What's the Big Deal About Lenses?

Camera lenses are perhaps the most overlooked component of any surveillance systems, but are without a doubt the most important. With incorrect lenses, the whole system is at minimum not performing to its full potential or capturing useable video footage. To help point you in the right direction, I’ll cover some of the important terms and features about lenses.

Lens Terms

Focal Length

The most frequently referenced optical term in CCTV is focal length. Focal length is measured in millimeters (mm), and is an indicator to the telephoto or magnification capability of a lens. As the focal length increases the field of view narrows. Conversely, as the size of the millimeters decreases the width of the field of view increases. As a benchmark that everyone can relate to, if you close just one eye, what you’re seeing through the other is approximately what a 4mm lenses will see.

Zoom Ratio

Zoom Lens

Zoom Lens

Zoom lenses are often described by the ratio of their longest to shortest focal lengths. For example, a lens with a 10-100mm focal length has a ratio of 10:1. To simplify information for the consumer market, this ratio is often expressed as a multiplier. The ratio of the lens in our example has a multiplier of 10×.

When selecting the proper lens for your application, it is important to keep in mind that both the focal length and the zoom ratio determine the telephoto capabilities of a lens. A 1-10 mm lens has the same zoom ratio as a 10-100mm zoom lens however the 10-100 mm zoom lens has more significant telephoto capabilities.

Consumers need to understand that there is a major difference between optical and digital zoom. Optical zoom is what the lens is capable of on its own, naturally without manipulation. Digital zoom is an artificial zoom where the size of the image is digitally enhanced to make the focal length appear greater.

Many manufacturers in the security, broadcast, and consumer industries will combine the digital and optical zooms to advertise a greater zoom ratio. It is important to remember that a camera with a 5x optical and 5x digital zoom is not the same as a camera with a 10x optical zoom. The camera with the combined optical and digital zooms will produce blurry images at 10x while the camera with the optical 10x zoom will produce clear, crisp images. When purchasing a lens for CCTV applications I suggest that you only take into account the optical zoom ratio for your application.

F-Stop

The f-stop of a lens is the lens focal length divided by the effective aperture diameter, or the diameter of the entrance pupil, of the lens. In essence, the f-stop measures the amount of light transmitted to the camera imager.

This is a very important number for low-light surveillance applications. The f-stop number is inversely proportional to the amount of light transmitted to the image sensor. So as the f-stop decreases, the amount of light delivered to the sensor increases. Lenses made for the security industry use a standard f-stop rating scale which corresponds to halving the light intensity of the previous stop. This is typically expressed as f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, etc…

Camera lux ratings are also impacted by the f-stop of a lens. Several camera manufacturers will advertise the lux rating at a specific f-stop. Selecting a lens with a different f-stop than that advertised than that with the lux rating will impact the ability of the imager to process images in poor lighting conditions. Be sure to select a lens with the correct f-stop to make the most of your application and its environment.

Electronic Shutter (Automatic)

An electronic shutter is a feature built into most video security cameras that acts like the auto iris feature in an auto iris lens. Electronic shutter is not a feature built into lenses. It works in conjunction with most standard CCTV lenses to compensate for very bright and very dark lighting conditions. The electronic shutter is a circuit inside the camera that adjusts the total amount of light exposed to the camera imager during video capture. It can be turned on and off, and is best used with applications requiring standard, non auto iris lenses.

Auto-Iris Lens

Auto-Iris Lens

Auto-Iris

Auto iris lenses operate in the same way as a camera’s electronic shutter. It adjusts the light exposure to the imager to maintain a consistent picture in varying lighting conditions. In many cases, auto-iris lenses respond faster and more accurately than the electronic shutter control built into a camera especially when exposed to extreme lighting conditions. Auto-iris lenses are best for any outdoor application or in applications with varying lighting conditions such as store fronts and doorways.

There are generally two types of auto-iris lenses, DC driven and video driven, available in the video security market. Both lenses perform the same functions with the same results.

DC driven auto iris lenses utilize coils to drive, or open and close, the iris, and are typically less expensive than a video driven lens. Video driven auto iris lenses operate via the camera output signal and can be controlled with the camera’s automatic light compensation (ALC) controls.

Many cameras work with a specific auto iris lens. When purchasing an auto-iris lens, be sure to select the correct auto iris lens that is compatible with the camera.

Megapixel

Megapixel cameras look and operate like most other security cameras except megapixel cameras have superior resolution to traditional analog CCTV cameras. Selecting the correct type and grade of lens is critical when using a megapixel camera.

Granted, almost any C or CS mount lens will screw onto a megapixel camera and will, for the most part, work however the resulting picture will not utilize the full benefits of megapixel video.

When purchasing a megapixel lenses, look for a resolution rating that is typically expressed as MP (megapixel) that is equal to or higher than the camera’s megapixel rating.

Types of Lenses

C and CS Mount

C and CS-Mount camera lenses are nearly identical – they have the same mounting thread count and outer diameter. However, C-mount lenses are made for cameras using a 1/2” imager while CS-mount lenses are typically used for cameras with a 1/3” imager or smaller.

