The choice of lens for your system is key in delivering pictures of evidential quality for your system, writes Mike Tennent, Director of Training, Tavcom Training.
I don’t wish to harp on between the differences of terrestrial television and our CCTV images, but one has to remember that for those of us used to watching analogue or the new digital television programmes expecting to have the same picture qualities from our CCTV, are for the most part disappointed. That is down to two prime reasons
1) Cost of a broadcast lens may be as high as £25,000 (that’s before we talk of cameras and the associated studio lighting). The security industry equivalent taking a typical decent zoom lens costs a fraction, at around £2,000.
2) Cost: the second reason is that we tend to go for the cheapest option when selecting a CCTV system and, in my view, not enough time is devoted to choosing a lens that would be really fit for purpose.
With the new technologies available, there really is no excuse for inferior pictures; it is your choice of quality that you will have displayed and recorded. Let’s take a look at the lens as a pivotal part of your system. The function of the lens is to gather light and focus it onto the pick up device (the camera’s CCD – Charge Coupled Device). We are all keenly aware that many CCTV systems do not offer high quality pictures because the correct lens has not been specified at the survey stage. To appreciate the lens’ role we need to understand:
l Light
l Fixed and variable focal lengths
l Manual and auto-iris lenses
l Varifocal lenses
l Manual and motorised zoom lenses
l Depth of field
l Lens filters
l Back focusing
Light
Light has two basic properties, its amount or quantity, which is measured in Lux in many parts of the world (the USA for example still use the foot-candle, there are approx 10 foot candles in a Lux) and its colour or wavelength, which we measure in nanometres 10-9 metre or a billionth of a metre.
Background history for us anoraks! The foot candle was measured using a foot square piece of greaseproof paper (or similar) and when the light from a candle illuminated the surface equally, this was then deemed to measure one foot candle. The Lux is a metric conversion measured as a square metre, hence the 10 fold difference in measurement. However light, like all electromagnetic waves, has three properties in addition to quantity: velocity, frequency and wavelength. Velocity is in metres per second in a given direction. Wavelength is the distance from one wave peak to the next and is measured in metres and Frequency is the rate that a waveform repeats and is measured in Hertz.
Therefore: frequency x wavelength = velocity
The speed of light changes when it travels from one medium to another: air to glass, or glass to air. And the speed change is dependant on the wavelength of the light. The frequency remains the same, so if the speed changes, so must the wavelength. It is the frequency of the light that our retinas are sensitive to. Light travels fastest in a vacuum, slightly slower in air and even slower in glass. So, when light passes from air to glass, the frequency remains the same, the speed is reduced and the wavelengths become shorter. Red light travels more rapidly through a block of glass than does blue light. In a prism blue light bends more than red light, which is why in a rainbow blue is on the inside of the bow and red is on the outside. The Home Office recommendation for recognition of a known person of 1.8 metres height, is that they should occupy 50 per cent of picture height. In this case total picture height would be 1.8 x 2 = 3.6 metres, with the picture width 4.8 metres. A one-third inch format camera has an imaging chip size of 4.8 mm x 3.6 mm. This means that the maximum distance, in metres, for recognition is the same number as the focal length, of the lens in millimetres. This does not take account of over-scanning of monitors.
F numbers
The F number (speed of the lens) can be critical in the surveying and design stage. The f-stop (aperture) or f/No determines the amount of light passing through the lens. The f-stop is not the one and only guide to light and sensitivity but is simply a mechanical ratio of the focal length of a lens to the effective diameter of the object lens. This decides the amount of light that could be passed to the sensor. However, there are many other factors that dictate the actual amount of light that is passed efficiently. Such as
l quality of the glass
l quality of the coatings on the glass elements
l the number of lens elements
l accuracy of the lens grinding
(A low cost lens with an apparent aperture of f1.4 may only pass the equivalent light of an f2 aperture).
If you were to buy a lens with a f1.4, that is fairly typical of a lens in use for the CCTV industry, you would only get 10pc of the available ambient light illuminating the CCD sensor. That’s pretty good really as even the most expensive spherical lens at say f1.0 will only enable 20pc of the available light through the lens and onto the CCD. Now zoom and varifocal lenses are something else. A typical zoom lens may operate from 12.5mm to 125mm (a 10:1 zoom) the specification may indicate that the f stop number is say f1.4. Remember that is taken when the camera lenses is zoomed to the wide angle, at the 12.5mm setting.
l When the lens is zoomed in the f stop increases
l When the f stop increases less light is allowed through.
So when your CCTV salesperson says the camera will work at full video at 0.01 LUX perhaps you could ask them if that is measurement is taken at the faceplate of the camera or the actual ambient light. The exposure in a normal photographic camera can be controlled by a combination of shutter speed and iris opening and film speed. CCTV cameras produce a frame (2 fields) every 1/25 second (mains frequency 50 Hz 1 field). Generally the exposure time is fixed and the amount of light passing to the imaging device is by adjusting the size of the iris. The sensitivity of the camera equates to film speed. One f-stop relates to a 50pc decrease or two times increase in the amount of light transmitted.
