Chapter 3.2: Lenses - CCTV Technology Fundamentals

Posted by Arowosegbe Olawale in General

The CCTV camera's lens is the first link in the imaging chain, which also includes the camera, transmission system, image management and analysis software, and the monitor. The lens directs visible or infrared light/energy to the camera's detector. The purpose of a lens is to provide a clear, sharp, and focused image to the camera's sensor. In order to get the best results from an imaging system, it needs lenses specifically designed to do so. The lens is the most critical part of the imaging chain, and it cannot be replaced if it fails.  

The focal length, field of view, image sensor size, and lighting conditions are all factors to think about when choosing a lens. The focal length, in millimeters, the maximum aperture, in f-numbers, and the size of the image sensor that the lens was designed for all help to uniquely identify a lens.

3.2.1 Types of Lenses

There are three primary varieties of lenses: those with a fixed focal length, those with a variable focal length, and those with a zoom function. A lens' focal length is defined as the distance from its optical center to the film or sensor that serves as its image. The camera's field of view (FOV) is established by the focal length of the lens and the resolution of the image sensor.

3.2.1.1 Fixed Focal Length Lenses

Lenses with a fixed focal length have a predetermined focal length that cannot be altered. When the camera won't be moving and the scope of the surveillance doesn't shift, these lenses come in handy.

3.2.1.2 Varifocal Lenses

Varifocal lenses allow for a range of focal length adjustments within a fixed range, but each adjustment must be made manually in camera. In addition, the iris and focus may require fine-tuning whenever the focal length is altered. In comparison to standard zoom lenses, varifocal lenses are more affordable and provide greater freedom to change the focus point depending on the subject matter of the scene. Once the f-stop and iris are set, the camera will keep the same field of view. The focal length range, the aperture range, and the image sensor size of a varifocal lens are the distinguishing characteristics.

3.2.1.3 Zoom Lenses

The zoom lens's focus setting is intended to be permanent, unlike that of the varifocal lens. In the CCTV industry, zoom lenses are frequently manufactured with internal motors that allow for focal length adjustment via remote control. Cameras with the ability to pan and tilt use these to keep an eye on a wide area. The focal length range, aperture range, and supported imaging sensor size all serve as unique identifiers for zoom lenses.

3.2.1.4 Optical Versus Digital Zoom Ranges

Optical focal lengths, which are a function of the lens's components, are indicated by the focal length ranges for varifocal and zoom lenses. Two common ways of describing the zoom range are by focal length range, as in "6 to 24 mm," and by zoom factor, as in "4x" (four times). This means that the zoom factor is 4x for the aforementioned 6–24 mm focal length range (6 x 4 = 24). It's important to remember that the zoom factor indicates only the focal length range and not the actual image magnification. It is the combination of the image sensor's resolution and the lens's focal length that determines the final image's magnification. With the longer focal lengths of zoom lenses, security personnel can magnify and inspect a specific area of the image for greater detail or to make an identification.

The ability of the camera or the processing software in a closed-circuit television system to select and enlarge portions of the full image is what is meant by the term "digital zoom" or "electronic zoom." Here, only the clicked-on pixels are blown up. Instead of improving on an image's detail, digital zoom diminishes it. Magnification factors of 2x, 6x, etc. are commonly used to describe the zoom capabilities of digital cameras. 

3.2.2 Features of Lenses

The scope of a lens's application is limited by its construction and individual features. The focal length, aperture and focus type, wavelength of light or energy, and image sensor size are all factors in a lens' performance. Also important to the success of a CCTV system are the cameras' resolution and the effect of noise on the captured image.

3.2.2.1 Focal Length and Imager Format

The angle from which the lens accepts light to focus on the image sensor is set by the focal length and the size of the image sensor. The image sharpness on various sized sensors can be optimized by using lenses of the same focal length. Image sensors in surveillance cameras typically have dimensions of 1/4, 1/3, 1/2, 2/3, or 1 inch. The diagonal size of the image sensor is what these numbers refer to. Focal lengths are measured in millimeters, while image sensor formats are denoted in inches. 

So that the image formed on the image sensor makes the most of the available pixels, a lens should be chosen to match the format of the image sensor. In the camera industry, lenses are typically marketed for use with image sensors of a certain size. The field of view of a camera depends on the focal length of the lens and the resolution of the image sensor.

Lenses with the specified focal lengths for various camera image sensors are listed in Table 3-1.

Table 3-1. Standard Lenses for Image Sensor Size

When the focal length of a lens is longer than the standard lens, we call it a telephoto lens, and when it is shorter, we call it a wide-angle lens. When used on a small-format camera, a lens with the same focal length will create a telephoto-like image, but when used on a large-format camera, the lens will create a wider image. The following applications all suit a lens of 12 millimeters in diameter:

• Camera with a standard 1/2-inch image format and quality output;
• A camera with a 1/3-inch sensor and telephoto optics;
• The camera has a 2/3-inch image format and can take panoramic pictures.

