Lenses - considerations and choices

December 2001 Surveillance

When choosing a lens to perform in a CCTV site, there are several considerations that must be taken before the actual decision is made. Choosing the lens is easy, yet it is the second most important decision of the entire job.

The first decision to be made is the actual area to be viewed at the monitor.

1. Does a wide area at short range need to be viewed? If so, a wide-angle lens is needed.

2. Does an average scene at medium distances need to be viewed? If so then a standard lens is needed.

3. Does a narrow area at long range need to be viewed? If so, a telephoto lens is needed.

4. Does an area representing 1, 2 and 3 need to be viewed? If so, a zoom lens is needed.

The second step depends on the type of camera that will be used:

1. Is the camera to be installed in an area where the lighting is fixed or minimal variable? If so, a fixed or manual lens is needed.

2. Is the camera to be installed in an area where the light is largely variably? If so an auto iris lens is needed. (If using a camera with built-in auto shuttering, it may be possible to use a fixed or manual lens.)

3. Is the camera a 1/3", 1/2" or 2/3" format? The format size of the lens must meet or exceed the format size of the camera.

4. Is the camera a colour camera? If so, a colour-corrected lens must be used. Is the camera a 'C' or 'CS' style camera? CS style cameras are new design cameras. Their chips (CCD chip) are located 5 mm closer to the lens mount. When using C type lens on a CS camera make sure the CS ring is used on the camera, as the lens will break the chip on the camera when it is screwed on.

Once the above questions are answered, a few simple calculations can be made on paper and the proper lens for the proper job can be ordered.

Focal length

The focal length of the lens determines the interior length of the lens, as measured in mm and will determine the 'field of view' that the camera produces.

A simple rule of thumb applies: the smaller the mm length of a lens, the wider the angle of view will be and the larger the mm length of a lens, the more telephoto it will be.

Therefore, if a 12 mm lens is standard for a 1/3" camera, a 8 mm lens would be wide angle and 16 or 25 mm will be a telephoto.

The following table of lenses lists those commonly used for various sizes of cameras.

It should be noted that anytime a zoom lens is used in a CCTV system, the lens should never exceed the 10 to 1 ratio. For example, if the low end or wide angle is 8 mm the long end or telephoto of the zoom lens should not exceed 80 mm. This is due primarily to light loss, depth of field and resolution that occurs when the lens is in full telephoto position.

Field of view

Field of view refers to the actual size of the picture, both height and width, that a specific formatted size camera will produce, with a specific lens, at a specified distance from the subject.

If too much of a view is produced by a camera, then the subject of concern will be too small and/or too far away for proper identification.

If a camera is mounted too high, with a lens that is too short (wide angle) then one will see the tops of people's heads. If one mounts the camera in such a way as to have a bright background, the foreground will be silhouetted. Therefore the field of view is extremely important and is directly related to the angle of view. To calculate the field of view, you can use the following method:

Scene width = Format (horizontal) x distance/focal length

Scene height = Format (vertical) x distance/focal length

Since the horizontal and vertical format sizes of the chips are not readily available, the following will assist in listing them:

Another method of calculating the field of view would be to use a lens selection wheel.

The third method of calculating the field of view would be to use a viewfinder. This is a specifically designed lens that one can look through.

Depth of field

Depth of field is a term that relates to what portion of a picture is in focus and what portion of the picture is out of focus at any given light level or iris opening at specific distances. The wider the angle of the lens, the more depth of field the picture will have. The longer the telephoto capabilities of the lens, the shorter or narrower the depth of field will be.

For example: the depth of field of a 25 mm lens focused on an object 7,5 m away in full daylight will be 1 m to infinity. Any object closer to the camera than 1 m will be out of focus. All objects beyond 1 m will be in focus.

A common rule of thumb is the shorter the lens length (2,8, 4,8, 8 mm) the wider the depth of field will be. The longer the lens (16, 25, 50 mm etc), the shorter or narrower the depth of field will be.

Zoom lenses

The term zoom, in lenses, refers to the ability of the lens to physically alter its view from wide to narrow. In simpler terms, a zoom lens can either make objects that are far away appear near and large, or it can make them appear far away and small.

The majority of zoom lenses used in CCTV systems are motor driven. The motors give the operator an opportunity to zoom in on a distant object and to observe with more detail, or back out of an object to see more of the surrounding area and actions. For this reason considerations must be given to the control of the zoom lens.

Iris and F-stop explanation

The iris of the lens is referred to as the 'centre focal point of the lens' and is mounted at the aperture of the lens. This is the point within the lens where the intensity of the light focused on the CCD chip is controlled and is measured in terms relating to 'F-stops'.

The iris is a simple mechanism used to vary the diameter of the aperture. Some irises are made of a number of thin, arc shaped metal blades that surround the aperture, each blade having a lower stud at one end and upper stud at the other end. The purpose of the iris is to adjust the amount of light or scene reflection that is allowed to pass through to the chip. During a bright period of light the iris should be closed, allowing a small portion of light to pass through. In a dark period the iris should be open allowing more light available to pass through. In a moderate period of light the iris would be partially closed, allowing a moderate portion of light to pass through.

F-stops are numbers, the smaller the number, the larger the opening at the aperture or iris. The larger the number, the smaller the opening or aperture. F-stops will always be measured with the following numbers: f-8, f-1.4, f-2, f-2.8, f-4, f-5.6, f-11, f-16 and f-22. Each number refers to the reduction of the aperture size of the lens. For example: f-22 means that the lens opening is f/22 as large as the actual aperture opening. It would be used in bright light where only a small amount was needed by a chip.

A fixed iris lens is just what it sounds like. It is a lens with an aperture of a specific size, with no ability to be adjusted manually or automatically. An auto iris lens is a lens that is designed to have an iris that is adjusted automatically ie through the use of an electronic controller or video sampler which is built into the lens or camera. The auto-iris lenses are of two types: video lenses or direct drive (sometimes referred to as DC type).

Video signals being emitted by the camera will change in amplitude (size) according to the amount of light of the scene being focused on the chip. The more light being focused on the chip, the larger the video signal that will be emitted from the camera. The less light, the smaller the video signal from the camera. As the video sampling system of the lens, senses increases in the amplitude (size) of the video signal that go beyond a preset size, it will command the iris to close. As the iris closes it will cut down the light passing through the lens onto the chip and cause the video signal to decrease in direct proportion until it fits into the preset calculations of the sampling system, at which the iris will stop and remain in a fixed position until it is again commanded to open or close. The same can be said about the video signal when it decreases. The iris will be commanded to open so that more light can pass onto the chip.

For further details contact Abie Ali, Frank Street on tel: (011) 838 4515, e-mail: frankstreet@icon.co.za

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