What is an Aperture?

Glossary Definition

A camera’s aperture is the size of the opening (and sometimes the internal light path) of the the lens. While the aperture can be measured on its own by measuring the diameter of the aperture, it is more commonly measured in comparison to the focal length of the lens and expressed as an f-number.

The smaller the f-number the bigger the aperture, which results in higher light collecting ability (or thermal infrared energy collecting ability). A high f-number results in lower amount light (or thermal infrared) collecting ability. Typically the lower the f‑number the better the lens is and the better the camera will operate in visible, NIR, SWIR and especially thermal infrared imaging. In visible, NIR and SWIR cameras a lower f-number results in a camera system with increased light sensitivity which allows the camera to capture more accurate images in lower light levels and respond better to IR or white light illumination. For thermal infrared cameras it increases the radiant heat gathering ability which improves range and performance for both cooled and uncooled thermal cameras.

Visible, SWIR and NIR camera lenses often include a motorized iris, which is an internal adjustable aperture that can change the amount of light coming through the lens as well as the camera’s depth-of-field. The f-number in these lenses is often referred to as the f-stop number, as it can be “stopped” down to reduce the amount of light reaching the sensor. The greater the focal length of the lens, the harder it is to obtain a low f-number as the lens diameter would need to be far too large. For example our 135X 15.5–2075mm lens at max zoom is an f/32 lens. This means that the aperture size is 1/32 of the focal length, or about 65mm (2½ inches). The f/32 spec reduces the sensitivity of a camera to about 1/700th of what it would be with an f/1.2 lens, which is often the assumed lens specs that manufacturers use to claim a lower light performance. For our 2075mm lens to be rated f/1.2 it would need to be at least 1.75m (nearly 5¾ feet) in diameter. This is why, despite manufacturer’s claimed lux ratings, there is no such thing as a long-range low light camera. For long distances at night for visible/NIR cameras, LED illumination is required for distances of about 100-500m or active laser illumination (ZLID) is required for distances beyond that up to 5km.

For thermal infrared cameras, having a larger aperture (lower f-number) increases the image contrast and clarity, which results in longer 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 Ge lens allows for 2.5 times more infrared thermal energy to be transferred to the infrared sensor than an ƒ/1.6 lens. Thermal cameras do not require illumination and can operate in 0 lux (complete darkness) since all of the photons they see are naturally emitted by the objects. 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 specs in determining the actual performance of the system. Infiniti uses mostly ƒ/1.0 and ƒ/1.3 lenses in our LWIR thermal cameras to ensure high contrast, long-range performance for detection, recognition, and identification, with the exception of our biggest LWIR lens which is 15-400mm and is f/1.5.

Infiniti optics is provides a wide variety of EO/IR camera solutions all the way from component level, open frame modules (lens and sensor attached) or as complete integrated PTZ cameras with visible, SWIR and thermal sensors for ultra long-range imaging optimized for 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.