The ability to control a camera’s iris opening plays an important role in image quality. An iris is used to maintain the optimum light level to the image sensor so that images can be sharp, clear and correctly exposed with good contrast and resolution. The iris can also be used to control the depth of field.
Iris control can be fixed or adjustable. Adjustable iris lenses can be manual or automatic (auto iris and P-Iris).
P-Iris is an automatic, precise iris control first developed by Axis Communications of Sweden and Kowa Company of Japan. It involves a P-Iris lens and specialized software that optimize image quality. The system is designed to address the shortcomings of an auto-iris lens. P-Iris provides improvements in contrast, clarity, resolution and depth of field.
Having good depth of field - where objects at different distances from the camera are in focus simultaneously - is important in the video monitoring of, for example, a long corridor or parking lot.
In bright situations, P-Iris limits the closing of the iris to avoid blurring (diffraction) caused when the iris opening becomes too small. This can typically happen in cameras that use DC-iris lenses in combination with megapixel sensors that have small pixels. Being able to avoid diffraction and at the same time benefit from an automatically controlled iris is highly valued in outdoor video surveillance applications.
A P-Iris lens uses a motor that allows the position of the iris opening to be precisely controlled. Together with software that is configured to optimize the performance of the lens and image sensor, P-Iris automatically provides the best iris position for optimal image quality in all lighting conditions.
In Axis network cameras with P-Iris, the user interface provides a scale of f-numbers that ranges between the widest and smallest iris opening. This feature enables the user to adjust the preferred iris position, which is the iris position used by the automatic control for most lighting conditions.
P-Iris allows fixed network cameras to reach a new level of performance in image quality. The advanced iris control will be especially beneficial for megapixel/HDTV cameras and demanding video surveillance applications. P-Iris is expected to replace DC iris as the standard iris control for fixed network cameras.
Auto iris (DC and video)
There are two types of auto iris lenses: DC iris and video iris. Both have a motor-driven, automatically adjustable iris opening that responds to changes in light levels. Both also use an analog signal (often analog video signal) to control the iris opening.
The difference between the two is where the circuitry to convert the analog signal into motor control signals is located. In a DC-iris lens, the circuit resides inside the camera; in a video iris, it is inside the lens. In bright situations, a camera with an auto iris lens can be affected by diffraction and blurring when an iris opening becomes too small.
This problem is especially prominent in megapixel and HDTV cameras since the pixels in the image sensors are smaller than standard resolution cameras. Therefore, the image quality is more dependent on getting the right iris opening (aperture).
In order to optimize image quality, a camera needs to have control over the position of the iris opening. The problem with an auto iris lens is that this control cannot be made available to the camera or user.
In indoor environments where light levels may be constant, a fixed iris lens can be used. With fixed iris lenses, the iris opening cannot be adjusted and is fixed at a certain f-number. The camera can compensate for changes in the level of light by adjusting the exposure time or using gain.
With manual iris lenses, the iris can be adjusted by turning a ring on the lens to open or close the iris. This is not convenient in environments with changing light conditions, such as in outdoor surveillance applications.