Image quality is vital in video surveillance, especially where the video is used to identify faces, recognise number plates or in automated image analysis applications. There are also many different surveillance situations ranging from people counting in small retail stores to helping identify trespassers in dark railway tunnels. This results in many different requirements when it comes to image quality.

There is a vast range of different network video cameras on the market, but the camera with the highest technical specifications and most advanced features is not always the best for the task at hand. Ensuring the images can be used for the purpose that the surveillance system is designed for is key, and this means focusing on the user’s needs and the application, as well as the specific camera features. Moreover, looking at the technical specifications alone is not enough; it is important to understand how the camera will perform in a specific surveillance situation.

One of the obvious factors that influences image quality is the amount of light in the area that’s monitored. This can vary greatly from day to night, depending on the weather or even the time of year. Some of the issues can be resolved by adjusting the positioning of the camera or its settings. Ultimately though, the most important thing is to choose the right camera with the right lens, tested for maximum performance based on how it will be used.

The role of the lens

The main role of a lens is to gather and focus incoming light onto the sensor. There are also many different aspects or characteristics of a lens that will determine how the camera will behave and what it can be used for. One important consideration when selecting a camera and lens is the field of view and resolution required, that is, how much of a scene and what level of detail are to be captured. The lens also controls the amount of light passing through to the image sensor and will affect the camera’s low-light capabilities.

But, the lens is only one component of a very complicated and finely tuned system. In a camera, each component and how it interacts with the other components plays a vital part in determining the camera’s performance.

Matching the lens and the camera

On a megapixel camera, a high quality lens is always required since megapixel sensors have pixels that are much smaller than those on a VGA sensor. It is critical to match the lens resolution to the sensor and image processing, in order to fully use the camera’s capabilities.

The lens also needs to be matched to the image sensor. If a lens is made for a smaller image sensor than the one that is fitted inside the camera, the image will have black corners. If a lens is made for a larger image sensor than the one used in the camera, the field of view will be smaller than the lens’ capability and part of the image information will be unused.

When it comes to the lens mount, there are two main standards used on network box cameras: CS-mount and C-mount. They both have a 1-inch thread and look the same, but the distance from the lens to the sensor when fitted on the camera differs. Some lenses require a spacer to be able to focus.

In low-light situations, an important factor to look for in a network camera is the lens’ light-gathering ability. This is determined by the F-number – also known as f-stop – which defines how much light can pass through a lens. In low-light situations, a smaller F-number generally produces a better image quality as the lens’ light gathering ability is better; however, there may be some image sensors that can’t take full advantage of this due to the way they are designed.

The ability to control a camera’s iris opening also plays an important role in image quality. The iris can be used to optimise the resolution and depth of field – criteria that are important in video surveillance applications, for instance, in monitoring an aisle or a corridor where a large depth of field necessary.

Fine-tuning the setup

This is not where the story ends though. With higher megapixel cameras in particular, image quality reaches a level where it is not sufficient to match the lens to the image sensor, mount, light conditions and camera resolution. There are other factors, too, that can influence the finely tuned interaction between the lens and the camera’s other components – and, if ignored, even a small detail can mean that the camera will not produce the expected image quality.

For a start, lens manufacturers use different criteria in specifying a lens – the megapixel amount is not always comparable. This is why the R&D team at Axis does its own measurements with an MTF (Modulation Transfer Function) machine.

MTF is the most widely used method of describing lens resolution and its performance. It refers to the ability of a lens to accurately reproduce an image of alternating black and white lines with a known spacing onto an image plane.

In addition to resolution measurement, the MTF machine can also measure distortion, chromatic aberration and curvature of field, all important parameters to consider when optimising a system.

The actual size of the sensor also varies from manufacturer to manufacturer, and for different sensor types. Two image sensors with the same size specification aren’t necessarily the same size. If the tolerance in the lens is too small then black corners can appear in the image.

Different lenses have different mount design and protrude into the camera. If the lens part that goes into the camera house is too long, the lens can damage other components in the camera.

Tilt tolerance is another important factor. To get a sharp image in the corners of the picture, the optical axis needs to be orthogonally aligned to the sensor surface. Both the lens and camera however have a slight tilt. So additional measurements and tests are necessary to ensure that the chosen lens and camera are indeed compatible.

Camera components will be affected by varying temperatures. Every component needs to be seen as part of a system that is perfectly matched. How the sensor – lens distance changes with the temperature is both camera and lens dependent. If the wrong lens is used or a lens replaced with a different one, the image focus may be affected with varying temperature.

Optimising glass and image pipeline

The amount of glass in the camera, between the lens and the sensor surface, must also be matched to the design value of the lens. This is increasingly important with higher resolution cameras. Unless the amount of glass is optimised, the video image can appear ‘milky’.

Similarly, the complete image pipeline in the camera including the lens needs to be trimmed and optimised with respect to resolution, contrast, colour representation, and gain. If not, it will affect image quality.

For example, to optimise the image quality in a camera, the ghost and flare effects resulting from the lens-filter-sensor combination must be minimised. This is especially important in a wide dynamic range camera. Axis has specially designed machines to measure these effects that will help find the best lens-filter combination.

When it comes to day/night cameras, the lens needs to retain enough resolution in Near Infrared (NIR) light. Again, the resolution in a lens is lens manufacturer dependent and different manufacturers use different criteria to determine what resolution is needed in NIR. Using an MTF machine to measure the actual resolution, it is possible to find an optimal combination of sensor and lens that works well in both day and night scenes.

Another challenge with changing a lens is the risk of contamination. Lenses should always be mounted in a special clean environment to avoid dust getting into the camera and onto the sensor. Installers and customers do not usually have access to clean enough environments and dust that has entered the camera can result in irritating ‘dots’ in the image.

Apart from the impact that changing the lens or using an inferior lens can have on the camera’s image quality, there is a support issue, too. Camera manufacturers will find it harder to help with any image quality issue if the lens was purchased and added to the camera separately, as it may be difficult to determine if a problem is due to the lens or the camera itself.

Summary

There is a reason for why some camera manufacturers ship their cameras with the lens already included. With a high-tech, high-megapixel camera, all the components need to be matched and optimally adjusted to achieve maximum performance. If not, there is a risk that image quality will suffer.

Wai King Wong is Country Manager Australia and New Zealand for Axis Communications. Wai King is responsible for the management of all sales and operational activities in ANZ and has held the position since November 2002.

Wai King joined Axis Communications following a three year stint as senior product manager – Networking and Support Services Manager (Malaysia) Technical Consultant (Australia) at Ingram Micro.

Wai King has extensive knowledge and experience in the networking and software deployment and support arenas and holds a bachelor of Business in Business Information Systems from the Royal Melbourne Institute of Technology (RMIT) and a Master in Business and Technology, UNSW.

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