In today’s CCTV industry, most cameras manufactured with a 1/3” imager. However, these cameras are backwards compatible and will accept a C-mount lens when used with a C to CS adapter ring.

When using a C-mount lens on a CS-mount camera, the focal length actually increases. For instance, if a 12mm 1/2” C-mount lens is used with a 1/3” CS-mount camera, the focal length increases by 1.4 to 16.8mm (focal length x 1.4).

Fixed Focal with Fixed Iris

Fixed Focal Length Lens with Fixed Iris

Fixed Focal Length Lens with Fixed Iris

Fixed focal and fixed iris camera lenses typically are “economy lenses” because they are low in quality, price and capabilities. These camera lenses are made with a single focal length and cannot be adjusted for the field of view or for lighting conditions. I rarely recommend one of these lenses – only when price is a significant issue.

Fixed Focal with Manual Iris

Like fixed focal length lenses with fixed irises, fixed focal length lenses with a manual iris are becoming obsolete due to advances in security camera technology. Fixed focal length lenses with a manual iris have a higher quality than their fixed focal length/fixed iris counter parts and are a good alternative when needing quality while sticking to a budget.

Fixed Focal with Auto Iris

Like the previous two lenses, fixed focal lenses with an auto iris do not have zoom capabilities; however they have the enhanced features of an auto-iris lens. On average, auto-iris lenses outperform the camera’s built-in electronic shutter in varying and extreme lighting conditions.

Varifocal

Variable focal length lenses, or varifocal lenses, come in either a manual or auto-iris format. Varifocal lenses are the number one choice of today’s security professionals because they remove the need to perform field of view calculations prior to purchase, and are more adaptable to varying field of view requirements and applications.

These lenses are available in a wide range of focal lengths, from the popular 2 mm – 12 mm, which covers most small to medium sized areas, up to 500 mm – 11,000 mm lenses for specialized law enforcement and military applications.

Zoom

Zoom lenses should not be confused with varifocal or motorized lenses. A true zoom lens has the mechanical and optical capabilities to zoom from one extreme to the other with very little need to manually adjust the focus.

Zoom lenses are rarely used in the security industry due to their price and that many cameras come equipped with inexpensive electronic components that automatically focus the lens as the user zooms in and out.

Motorized Varifocal

Motorized varifocal lenses are essentially varifocal lenses with a DC motor assigned to each of the lens functions – zoom, focus and iris. Control of these functions is accomplished with a lens controller that supplies each individual motor with 12VDC.

Today, these lenses are typically only used in specialized, long range surveillance applications such as border security and military surveillance. A PTZ camera is more efficient, easy to use and less expensive option for most common security applications.

Doubler

A doubler is used in conjunction with lenses and cameras. They essentially double the focal length of any given lens. Doublers are most often used in surveillance kits where the application distance varies and is generally unknown. A doubler is also used in applications where the proper lens focal length is not available.

C and CS-Mount Pinhole Lenses

C and CS mount pinhole lenses are also an aging technology due to the reduction of size in micro video cameras and board cameras. The zoom feature still makes C and CS-mount pinhole lenses a viable choice when selecting lenses and cameras for covert surveillance.

In applications where covert surveillance is fundamental, but is impossible to get a smaller covert camera close enough, a C or CS-mount pinhole lens is the perfect solution. Some of the lenses available have a 4 to 20mm or a 5 to 50mm zoom capability. Either lens makes watching a keyboard or cash register from afar easy without risk of detection.

The two downsides to these types of lenses are size and expense. C and CS mount pinhole lenses can range from 3 – 12 inches in length which limits where they can be concealed; usually this means using an adjacent room, drop ceiling or another larger object to conceal the camera and lens. Also, they range from $100 to $800 in cost due to the limited and specialized manufacturing of these lenses.

Micro Video Lens

Micro Video Lens

Micro Lenses for Micro Video and Board Cameras

Technically known as M12 lenses, micro lenses are used with micro video cameras, board cameras and some bullet cameras. The small form factors of these cameras and micro lenses make an excellent choice for normal surveillance needs, scientific applications and even medium range covert application.

Pinhole lenses aren’t the only lenses for covert applications. Keep in mind that at its largest, a micro video lens is a half inch in diameter. At twenty feet or more, these lenses will appear as just a dot and can be easily concealed.

Micro lenses are available in a variety of sizes. Normally they can be found in focal lengths ranging from 2.5mm, great for monitoring wider scenes to 25 mm for a more telephoto view.

Hopefully this blog has been educational and helped you in selecting the right lens for your application. If you need additional assistance in determining the best lens for your project, please feel free to send me an email.

Jake Lahmann is an expert in video security technology. He is widely known for his industry-leading views on the use of covert camera technology and has written articles for publications as diverse as Security Product News and Law Enforcement Quarterly. Jake spent 6 years in federal law enforcement, including 2 years drug enforcement in San Diego.