Selecting the most appropriate lens can be a complex task because lenses have a number of characteristics that must be considered to match a particular requirement with the best lens for the job.
- Fixed focus lenses are the simplest type of Lens. The preset focal length means a precise calculation is required to select the lens most suitable for the location, based on the desired size of viewing area and its distance from the camera. Typical Lens sizes offer either 30 degree view – narrow to allow more detail at distance – or 60 degree, which offers a much wider angle of view.
- Varifocal lenses offer more flexibility, allowing the field of view to be adjusted manually to get the view required rather than the limited by the constraints of the fixed Lens.
- Zoom lenses are the most complex type, offering the widest choice of associated features and technologies. Zoom lenses can be remotely adjusted to allow variation of the focal length. This means that a single lens can be used to view a wide area until an intruder is detected whereupon it can be zoomed into capture facial details. Generally Zoom lenses incorporate an Auto Iris mechanism to permit 24-hour usage.
Lenses are also categorized according to size format. As Camera technology has advanced, sensor chips have reduced in size, requiring lenses to produce smaller images at the focal point. This has made smaller lenses possible (less glass resulting in less physical size and weight) although the requirements of precision manufacturing doesn’t permit a proportional price reduction – the component materials of a Lens being a very small proportion of the overall manufacturing cost. The quoted format of the Lens (1″, 1/2″, 1/3″ and now even 1/4″) is derived from the ratio of diameter to the viewing image produced. Whilst it is often most cost effective to match the lens format to the camera sensor size, it is possible to use a larger lens on a smaller size camera since the image only needs to be at least as large as the sensor. Using a larger lens can often be advantageous, since it offers greater depth of field (the range of distances from the lens before objects are too close or too far away to be in focus). Larger lenses also mean that the area of the image that is used is taken entirely from the central, flatter part of the lens causing much less corner distortion and better focus.
A lens with a manually adjusted iris can be used in indoor applications, where the lighting level is controllable and consistent. Both manual and fixed iris lenses can be used with cameras which offer a feature known as ‘electronic iris’ – an on-board technology to effectively reduce the sensor exposure to compensate for the lack of iris control. This can be cost effective, but does not provide the increased depth of field offered by a correctly sized iris.
For external use (where conditions generally vary the most), an automatic iris lens offers the best performance, as the iris aperture automatically adjusts to create the optimum image by monitoring the output signal from the camera. There are a number of different lens types offering this method of iris control. The original design for automatic iris lenses was wholly self-contained, with the image analysing technology built into the lens and an iris that was adjusted by servomotors.
The final lens characteristic to take account of is the light-gathering speed of the lens-expressed as an f-stop number. This literally measures the amount of light captured by the lens in a given period of time; the lower the f-stop range, the more light that can be transmitted.