Lens Size and Design Guide

A little preplanning can make all the difference when it comes to designing a surveillance system. Two cameras with the correct lens choice and correct installation position can be just as effective (or more effective) than installing more cameras.

After the decision to buy has been made, but before the purchase is complete, the most important step is to thoroughly determine what you want that system to see. There are no right and wrong answers when it comes to lens choice and placement.

Having clearly defined expectations will help Seon sales and installation reps determine the best plan for each individual fleet or bus.

Ask yourself what kind of surveillance coverage you want:
• the faces of everyone who enters and exits the bus?
• the backs of seats to monitor vandalism?
• stop-arm violators?
• the bus driver and the control panel?
• the outside of the bus and entry and exit points for liability?
• faces in low-light situations?

Other points to consider:
• Do all routes need the same set-up?
• Would long distance routes be served by adding an IR Illuminator for early evening and late night runs?
• Is there one route that consistently has stop-arm violators?

The first step is to determine potential camera locations given your answers above.

Potential placements for a school bus and a transit bus

The simplest installation is a system featuring one camera installed at the front and facing the back. For multiple camera systems, the current trend in the industry is to move away from mid-cabin camera placements because they are more susceptible to vandalism and blocking. Instead, two cameras placed up-front near the driver are proving just as effective. One camera has a wide 2.9 mm lens, while the other has a narrower 6 or 8 mm lens to capture the rear of the bus.

The desired locations combined with the type of lens will ultimately determine what is recorded.
Check out the Seon Lens Guide.

The next step is to figure out exactly what width of coverage each camera will capture. This is called determining your Field of View (FOV). The table below displays some common distances. It shows that a 6 mm lens will have an eight-foot FOV at a distance of 10 feet. A 12 mm lens will have a four-foot FOV at a distance of 10 feet.

To figure out exactly the requirements of your fleet use the formula below:

Width = 2x (distance X tangent of the 0.5 horizontal angle)

The angle can be found in Lens Guide, in the manual or in this chart:

When choosing the Field of View, consider the images below taken by a stop-arm camera using different lenses. Keep in mind, your answers to the questions above to help determine your expectations.

Each is a good image, depending on what you want it used for. The image on the left captures two lanes of traffic. The image in the center captures one lane of traffic and would give a clear view of the person driving. The image on the right provides a clear view of the license plate to catch stop-arm violators. Each image is valuable, but each meets different priorities.

Once you have determined your lens size and the placement, there are a few things to consider. The image is recorded in a ratio of 3X4. All Seon cameras are designed to easily rotate to adapt to each individual situation.

For a fleet that wants to record the length of the bus, the camera can be rotated so the long-side is vertical, maximizing the usable Field of View.

For a situation monitoring the driver, the control panel and people exiting and entering the bus, it is important to keep the long-side horizontal. Rotating the camera is a quick alteration that vastly changes the set-up of your surveillance system.

This method will give you larger images, as opposed to going with a wider lens, which gives you smaller images. However, when viewing the image on playback, it is also rotated. In some situations this will work well, but in others it may make viewing footage a challenge.

When finalizing camera placement, there is one major issue to keep in mind: avoiding backlighting. This might be as simple as pointing the cameras down to avoid ceiling lights or a bright white ceiling, or it might mean rotating the camera to avoid bright windows.

For more information on FOV and camera placement call 1-877-630-7366.