Aperture, expressed as an f-number like F1.6 or F2.0, controls how much light reaches the sensor — a lower f-number lets in more light, which matters most when a camera has to perform in low light.
The f-number is the ratio of focal length to the diameter of the lens's light-gathering opening. A lower number (F1.0, F1.2) means a physically wider opening relative to focal length, so more light reaches the sensor in the same amount of time. A higher number (F2.8, F4.0) means less light gets through. This is inverse — F1.2 lets in noticeably more light than F2.0, even though "1.2" looks like the smaller number.
Security cameras are often asked to produce a usable image at dusk, at night, or in dim indoor spaces with no control over lighting. A wider aperture (lower f-number) keeps the image brighter and less noisy in those conditions without relying entirely on IR illumination. The tradeoff is a shallower depth of field and sometimes softer image quality toward the edges of the frame, so it's not "lower is always better" — it's a real tradeoff against your lighting conditions.
A fixed-iris lens has one aperture setting, chosen for a space with consistent lighting (indoors, under controlled lighting). A DC auto-iris lens automatically opens and closes its aperture in response to changing light — necessary for outdoor cameras or anywhere light swings between bright daylight and near-darkness, since a fixed aperture would either blow out highlights in daylight or go dark at night.
Important: Check your camera supports DC iris before you order. A DC auto-iris lens only works if the camera itself provides DC iris support — the camera supplies the control signal that drives the lens's motor, typically through a dedicated DC iris connector. If your camera doesn't have that connector or that feature, a DC iris lens's auto-iris motor simply won't be driven, and the lens won't adjust. Check your camera's spec sheet or connector type before ordering a DC iris lens; if you're not sure, a fixed-iris lens is the safer choice for a controlled-lighting environment.
| Lighting Condition | Recommended Aperture | Notes |
|---|---|---|
| Low light / night, no IR illuminator | F1.0–F1.6 | Maximizes available light gathering |
| Well-lit indoor (retail, office) | F2.0–F2.8 | Plenty of light; depth of field matters more |
| Outdoor, variable daylight | DC auto-iris lens | Aperture adjusts automatically through the day |
Our PT-1612 (16mm, F1.2, "Large Aperture CS Mount Lens") is built specifically for low-light performance — compare that to PT-1612NI, the same 16mm focal length but F1.6 with a fixed iris, intended for better-lit, more consistent environments. Same focal length, same field of view, but a different aperture for a different lighting situation.
Browse our full range of M12 and CS mount lenses, or check our guides on focal length and infrared imaging to finish specifying your camera build.
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