A part of the lens controlling the amount of light reaching the image sensor through the lens. In more precise terms, it adjust the relative aperture to control the amount of light reaching the image sensor through the lens. The centre of the aperture corresponds to the optical axis of the lens system. The ratio of the system's focal length to the diameter of the entrance pupil is referred to as the f-number.
High f-number (e.g. f/22) means that the relative aperture is smaller and allows less light to pass, whereas low f-number (e.g. f/1.4) means that the relative aperture is large and allows more light to pass through the lens.
The following lens types are available:
manually adjustable iris (manual iris),
automatically adjustable iris (auto iris).
The lens is selected depending on the application. In outdoor applications with variable luminous intensity, lenses with automatic iris (auto iris) are recommended. In indoor applications where daylight is not the prevailing light source, lenses with manual iris (manual iris) or even less expensive fixed iris can be used.
The f-number is a sequence of the following numbers: f/1.0, f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8.0, f/11.0, f/16.0, f/22.0, f/32.0... A subsequent value in the sequence increases the amount of light reaching the image sensor by a factor of 2 i.e. f/16 lens will allow twice the amount of light than f/22.
The f-number does not affect the amount of light passing the lens. The f-number also directly affects the depth of field i.e. the distance between the nearest and the furthest objects giving a focused image. In simple terms, the depth of field increases with the increase in the f-number (at decreasing amount of light passing the lens) and decreases with the decrease in the f-number.
Fig. 1. f/2.8
Fig. 2. f/5.6
Fig. 3. f/16
For lenses with automatic iris (auto iris), the depth of field will change with any change in lighting conditions. The focus set in outdoor conditions during the day when the amount of light is relatively high (small aperture - high depth of field), will be lost with any change in lighting conditions, e.g. after dusk. The iris will try to adjust to the decreasing amount of light and will increase the relative aperture, reducing the depth of field. It can be prevented by using a special device forcing the iris to remain open and fine-tuning the focus.