What is depth of field?
Imagine you are in a large estate with gardens stretching to 150m to the rear fence. During the day you can clearly view the fence and indeed everything else from you to the fence! When the light falls, maybe dusk, the fence becomes invisible and you can only focus on items close enough for your eyes to focus upon. The distance in sharp focus immediately in front of and behind the target is known as "the depth of field". This will change according to the amount of light available at the target and is the reason why the back focus must be set up correctly.
Format compatability
In the beginning we only had a choice of one lens, a one inch. When the tubes became available in the two-third inch size, manufacturers provided a two-third inch lens. CCDs became part of our life in circa 1986 and these chip sets began to become smaller and smaller. Now we have a vast selection of lenses, from one inch, one-third inch, two-thirds inch and quarter-inch. All the physical screwing on bits are identical, this means of course that all lenses fit all cameras. That’s the good news! The bad news is that if you use a smaller format lens on a larger format camera. For example purchase a new camera with a half-inch chip set and use a nice cheap one-third inch lens you will then have a ‘port hole’ effect. That is the top and bottom corners of the picture will be black.
Other bad news
The other bad news is that all the lens calculations go out of the window if you do not use the same format lens for the appropriate camera. That is for example if you use an existing one-inch lens, that is perfectly good, on a one-third camera the images will appear smaller than you would have expected from a proper match!
There is even more potential bad news! There are also C and CS mount lenses. The more modern lenses are mainly CS (that is, smaller length) than the C mount. Originally the standard sized lens was a one inch format using a C mount screw-in configuration. When screwed into the camera the distance between the lens and CCD device was 17.5mm. However there are now several lens formats available depending on the camera used and the field of view will be different for each one. Modern CS mount lenses when screwed into the camera have a distance of 12.5mm between the lens and CCD. By fitting a 5mm adapter ring to a C mount lens we increase the distance between the lens and CCD from 12.5mm to 17.5mm.
Pen test
In practice, as the CS mount is the more modern type: a C mount lens can be used with a CS mount camera providing a 5mm adapter ring is fitted. A CS mount lens cannot be used on a C mount camera. While it will fit physically, you may not be able to focus the camera. A good test is to put a finger or a pen close to the lens, if this comes into focus you probably have a wonderful mismatch of camera and lens. A lens designed for a larger format camera may be used on a smaller camera but a lens designed for a smaller format camera may not be used on a larger camera. So, great care must be taken when electing the camera and lens combinations.
Focal lengths
The focal length of a lens is usually stated in mm and determines the angle of view. Typically with a one-third inch format camera, the angles are as follows.
l A 4.00 mm lens will give a horizontal angle of view of around 62 degrees.
l An 8.00 mm lens will give a 30 degrees angle of view.
l A short focal length produces a wide angle of view.
Fixed focal length lenses are generally available in a range from 2.8-75 mm. They may be referred to as monofocal. It is not possible to alter the focal length unless the fixed lens is changed.
Auto-iris
Automatic iris lenses are to be used everywhere that the ambient lighting will vary. This is particularly relevant for external applications. These lenses are controlled by light input levels resulting in differences in amplitude so they adjust in relation to the light available. This is in direct reference to the composite video level. In theory the auto-iris lens controls the amount of light that reaches the camera in the same way that the pupil of the human eye contracts and dilates so as to optimise the vision in changing light levels.
Automatic iris lenses are now completely self-contained containing very sophisticated electronics and microscopic motors. As the lighting changes an influence is made on the reference voltage. The minute voltages are amplified and constantly adjust the iris to ensure that a 1 volt peak to peak remains constant. The auto-iris may be
l Direct drive, which uses the electronics within the camera.
l Video drive, which have an amplifier built into the lens and draw the low power needed to control the minuscule motors from the camera.
Spherical
Traditionally, spherical lenses were the only type there was. They are made from the equivalent of a slice from a sphere and providing we are happy to only use the centre part of the lens they are perfectly satisfactory. However, if you wished to use the entire area of the lens, in order to get the maximum light transmission, light coming through the edges of the lens would focus at a different point to light coming through the centre of the lens. This effect is called spherical aberration.
Aspherical lenses
Due to "modern technology" it is now possible to construct a lens of a particular shape, other than spherical, which overcomes these aberrations, and allows us to use a greater part of its surface area. This enables us to get another ‘f’ stop, or 50 per cent more light to the camera’s imaging chip. On the other hand, it could enable the lens manufacturer to make a physically smaller lens while still giving us an f/1.4 aperture and charging us more for it. Because these lenses do not have a spherical profile, they are called aspherical lenses. On a recent Tavcom bench test we found that aspherical lenses do do what they say they do on the box! Whether they have been used to miniaturise lens to fit domes and produce good results or to provide a much better light gathering solution in conventional camera housings, the manufacturers have come up trumps. In particular we were most impressed with the Infra Red focusing with many of the lenses.
