“The lens is the second most important choice in a security system after the camera,” asserts John Donaldson, president, Summit Sales and Marketing Ltd., Davenport, Iowa. “It’s the most difficult choice for dealers because their customers often have trouble describing exactly what they want to see.”

Here’s a checklist of considerations that can help ensure you buy the right lenses for the range of your customers’ needs.

Lens size

Buy lens sizes based on camera formats you install the most, and on the fields of view and distances to objects customers frequently request. Make sure the lens is the same size, or larger than the camera format. (Format size = image size.) If you use a smaller lens, such as a 1/3-inch lens on a half-inch camera, you will get a black ring or “porthole” around the image. The wider the view desired, the smaller the lens size (millimeter number) required; the longer the distance to the object, the larger the millimeter number.

“Dealers can determine correct lens sizes by asking customers three questions,” suggests Steve Beaulieu, national sales manager, Tamron USA, Commack, N.Y.

- What is the lighting in the installation area (indoor and outdoor)?

- Do you just want to see activity in an area or to specifically identify someone or something?

- How far away will the person/object to be identified be?

Fixed or varifocal lens

Fixed focal lenses have a single millimeter number and can be slightly less expensive than varifocal lenses, which have a range of settings, such as 2.6 – 6 mm (wide angle), 3.5 – 8 mm (medium angle), 5 – 50 mm (long distance). The 3.5 - 8 mm lens is widely used because it combines the two most popular fixed focal lengths.

Jesse Abbott, president of Advance Technology Inc., in Scarborough, Maine, stocks up to 200 fixed-focal lenses in sizes ranging from the widest to the narrowest. “We keep various size lenses on our trucks,” he says, “so installers can easily change lenses based on what customers want.”

“Even though they cost a little more, we use varifocal lenses in all installations except those where the desired object is a long distance away,” states Paul Kusnierz, national installation manager, Fire Protection Service, Ogden, Utah. “Customers may choose one focal length in the planning process, but inevitably want a wider or narrower view at installation. Varifocal lenses give our installers the flexibility to fine-tune the view quickly and easily. They also reduce the number of lenses we need to inventory.”

f-stop

The camera lens has one key function: collect light reflected from a scene or object and focus it onto an image in the camera. In the often low-light locations of security cameras, light is a critical issue. The more light collected, the better the image. How much light passes through the lens is a function of its f-stop, which is expressed as a number. Dial down one f-stop on a lens, from 2 to 1.4 for example, and gain 100 percent more light. Go the other way, and there’s a 50 percent light loss. The smaller the f-stop number, the more light the lens transmits to the camera. The once-smallest f-stop, 1.6, has been surpassed by the 1.4, 1.2, and even 1.0 f-stop lenses available today.

Aspherical or standard

Aspherical lenses have highly polished, computer-designed convex surfaces that let in more light than standard lenses and hold image focus better from center to edge. This high-precision technology is becoming the standard because it is smaller in size, can correct for color aberration and lowers the lens f-stop number for increased light transmission. “Because the curvature in aspherical lenses gathers more light, we recommend them for cameras that need to boost their low-light capabilities,” comments Erron Spalsbury, regional sales manager, Electronics Line USA, Boulder, Colo. “Aspherical lenses are higher end, but not necessarily higher price.”

Manual iris or auto-iris

The iris in a lens is like the iris in the human eye; it opens and closes in response to light. Irises in manual-iris lenses are set to one fixed f-stop opening. These lenses are best suited to applications in which lighting is relatively constant, such as rooms without windows.

Auto-iris lenses have an electronically controlled iris that opens and closes as the light changes to maintain a constant video level image. There are two types of auto-iris lenses.

1. Video lenses have a built-in computer that monitors the camera’s signal and opens or closes the iris based on what the camera is seeing. These lenses may be required if you are retrofitting older cameras. Connectors must be soldered to pins according to the pin layout of each camera.

2. DC lenses, electrically connected to the camera, depend on the camera to tell it how to adjust the iris. All cameras today can support “plug-and-play” DC lenses, which are less expensive than video lenses, and some cameras can support both video and DC lenses.

Auto-iris lenses cost more than those with manual irises, so the key is to use them only where necessary. Built-in electronic shutters automatically adjust the light-gathering ability of cameras today – and do it very well in many applications. Auto-iris lenses do the same thing, but to a greater extent. Thus, they can be an appropriate supplement to cameras in areas with lighting extremes, such as bright sun on snow or water.

Visible or infrared light

Infrared (IR) light travels through lens optics differently than visible light, which can lead to focus point shifts and fuzzy images. IR lenses focus both types of light on the same focal plane for sharp images. These lenses can be particularly useful on day/night cameras and black-and-white cameras sensitive to both visible and near IR light.

Coated optics

Most quality lenses have up to seven coating layers to minimize flare and glare, particularly in outdoor applications. Coated lenses are recommended for both black-and-white and color cameras, but are especially important for optimal color balance of lenses. “Manufacturers use a variety of coatings that can react differently to different cameras,” advises Gary Perlin, vice president of video products, Speco Technologies, Amityville, N.Y., “so testing lenses on your cameras before installation can help avoid problems.”

C or CS mount

C mounts have been around longer, but CS mounts, introduced about 10 or more years ago as equipment became smaller and lighter, are now the standard for full-body CCTV camera lenses. All new cameras use CS lenses. Older C lenses still can be mounted on CS cameras using an adapter or a switch on the camera that mechanically moves the CCD in and out. If a CS lens is put on a C-mount camera, the image will be out of focus.

Manufacturer relationship

"We want to put in a lens and not have problems with it,” explains Tom Nealer, director of technology and training, Tech Systems, Duluth, Ga. “One service call to replace a lens that doesn’t work, or that is wrong for the camera or the application, can wipe out the profit margin on that lens.”

Jeffrey Slatken, senior installation manager, Mutual Central Alarm Services, New York, says his company sticks with just a few lens manufacturers for several reasons. One is cost. “Buying in bulk lowers the price,” he notes. “Plus, buying lenses from many different manufacturers is hard on installers who have to learn each brand. Jobs take longer and there is more room for error.”

Sidebar

Free Online Lens Calculator

Use this tool to quickly determine the right lens for your application. Go to www.specotech.com, click “Reference” and then “CCTV Camera Lens Calculator.” Enter the distance to object, camera format size and focal length of lens to learn the field of view. Or enter field of view, distance to object and camera format size to see the focal length lens you need to get that view.