What is an f-number (ƒ/#)?

Glossary Definition

An f‑number (ƒ/#) or f‑stop refers to the ratio of a lens’s focal length to its aperture diameter and indicates the amount of light coming through the lens. F-numbers can change throughout a lens’s zoom range, lenses are typically specified with their maximum aperture ability.

A low ƒ/# means a larger maximum aperture (also called a fast lens), which results in a higher light or infrared collecting ability while a high ƒ/# means a smaller aperture (slow lens), which has lower light or infrared collecting ability. The lower the f‑number, the better the lens is for visible and NIR imaging, as it results in a system with increased light (or Lux) sensitivity and allows the camera to capture more accurate images in lower light levels. (For a comparison of common f-numbers and how much they affect the amount of light reaching the sensor, see the PDF below.)

Long-range cameras due to their very long focal lengths have much higher f-numbers (smaller apertures in relation to their focal length). For a long-range camera to have a lower f-number it would need an aperture of a very large diameter. Large diameter apertures exacerbate atmospheric distortion, which is why there are no long-range low-light cameras. (This one reason why long-range cameras are either thermal or require our ZLID™ Illumination in order to have night vision.)

For thermal infrared cameras, having a lower f‑stop number increases the image contrast and clarity, which results in higher detection distances. This is particularly true for uncooled LWIR cameras where lower f‑number lenses like ƒ/1.0 to ƒ/1.6 are required. An ƒ/1.0 lens allows for over 2.5 times more infrared thermal energy to be transferred to the infrared sensor than an ƒ/1.6 lens.

When looking at thermal imaging it is very important to look at the lens’s f‑number, as this is often as important the thermal sensor in determining the actual performance of the system. Infiniti uses mostly ƒ/1.0 and ƒ/1.1 lenses in our LWIR thermal cameras to ensure high contrast, long-range performance for detection, recognition, and identification. Infiniti Electro-Optics provides a wide variety of EO/IR camera solutions all the way from component level, open frame modules (lens and sensor attached) to complete, integrated PTZ camera systems. These integrated systems can include multiple sensors for visible light, thermal, SWIR and more to achieve the type of ultra long-range imaging performance required by a host of military and defense applications.

For more information about thermal infrared imaging for night vision and surveillance see our Thermal Imaging Explained page.