Manual iris
The manual iris lens should only be used where the light levels remain fairly constant. The iris is manually adjusted by a control ring on the lens. Coarse adjustments are available ranging from wide open to fully closed. Similarly manual zoom lenses are adjusted by means of a knurled ring on the lens.
Colour corrected lenses
The overall quality of the CCTV image is also governed by the quality of glass and the accuracy to which the lens is manufactured as these all affect the ability of the lens to create a sharp image with good contrast. Astigmatism is caused by the curvature of the lens being different in the horizontal and vertical planes in relation to the incident light. It is more apparent in wide angle lens configurations and is much less if the iris aperture is reduced. A further characteristic which influences the quality of the image is whether the lens is colour corrected. Most quality manufacturers produce lenses which are compatible with colour use but there are other manufacturers that produce low cost lenses that do not perform well with colour cameras. The day-light we see is not white light as we imagine but made up of all the visible colour spectrum. Each colour is a different wavelength. Blue for example has a lower wavelength than red while green is right in the middle of the visible light spectrum. It is interesting that a forest of trees and pasture land with a multitude of various green shades is rather pleasing to the eye. Red in contrast has strikingly different shades, imagine scarlet and post office red side by side.
Zoom lenses
Are lenses with a variable focal length so that an object or person being viewed can be made to appear larger or smaller on monitor screen. Varifocal lenses do the same thing, but the important difference between zoom lenses and varifocal lenses is that a zoom lens is designed to remain in focus throughout its entire range without any further adjustment but a varifocal is not. The adjustments can be made manually, remotely or automatically. If a lens is to be adjusted other than manually it would be motorised, with the motors controlled by a telemetry system from a control panel or by means of some automatic triggering system. If the lens is being used in this way it would be fitted with pre-set potentiometers which provide a reference or a means of identifying where the zoom and focus functions are positioned within it’s range. A zoom lens is described by the ratio of its longest focal length to its shortest. For example, a lens with a longest focal length of 120 mm and a shortest focal length of 7.5 mm would be described as a 16 to 1 zoom lens, usually written as 16:1, ( = 16). Most zoom lenses, what ever their zoom ratio is, are designed to have a zoomed out horizontal viewing angle of about 45o, this allows a reasonable area to be observed, thus providing a number of possible scenes to zoom in to. However, if a lens designed for a 1/2" format camera is used on a 1/4" format camera the zoomed out horizontal viewing angle could be as little as 20o, which greatly restricts the choice of subjects to zoom in to. Most large lenses have a threaded mounting point that is the same size as the standard 1/4" camera mounting. This enables the lens to be mounted in the camera position and the camera screwed on to the back of the lens. The mounting point can then be adjusted to bring the camera to an upright position so that the picture is level. Varifocal lenses have to be refocused every time the lens is adjusted for a different scene position.
Keeping the picture in focus
Too many times we have been asked to comment on pictures defocusing when the light levels drop or when infra red lights are used. There is a simple setting up procedure that eliminates the need to go to site at night time to reset the focus and to ensure the focus remains excellent at all times. In common with other lenses, the depth of field increases when the iris is smaller (high f/No.) and reduces when the iris is bigger (low f/No.). So it follows that when the iris is at its widest, focusing is at its most critical and it is then that the lens/camera combination should be set-up. On an auto-iris lens, one way to ensure the iris is fully open, is to wait until it is dark then make the adjustments. A better way is to place a neutral density (ND) filter over the lens. This reduces the amount of light, simulating low light conditions, and the iris opens thereby minimising the depth of field and making focusing more accurate. If the camera is monochrome and IR sensitive it may be necessary to use an IR cut filter in addition to ND filters, as some ND filters are transparent to infrared. When the light level falls the lens auto iris opens and the depth of field is found to change. This is illustrated when cameras that are in focus during good lighting conditions tend to de-focus when the light level falls. The back focus is the mechanical adjustment of the position of the imaging device in a camera used to set the distance between the back of the lens and the pick-up. This must be correctly set up.
Back focus for a zoom lens
1) Point camera at a far object , say at about >30 metres
2) Set lens focus ring to mid point
3) Place ND (neutral density with Infra Red Cut) filter in front of lens to force the iris fully open. Note that a higher ND number filter will need to be used for monochrome as the cameras are generally more sensitive.)
4) Set lens zoom to widest angle
5) Adjust back focus control on camera to give the sharpest focus of image
6) Set lens zoom to narrowest angle (zoomed in).
7) Adjust lens focus to give the sharpest focus of image
8) Repeat steps 4 to 7 to ensure that the back focus is still in perfect focus
9) Remove ND filter and lock back focus control. Check and test with filter removed to ensure tracking remains in focus.