It's possible to use a lens designed for a large-format camera on a small-format camera, but not vice versa; the image will become darker toward the edges and corners otherwise. A lens made for a 1/2-inch sensor, for instance, will also work on a camera with a 1/3-inch sensor, but not vice versa. In order to get the best results from your camera, it's important to use a lens that's optimized for your camera's image sensor.

3.2.2.2 Field of View

FOV can be calculated using the focal length and the size of the image sensor. The field of view (FOV) is the swath of scenery captured by the camera. A lens calculator is typically used to determine this region.

The formulas for determining the FOV's horizontal and vertical dimensions are provided in Table 3-2. The field of view is determined by the focal length of the lens, the distance from the camera to the subject, and the imager format of the camera.

Table 3-2. Calculating the Horizontal and Vertical FOV

 

A handheld, wheel-style lens calculator is a common tool in the CCTV industry. A quick and easy way to get the dimensions of a place. CCTV vendors commonly provide wheel-style lens calculators on their websites for free. There are also numerous lens calculators available as software and on the web. They start with very basic calculators that are functionally equivalent to handheld calculators and progress to more complex items that offer advanced features like additional illustrations and simulated camera views.

3.2.2.3 Iris and Aperture

The term "aperture" refers to the slit-like opening in a lens that lets light in. Within lenses that allow for aperture control is a multi-leaved mechanism known as an iris diaphragm. Adjusting the aperture affects the size of the iris. The iris controls how much light enters the lens and reaches the camera's image sensor.

The iris controls how much light enters the camera through the aperture, allowing for the best possible image. To see images in their best light, this is essential. The image will look washed out if there is too much light falling on the image sensor. If there isn't enough light, only the brightest objects in the field of view will be discernible.

A lens' aperture setting is indicated by a number that is the ratio of the lens' focal length to its aperture size. An f-number is the result of the following equation:

f-number = focal length / effective aperture

The focal length of a lens is expressed as a fraction of the diameter of the aperture opening, so that an f/2 lens has a focal length two times as long as the aperture.

One key distinction between lenses is whether or not the iris is fixed or can be adjusted manually or automatically. In situations with consistent lighting, a lens with a fixed aperture may prove useful. Manually adjustable aperture lenses are best used in static viewing situations with consistent lighting. Manually adjustable lenses are more affordable than automatically adjustable lenses, but they must be adjusted by a technician. Lenses that are motorized let the user adjust the iris, focus, and zoom from a distance.

Auto-iris lenses, so named because of their electronically controlled apertures, are a popular type of auto-focus lens. These lenses feature a solenoid that can be activated by either the camera's video signal or direct current. They work best in areas with a lot of natural light or dramatic changes in lighting.

One way to measure a lens's ability to perform in low-light situations is to look at its aperture, which is measured in f-stops. If the f-stop number is low, the aperture is large. Manual lenses typically have an f-stop of either 1.4, 2, 4, 5.6, 8, 11, 16, or 22. When set to f/4, twice as much light enters the camera as when set to f/5.6. Because they allow more light to reach the sensor than lenses with smaller apertures (such as f/8.0 or f/11), lenses with large apertures (such as f/1.0, f/1.2, and f/1.4) are often called "fast" lenses. Greater f-stop numbers indicate a slower lens.

Depth of field (the distance in front of and behind an object that appears to be in focus) is also affected by the aperture settings. As the f-stop number increases, the depth of field also grows.

3.2.2.4 Back Focus

Focusing a lens so that the image it transmits is precisely in the center of the image sensor is essential for effective photography. Back focusing is the term for making this adjustment to the camera. The distance between the camera's imager and the lens mount can be adjusted in a number of ways. The image sensor can be panned left and right in most cameras. The image sensor can be moved in some by turning a wheel, while in others, a screw is used to activate an internal mechanism. A black focus adjustment is also included in some lenses. For the most part, zoom lenses suffer from the problem of back focus.

Figure 3-2. Focus Chart

When back focusing with a zoom or fixed lens, the largest aperture should be used, and the longest focal length setting should be used if available. A gray neutral density filter or low-light conditions are recommended for using the lens's maximum aperture. Using a portable monitor and a detailed, stationary object in the scene, a seasoned technician can often zero in on the camera's focus. The Moiré effect, or central blurriness when moving, can be reduced if, for instance, technicians insert a focus chart into the scene and adjust the camera's focus accordingly.

3.2.2.5 Image Quality and CCTV Lenses

Distortion is one of several factors that affect how light travels through the lens and forms an image on the sensor. Barrel distortion and pincushion distortion are the most common forms of distortion in photographs. In general, wide-angle lenses and a wide-angle zoom lens are associated with barrel distortion. Images with this effect will have rounded corners, as if they were wrapped around a sphere.

