understand that a prime lens has a fixed focal length, unlike a zoom. What confuses me is focus: if the focal length is fixed, why can a prime lens still focus on subjects that are near or far away? Shouldn't a lens with one focal length only form a sharp image at one distance? What is changing inside the lens when I turn the focus ring?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
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A prime lens still has a moving focus element allowing you to change the focal plane through the range of the focusing ring's range. A prime is a lens that has a fixed focal length (100mm, 50mm etc) as opposed to a zoom which will allow you to cover a range of focal lengths (70mm-200mm for example).
A fixed focus lens cannot change its focal plane, but this is not the same as a prime lens.
Originally by user3664. Source · Licensed CC BY-SA 4.0
user3664
15y ago
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A prime lens has a fixed focal length, not a fixed focus distance. Those are different things.
Focal length is a property of the lens design/optics. In a zoom, the focal length changes; in a prime, it does not. Focusing is done by changing the spacing between lens elements and/or between the lens and the sensor/film plane.
For a simple lens, focus follows the lens formula:
1/f = 1/s1 + 1/s2
where f is focal length, s1 is the lens-to-image distance, and s2 is the lens-to-subject distance. If f stays the same and the subject distance changes, the image distance must change too. That is what the focusing mechanism does.
At infinity focus, the image forms about one focal length behind the lens. For closer subjects, the lens must be positioned slightly farther from the sensor (or internal elements are moved to achieve the same effect).
So a prime lens can focus at many distances while keeping the same focal length. A fixed-focus lens is the type that does not allow that adjustment.
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