IMAGE IS EVERYTHING
lens standpoint, says Jeff Gilman, president of Theia Technologies, based in Wilsonville, Ore. Lenses that are ideally suited for the highest-resolution cameras require more physical elements in their construc- tion, making for a larger lens — and a greater cost. So as megapixel resolution has grown in popu-
larity, other factors have come into play, as well. Failure to pay attention to those factors — even though some of them can be expensive — is a risky game for installers and integrators, Peterson says.
Quick Guide to Lens Selection
Lens size vs. chip size The format size (1/2-inch, 1/3-inch, etc.) of a lens has to be equal to or greater than the format size of the camera to avoid distortion at the edges of the image.
Mounts
Both C mounts and CS mounts look similar, and there is no physical measurement to determine which type of mount should be used for a particular lens. The only difference is the distance between the lens and the CCD image sensor (C mount = 17.5mm, CS mount = 12.5mm). Most new lenses are CS mount. While the mount and the lens should be the same, it’s possible to put a C-mount lens on a CS camera with an adapter ring.
Focal length
The shorter a lens’ focal length, the wider the field of view it provides. A wide view is ideal for covering a large area or for a situation where a close-up is necessary. Higher focal lengths provide a narrower field of view, making them ideal for areas where a limited view is desired (entrances, hallways, etc.).
Iris
The body of a manual iris includes a ring that is turned to alter the lens aperture directly. Manual is best for offices or convenience stores where there are fixed-lighting conditions or where continual lens adjustment is convenient. Conversely, auto-iris lenses are designed to control the light that is allowed into the camera by changing the iris automatically. They are best for outdoor or other locations with changing light conditions.
f-Stop
The f-stop of a lens indicates its ability to gather light depending on the aperture and focal length. The smaller the f-stop, the more light the lens can allow in.
Depth of field
Depth of field is higher when focal length is short, f-number is large or object distance is longer. The larger the depth of field, the larger the percentage of the field of view that is in focus. Shorter depths of view provide focus on only a small section of the field of view.
Lens calculators
If all else fails, most lens manufacturers provide a lens calculator on their websites. When you enter various information, the calculator determines which lens is appropriate, and provides an example of the quality, focus, field of view and more you can expect to get out of that lens.
80 November 2010
The bottom line is that there are many advantag- es to using megapixel cameras as long as you have the right lens for the job, which at its most basic means making sure to use a megapixel lens with a megapixel camera, says Cheryl Bard, product marketing manager, Bosch Security, Fairport, N.Y. The first and most important consideration when choosing the right lens is image quality, says Chuck Westfall, advisor, technical information, CIG Professional Engineering and Solutions Divi- sion, for Canon USA in Great Neck, N.Y. “This is security, so it’s crucial that you’re able to get an image that’s as clear as possible across the entire field of vision — not just at the center, but edge to edge,” he says. Andrea Iniguez, Theia Technologies’ vice presi- dent of business development, suggests installers and integrators work with their customers to under- stand the specific needs for each situation, and then find a lens that will deliver what that customer is looking for. “It depends on the level of surveillance you need in an image. Are you going to use it in a courtroom, or in a home or other lower-priority situation?” she asks. “We recognize that there are cases where a lower-quality lens is good enough.” Achieving the desired image resolution and clar- ity depends not only on the lens, but on the cam- era’s resolution, sensor, focal view and a number of other factors. That’s why Theia Technologies, as well as other lens manufacturers, offers an online lens calculator to help installers and integrators determine the best lens for a particular job. “A lens calculator is a great tool for providing a visual rep- resentation of what you can expect from different lenses,” she says. For example, in an enclosed space or a limited- mount space, Westfall says an ultra wide-angle lens is needed to provide more detail in the overall space you’re trying to image. In a longer-distance situation where there are no opportunities to mount a camera closer to the desired area, you need a lens that can zoom in on a tighter area without losing image quality.
“If you use the wrong lens in the wrong situation, your image isn’t usable for security,” he says.
DISTORTION Westfall says wide-angle lenses can be very ver- satile in that they have a number of potential uses. Because one wide-angle camera can provide the same coverage as several standard cameras, they can also help installers and customers cut costs. But that’s not to say they’re right for every situa-
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