This effect, known as pincushion distortion, causes the images to look pinched in the middle and is most commonly seen with zoom telephotos. The further away an object is from the lens' optical axis, the greater the distortion. This type of distortion, known as pincushion distortion, occurs when the lens instead of the image is stretched.

All objects within the field of view appear distorted, as do their spatial relationships. Maintaining accurate perceptions of space and objects requires a level of distortion that is minimal. Although distortion is not the only criterion for quality, high-quality CCTV lenses have low distortion values.

3.2.2.6 Resolution

How well a camera can make out finer distinctions in an image is measured by its "resolution." A camera's horizontal resolution can be measured with a resolution chart. Multiple nearly parallel lines make up the resolution chart, with the lines getting thinner and closer together as one descends to the chart's base. The horizontal resolution is determined by the camera's ability to distinguish between lines as it pans from top to bottom, and this is represented by the horizontal resolution scale. The vertical resolution of a TV is limited by the number of scan lines used to create the image, and anything beyond that is typically ignored because it is beyond the visual capabilities of the human eye.

3.2.2.7 Signal-to-Noise Ratio

Noise in electronic systems is a natural byproduct of any circuit. Since amplifiers and processing components can both amplify and introduce new errors to a signal, the signal-to-noise ratio (SNR) becomes increasingly important whenever amplification or processing is required. A video signal with noise will appear grainy or a snowy white on the screen. When a camera is pushed to its limits, such as when shooting in low light or dusk with a daylight color camera, noise in the video signal becomes more noticeable.

Higher SNR values are preferable to lower SNR values, and SNR values are typically expressed in decibels. The camera and the system are not the only possible sources of noise. Electronic noise is difficult and expensive to remove once it has entered a video system.

3.2.2.8 Color Corrected and Monochrome Lenses

The lens you use must be compatible with the camera you buy. Color-corrected lenses are engineered to bring all wavelengths of light into focus on the same image plane, resulting in sharp, high-quality photos taken with both color and daylight monochrome cameras. Nowadays, color-corrected lenses are by far the most popular type of lens sold.

When used with low-end monochrome cameras, monochrome lenses can produce a passable image, but they should never be attached to a color camera. The best lenses for high-end monochrome cameras, which process reflected infrared light, are optimized for use in that spectrum. 

3.2.2.9 Lenses with Aspherical Elements

Lenses are constructed with spherical pieces of glass or plastic. Lens designers now have more leeway in adjusting the lens's light transmission thanks to advances in computerized lens production technologies that have made surface manufacture easier and cheaper. To prevent spherical aberration, aspherical lenses were developed. Light entering the lens from the center of a spherical optical element will converge at a different point than light entering from the outside, a phenomenon known as spherical aberration. A modified curved lens containing aspheric elements can rectify the blurring.

CCTV lenses with wide apertures that still produce usable images at full aperture can be designed by engineers. Aspherical lenses may be more practical than conventional ones for producing images with large depths of field (f/1.0 or faster). High-quality zoom lenses covering wide focal ranges may use aspherical elements due to the complexity of their design and engineering. To top it all off, aspherical lenses can mimic the performance of spherical lenses despite being smaller or having fewer elements.

It is debatable whether or not the image quality of lenses with aspherical elements improves over that of lenses made entirely of spherical elements. In some cases, the performance of a lens with spherical elements may be superior to that of a lens with aspherical elements because of the superior design and construction of the former. There is no guarantee that an aspherical lens will perform better than a lens made entirely of spherical elements.

3.2.2.10 Filters

Filters modify the light intensity or color spectrum reaching the camera's photosensitive chip. They typically attach to the front of a lens via a screw, though some are built right into the lens and can be rotated in and out of the optical path. Filters with neutral density (ND), polarization, and infrared (IR) cutoffs see widespread use in the CCTV industry. 

Neutral density (ND): In situations where there is too much light for the camera's image sensor, a neutral density (ND) filter can be used to reduce the amount of light entering the lens. Light reduction can be fine-tuned with ND filters, which come in a range of densities. Normal practice calls for attaching ND filters to the lens's front element.

Polarizing filters direct the light entering the lens in a specific direction. Reflected light and glare from shiny surfaces like water or glass can be significantly mitigated with the use of such filters. A polarizing filter can be attached in front of a lens and turned to reduce the amount of glare and boost the contrast.

 

IR-cut: Most of the NIR light energy that would otherwise reach the image sensor is filtered out or blocked by using an IR-cut filter. The majority of modern image sensors are extremely sensitive to near-infrared light. If the image sensor weren't protected by the filter, it would be exposed to IR radiation from a wide variety of daytime sources, including the sun, resulting in an image with garish, inexplicable colors and low contrast. The IR-cut filters used in color day/night cameras are typically housed in a rotating filter ring within the camera, which is used to direct NIR light onto the image sensor when it is required.