Focal length sets how wide or narrow your lens's field of view is: shorter focal lengths see more of a scene up close, longer focal lengths see farther but across a narrower slice of it.
Focal length is the distance between the lens and the point where it focuses light onto the sensor. A short focal length (1.2–4mm) spreads that light over a wide angle, so the lens captures a broad scene but each object in it appears small. A long focal length (16mm and up) narrows that angle, so distant objects appear larger but the lens covers less area at once.
The same lens produces a different field of view on a different sensor size, because angle of view depends on both numbers together:
where θ is the angle of view, d is the sensor's width (or diagonal, for the full diagonal angle), and f is the focal length.
This is why two lenses with the same focal length can list different angles of view in our catalog — always check the angle of view spec for your specific sensor size, not just the focal length number.
| Focal Length Range | Typical Angle of View | Best For |
|---|---|---|
| 1.2–4mm | 90–195° | Wide-area overview: entryways, small rooms, fisheye coverage |
| 4–8mm | 40–90° | General-purpose monitoring: retail floors, parking areas |
| 8–16mm | 20–40° | Mid-range identification: hallways, perimeters |
| 16mm and up | Under 20° | Long-range identification: license plates, fence lines |
A fixed-focal lens has one angle of view, set at the factory — simpler, usually sharper edge-to-edge, and the lowest cost. A varifocal lens lets you adjust the focal length by hand during installation to fine-tune coverage. A motorized zoom lens does the same adjustment remotely and can refocus automatically after zooming, which is useful when a camera's field of view needs to change after it's mounted, or when it's in a spot that's hard to physically reach.
To show how much the same change in focal length moves the angle of view: our PT-01224XFL (1.2mm, F2.4 fisheye, 1/3″ sensor) covers a 170° angle of view — almost the whole room. Our PT-1620BMP (16.0mm, F2.0, 1/2″ sensor) covers just 20° — narrow enough to pick out detail well down a hallway or across a parking lot. Both are fixed-focal M12 lenses; the ten-times difference in focal length is what separates "see everything nearby" from "see one thing far away."
Start with how far the camera needs to see and how wide an area it needs to cover, then use the table above to narrow down a focal length range, and check the angle-of-view spec against your camera's actual sensor size before you order.
Browse our full range of M12 and CS mount lenses, or check our guides on aperture and infrared imaging to finish specifying your camera build.
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