When it comes to lenses, talk of quality centers on two things: image quality and the quality of the lens itself. A low-quality lens will produce a low-quality image, which in many security applications is just not acceptable.
WHICH LENS IS THE RIGHT LENS?
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However, as in most, if not all, areas of the industry in general, some buzzwords in the lens space are IP and megapixels. As cameras’ megapixel counts continue to climb, those higher-resolution images become a challenge from a 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 construction, making for a larger lens — and a greater cost.
So as megapixel resolution has grown in popularity, 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.
The bottom line is that there are many advantages 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 Division, 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 president of business development, suggests installers and integrators work with their customers to understand 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 clarity depends not only on the lens, but on the camera’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 representation 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 versatile 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 situation. “Often a wide-angle straight lens is rounded out because of the distortion of the lens,” he says.
Until recently, the only option for a wide-angle camera was a fisheye lens. So these lenses were used almost exclusively despite their tendency to lose resolution at the edges of the field of view, creating a distorted image. The problem arises in attempting to provide a two-dimensional representation of the three-dimensional world around us. Think of the difference between a globe and a map of the world. On a globe, continents appear in the correct proportion based on their position on the Earth and their relative sizes compared with other continents. Depending on the type of projection that’s used to translate that 3D sphere into a 2D map, some countries and continents appear distorted and out of proportion.
In a rounded representation, objects located at either pole are squished and it may be difficult to read any print that appears on them. This is the same kind of effect a fisheye lens produces. With a fisheye lens, the farther an object is from the center of an image, the more narrow (distorted) it becomes. This can be overcome with software, Peterson says, but because of the required time and processing power, that may not be the best option. Additionally, he says, those objects at the edges of the image already have been compressed and the information lost as it traveled through the lens.
Another possible solution is to use a newer kind of lens called a rectilinear lens. Revisiting the globe-to-map example from above, the effect produced by a rectilinear lens is the same as a world map on which objects at the poles appear proportionally larger than they should be. Think of the monstrous size of Antarctica and Greenland on some maps and you’ll have an idea of this.
However, unlike on a map, enlarging objects at the edges of an image is actually a good thing, Peterson says. The 3D “stretching” created by the lens creates increased resolution at the edges of the image, making it more valuable from a security standpoint.
Unfortunately, says Glenn Wolk, president of Tekstar Optical, based in Kings Park, N.Y., distortion isn’t just limited to wide-angle lenses. Aside from pairing the wrong type of lens with the wrong type of camera, Wolk points to light levels as one of the main culprits.
According to Wolk, a lot of installers or integrators don’t think about the variation of the camera’s aperture across the full range of the lens. When they look at the f-number (which compares the length of the lens to its diameter), they look for the lowest number — assuming, naturally, that a lower f-number indicates a faster aperture and brighter light. However, he cautions, that’s not always the case because that f-number — and hence the lens’ light quality — is achieved only at the widest angle.
So if you think you can judge a lens’ performance by looking at the spec sheet, think again. The variation in performance across the entire lens — and the way it’s reported — has become somewhat of a frustration for many in the industry, Bard says. “A lot of times, it’s hard to even find what’s being disclosed as far as specs,” she thinks.
A specific piece of information Bard says is often conspicuously absent is the pixel count between the camera and the lens. Because some manufacturers bundle the two, it’s easy not to have that listed on the data sheet, she explains, but that means an installer has to call the manufacturer to get that in-depth information — if they’re even aware of its importance.
“You really have to put your faith into your manufacturer,” Bard relates. “There’s a lot more information available now than a year ago, but it’s still not all there.”
In the same vein, Iniguez points to the claims lens manufacturers make about resolution, which they express in terms of line pairs. Unfortunately, that number refers to the performance of the lens “at the dead center” and may not represent its performance at the edges.
“You could have a severe drop-off in image quality at the edges,” she says. “You can’t believe all the claims out there, but unfortunately, it becomes a matter of buying and trying different lenses.”
