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Choosing The Right Lens

Choosing the right lens
Once you have selected a camera, the next step is to select the appropriate lenses and any other relevant components necessary in the system.

There are two main lens mount standards called C-mount and CS-mount. They both have a one-inch thread and they look the same. What differs is the distance from the lenses to the sensor when fitted on the camera:
* CS-mount. The distance between the sensor and the lens should be 12.5 mm
* C-mount. The distance between the sensor and the lens should be 17.5 mm. A 5 mm spacer (C/CS adapter ring) can be used to convert a C-mount lens to a CS-mount lens

The initial standard was C-mount, while CS-mount is an update to this, allowing for reduced manufacturing cost and sensor size. Today, almost all cameras and lenses sold are equipped with a CS-mount. It is possible to mount an old C-mount lens to a camera with CS-mount by using a C/CS adapter ring. If it is impossible to focus a camera, you probably have the wrong type of lens.


Sensor size
Image sensors are available in different sizes, such as 2/3", 1/2", 1/3" and 1/4", and lenses are manufactured to match these sizes. It is important to select a lens suitable for the camera. A lens made for a 1/2" sensor will work with 1/2", 1/3" and 1/4" sensors, but not with a 2/3" sensor.

If a lens is made for a smaller sensor than the one actually fitted inside the camera, the image will get black corners. If a lens is made for a larger sensor than the one actually fitted inside the camera, the angle of view will be smaller than the default angle of that lens – part of the information being "lost" outside of the chip (see illustration below).


Focal length
Focal length determines the horizontal field of view at particular distances – the longer the focal length, the narrower the field of view.
Examples of focal length needed to achieve an approximate 30° horizontal field of view.

Most manufacturers provide simple-to-use slide and rotary calculators that calculate the lens' focal length from the scene size and focal length.

To detect the presence of someone on a display, they should make up at least 10 per cent of the height of the image. To accurately identify them, they should make up 30 per cent or more of the image.

For this reason, it is important to check the capabilities of selected cameras and view the resulting images on screen before going live.

Calculation - feet What width of objects will be visible at 10 feet when using a camera with a 1/4" CCD sensor and a 4 mm lens? H = D x h / f = 10 x 3.6 / 4 = 9 feet.

Calculation - meters What width of objects will be visible at 3 meters when using a camera with a 1/4” CCD sensor and a 4 mm lens? H = D x h / f = 3 x 3.6 / 4 = 2.7 meters.


Lens types
* Fixed lens: The focal length is fixed, e.g. 4 mm.
* Varifocal lens: This lens allows for the manual adjustment of the focal length (field of view). When the focal length is changed, the lens has to be refocused. The most common type is 3.5-8 mm.
* Zoom lens: The focal length can be adjusted within a range, e.g. 6 to 48 mm without affecting the focus. The lens can either be manual or motorized, so that it can be controlled remotely.

           


Iris
Generally network cameras control the amount of light passing to the imaging device via the iris or by adjusting the exposure time. In conventional cameras, exposure time is fixed. The role of the iris is to adjust the amount of light passing through the lens. There are different types of irises on lenses:


Manual iris control
The iris on a manual iris lens is usually set up when the camera is installed to suit the prevailing lighting conditions. These lenses cannot react to changes in scene illumination so the iris is set to an "average" value, which is used in conditions with varying light.


Automatic iris control
For outdoor conditions, and where the scene illumination is constantly changing, a lens with automatically adjustable iris is preferred. The iris aperture is controlled by the camera and is constantly changed to maintain the optimum light level to the image sensor.

* DC-controlled iris: Connected to the output of a camera, the iris is controlled by the camera's processor
* Video-controlled iris: The iris is controlled by video signal

Auto iris lenses are recommended for outdoor applications. The iris automatically adjusts the amount of light reaching the camera and gives the best results, as well as protecting the image sensor from too much light. A small iris diameter reduces the amount of light, giving a better depth of field (focus over a greater distance). A large iris diameter, on the other hand, gives better images in low light. The iris is defined by the F-number.


F-number = Focal length / Iris diameter
The f-number of a lens is the ratio of the focal length to the effective object lens diameter. It affects the amount of light energy admitted to the sensor and plays a significant part in the resulting image. The greater the f-number, the less light admitted to the sensor. The smaller the f-number, the more light admitted to the sensor, and hence better image quality is achieved in low-light situations. The table below shows the amount of light admitted to the image sensor at sample f-values.

In scenes with limited light, fitting a neutral density filter in front of the lens is recommended. This works to reduce the amount of light entering the lens evenly over the whole visible spectrum and forces the iris to open fully to compensate for this. Many network cameras today offer automatic iris control to ensure that the image remains clear throughout the year and time of day as light levels constantly change.