The role of the iris is to adjust the amount of light passing through to the lens. There are three different types of irises on lenses;
Manual Iris. On a manual iris lens the iris is fixed or adjusted by manually turning a ring.
DC Auto-Iris. The Iris is connected to the output of the camera and is automatically adjusted by the Digital Signal Processor (DSP) in the camera.
Video Auto Iris. This is sometimes referred to as a servo lens. The iris is driven by the video signal from the camera (the voltage if the video signal is proportional to the amount of light on the imaging device).
Auto iris lenses are recommended for outdoor applications. The iris automatically adjusts the amount of light reaching the imaging sensor for optimum results and also protects the sensor from too strong light.
A small iris diameter reduces the amount of light and gives a better depth of field (the image is in focus over a greater distance). A large iris diameter will give a better image under low light conditions.
The size of the aperture is known as the “f-number” of the lens. This is the ratio of the mechanical components of the lens and is defined as:
f-number = focal length/effective diameter
The effective diameter is related to the size of the front lens and is not the actual diameter. The f-number is a measure of the amount of light that the lens will pass to the imaging device. The smaller the number then the larger is the aperture.
The table below shows the amount of light falling on a lens that is passed to the sensor, as a function of f-number.