Iniguez adds that this only serves to underscore the fact that there are no overriding standards for measuring and reporting lens quality. “There’s not a standard definition among manufacturers or within the industry about the true definition of what, say, a 5-megapixel or 3-megapixel camera really is,” she says.
DAY/NIGHT
In addition to megapixel lenses, day/night lenses are also gaining traction in the industry, Wolk says. The main issue around these lenses is ensuring that they have the proper infrared (IR) coating to reduce the focal shift between day and night.
Day/night cameras are designed to “see” both visible light and IR light, both of which are present in most light sources. Without the proper IR coating, the red, green and blue color filters that light passes through also allow IR light to pass through, creating an image that is clear at night but blurry during the day, Wolk says.
“If you’re only looking 20 feet or so away, it’s not that big of a deal,” he claims. “But the farther out the view is, the more drastic the focus shift you’ll notice.”
According to Peterson, a “true” day/night camera physically switches its IR filter out of the lens’ light path, enabling the camera to see IR light, whether naturally occurring or artificial. And because street lights, the sun, the moon, and other visible light sources include IR light, a day/night camera can easily capture and record its field of vision. The day/night lenses required by these cameras to maintain proper focus in all light are more complex and therefore more expensive.
So when it comes to day/night, Peterson says that before investing in a more expensive IR camera and lens, installers should ask themselves whether their customer actually needs a day/night setup.
The answer, he believes, is “not necessarily.”
While a day/night camera has to be able to transmit IR light through the lens, day-only lenses have an immovable IR filter. Most security cameras, Peterson says, are designed for day-only use, so by keeping out the IR light, the IR filter improves an image’s color quality and sharpness.
With this in mind, IR illumination could also be used to augment that visible light found in the camera’s field of view. IR LEDs mounted to shine on the object or area the camera is covering can make it overkill to use a day/night camera and lens.
IF ALL ELSE FAILS…
Considering the wide variety of lenses that are available, coupled with the lack of standards for measuring and reporting specs, Wolk says the best method for finding the right lens for the right situation is an old-fashioned one.
“I always tell people to try out lenses,” he says. “All the numbers and specs in the world are no substitute for a real-world, side-by-side comparison. Testing various lenses under the same conditions is still the best way to make a decision.”
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.
Do You Need a Megapixel Lens?
When using a megapixel camera, you have two choices: use a megapixel lens or use a standard lens. Both will technically work, but a megapixel lens will work much better, and as a result is more expensive.
Todd Pinnell, product manager at Amityville, N.Y.-based Speco Technologies, cautions against using price as a determining factor. Rather, he suggests that if you require a high-resolution image, quality matters. So you should be looking at a higher-end lens from a trusted manufacturer, even if it’s more expensive. After all, he says, you get what you pay for.
“The smallest imperfection on a megapixel lens will cause the shot to look terrible,” he describes.
Although you can use a standard lens with a megapixel camera, Pinnell warns against it. However, if you must to go that route, be aware of the potential for image deterioration at the outer ridge of the shot. Megapixel lenses, he says, correct the issue.
What’s Old Is New Again
Combating image distortion doesn’t necessarily require the “latest and greatest” technologies, says Glenn Wolk of Tekstar Optical. Instead, you can “trade down” lenses by formatting a larger-format lens for a smaller-format camera.
For example, he says, using a 1/2-inch lens on a 1/3-inch camera produces results that may be surprising to some.
“By hitting the center portion of the larger lens with the smaller chip, it allows the camera to hit the sweet spot of the lens,” he says. “With a megapixel camera, you’d be amazed at the quality of the picture.”
Beyond being a more-than-adequate solution to the problem of distortion, he says, using older, larger-format lenses is both cost-effective and timeless.
“When people are swapping out older cameras, I always tell them, ‘Don’t throw out the lenses. Clean them up and they’ll be absolutely amazing on a smaller camera,’” Wolk says. “Toss the camera but keep the glass; its shelf life is indefinite.”
Reference: " Image is Everything " by Derek Rice, SDM Magazine Dec 2010
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