A CCTV lens is a camera’s window on the world. Simon Lambert explains what you need to know about CCTV lenses.
The old advice that “a chain is as strong as its weakest link” is certainly applicable to your CCTV. That’s why you must take care at every point in your system. These small, hidden, unsexy devices are often where corners are cut. People fail to appreciate how poor choices let them down. It isn’t difficult to make good choices.
Every lens should create a clear image of the scene for the camera to turn into video. How clear is “clear”? Your Operational Requirement details the job your CCTV must satisfactorily perform. Each lens must be a strong enough link in your CCTV chain to make sure it can ‘pull its weight’, so let’s understand your choices.
To begin, how can you fit any lens to any camera? ‘C’ and ‘CS’ mount devices all have the same screw thread. Some older or larger lenses are C-mount so will fit on both C and CS-mount camera bodies (with a 5mm adaptor ring).
Most recent small lenses are CS-mount so are fine on CS camera bodies but will never achieve focus if used on a C-mount camera. Be careful.
Basic lens set-up
The simplest CCTV lens is a piece of glass (or plastic) to focus the light from the outside world onto the camera’s imaging sensor.
This lens’ only adjustable mechanism is its focus-ring, part of the barrel body, which you turn until your image is as clear as possible. Incidentally, use the sharpest test monitor you can. For instance, black & white tubed monitors can be much sharper than colour LCDs. The amount of light let through to the camera cannot be controlled by this simple lens.
Upgrading to a lens with a manually adjustable ‘iris’ is only a benefit during set-up, so both are best suited to scenes where lighting is constant. To a limited degree, an automatically adjusting electronic shutter in the camera might accommodate changing brightness. However, in scenes where it varies significantly, such ‘electronic iris’ features are better replaced by an ‘auto-iris’ lens.
Automatic iris lenses are controlled by the camera to make sure that the right amount of light gets through to the imaging chip.
Too little and the iris opens up. Too much and the iris shrinks.
During CCTV set-up this lens/camera automatic level control (ALC) needs tweaking so as to aim for the desired brightness. The smallest auto-iris lenses have few electronics inside because the driving amplifier is in the camera.
These ‘direct drive’ (DD) lenses have a built-in cable usually carrying a widely-used square 4-pin connector plug which connects to the camera. The video level adjustment is done by twiddling a rotary ‘pot’ (potentiometer) on the camera’s body or using its internal software menus.
More sophisticated auto-iris (AI) lenses contain their own amplifier which provides two pots. One is for adjusting video level, as described above. The other is called ‘peak/average’ and when turned fully to ‘peak’ it considers the brightest part of the scene when controlling the iris. When turned fully to ‘average’ it considers the whole scene’s brightness when controlling the iris.
This latter case is most common in normal use, but the peak function can be vital where brightest areas must retain details, even at the expense of shadow details, especially under night time lighting.
Traditionally, the heavier auto-iris lenses contained servo motors to drive the iris mechanism, especially inside large zoom lenses. Small direct drive lenses use lightweight ‘galvometric’ electromagnetic actuatorsfor the iris, as do some AI lenses now.
Lens speed – the F-stop
A large iris delivers more light into the camera, obviously. Rather unhelpfully, however, the more ‘telephoto’ the lens the less light will get through. So how do we quantify the practical effect of these contradictory factors?
Every lens has an ‘F-stop’ number describing its maximum capability. For instance, F1.0 can pass four times as much light than an F2.0 lens. Half the F-number, quadruple the light!
Smaller F-numbers are better, which is particularly important for low-light CCTV performance. Wide-angle lenses commonly range from F0.8 to F1.8 while zoom lenses often range from F3.5 upwards, which means much less light gets into the camera.
Be aware that such a zoom lens claiming F3.5 will give this at its wide-angle position, but when fully zoomed it could drop to F8.0! This is called ramping and zoom lenses that minimise it cost a lot more money than budget lenses that only look good on paper.
Better quality optical elements will give you better contrast in your images, and fewer colour fringes around object’s edges (chromatic aberration).
With better resolution tiny details will be clearer and brighter, especially at the edges of the image where all lenses have imperfections, and straight lines will show less curvature (geometric distortion).
‘Aspherical’ optics are particularly finely crafted to maximize light transmission, hence impressive F0.8 capabilities.
Also, infrared in daylight and electric light can focus in a different place to visible light thereby defocusing your image. IR-corrected lenses reduce this effect.
A poor lens on a good camera is a waste of a good camera.
Video surveillance resources
Field of view
The field-of-view (FoV) your lens provides is fundamental to your meeting the Operational Requirement. The focal length of any lens is measured in millimetres (mm).
In CCTV where a camera’s chip can be, for instance, 1/3-inch format, a ‘wide-angle’ 2.8mm lens offers a horizontal field-of-view around 74 degrees; a ‘standard’ 8mm lens offers around 30 degrees, whereas a ‘telephoto’ lens of 50mm narrows the FoV down to less than 5 degrees.
The shorter the focal length, the wider the view. Megapixel cameras may have 1-inch format sensors meaning that large professional photographic lenses are used.
Fixed focal length lenses, known as ‘prime’, have no FoV adjustment but can perform very well owing to their simplicity.
Adjustable ‘fixed’ lenses
Adjustable ‘fixed’ lenses
The practicalities of CCTV installation can seek adjustable ‘fixed’ lenses. These are called varifocal and offer, for instance, a range of 10mm to 40mm which is manually adjusted during installation prior to focusing.
The downside to this versatility is often lower optical quality than a correctly selected prime lens. Motorising a zoom lens adds more remote control versatility to its applications.
Bigger lenses in large motorised zooms make them heavy, but greatly improve low-light performance. Incidentally, if you intend to drive such lenses to pre-programmed zoom and focus positions, e.g. in response to an alarm trigger, then make sure the servo mechanisms are built in.
Zoom lenses are often advertised by their zoom ratio, e.g. 30:1, which seems impressive but actually tells us very little of practical use. Its shortest and longest focal lengths are what we need to know to make sure our desired FoVs will be achievable.
Setting up requires the right tools and understanding, particularly the camera’s back-focus adjustment for correct zoom-tracking which is necessary for your image to stay in focus while zooming from maximum telephoto to wide-angle.
So many don’t owing to poor work by CCTV technicians, so should be checked.
Lenses are impressive feats of technology. Many modern lenses can perform amazingly well. Poor lenses are, however, weak links that undermine the strength of your CCTV. The price of good lenses is tiny when compared with their value in your system.