Security cameras are an important tool in the protection of facilities and critical assets. However, the phrase “what you see is what you get”…applies…literally. Proper set up of security cameras directly relates to the level of effectiveness they will provide.
One aspect of setting up a security camera is its field of view (FOV). In simple terms, the camera’s field of view refers to the image you see from the camera in either the live or the recorded view. That sounds easy enough, but several mistakes happen repeatedly, with one of the most prolific failure modes being the “Rule of Thirds.”
Thinking Like A Photographer
We all remember our high school photography class and the rule of thirds: two thirds of the scene should be ground and subject, one third should be reserved for the sky. We never seem to execute it correctly when we are taking Hawaiian vacation photos or snapping that selfie, but it is amazing how many times we get it right when positioning security cameras. Although the “rule of thirds” is pleasant to the viewer’s eyes, in the security world, there are many reasons to avoid this practice.
Importance Of Data Within A Scene
One third of any view represents over 33% of a camera’s image. That means that in the case of an intrusion, you are giving away one third of your sensor’s detection capability,through the viewing of the sky. When confronted with a security incident, the words you don’t typically hear are “we have more than enough video footage, you can go ahead and ignore that last 33%.” This is essentially, what you are saying when following the rule of thirds for a security application. You are willing to assign one third of your potential data to an area that you know will not have any value. Even if that added view is merely the top of the intruder’s head, this is providing you more information than the blue sky above.
Storage & Bandwidth Impacts
The cost of this data is more than just a theoretical cost. Bear in mind that allocating 33% of the image to the sky, also allocates 33% of your storage. A typical 4 CIF image in 704 x 480 or 337,920 pixels. If you assume 24 bits per pixel, you are allocating 34Kb per frame to…well…nothing. Now add in 15 frames per second, 24/7 and you start to see the impact on “real” storage.
The corollary to storage is, of course, bandwidth. Storage may be relatively affordable, but bandwidth is always a precious commodity. This is especially true as cameras tend toward higher and higher resolution. Now you can certainly argue I-frames and P-frames, or only recording on motion, but it still comes down to the fundamental question as to how much of your video storage and bandwidth you want to allocate on interesting cloud formations.
Avoiding Glare From The Sun
Another reason to avoid including too much “above the horizon” video is the issue of glare. Security cameras, especially visible light cameras, typically perform poorly when positioned directly into the sun. The impact of the sun on a particular camera obviously changes by the time of the day, and the season of the year, but sunrise and sunset create the biggest issue, as these are the times when the low angle of the sun can interfere with the cameras view.
If following the rule of thirds, 33% of your camera’s view may also be subjected to the glare of the sun as it rises or sets. If you are working with a lens that provides vertical field of view of 36 degrees, you have exposed 12 degrees of the camera view to potential glare from the sun. The sun moves about 15 degrees per hour, subjecting your camera view to glare issues for almost an hour! Reducing the amount of sky in the view to 10% or less, limits this time to about 15 minutes.
Consider Horizontal FOV
To be fair, it is worth noting that field of view and the rule of thirds has a horizontal component. It doesn’t get nearly the bad rap that vertical field of view receives, but the same rules apply. An example is a field of view aimed at detection along a fence. If the outside of the fence is of no concern, perhaps a busy sidewalk street, does it make sense to include that in the camera’s view and record that data? For humans, only seeing one side of a fence seems spatially unbalanced. We want to see the other side as well. Whether the other side of the fence has value, is a security decision. However, if it is determined that it is not of concern, then steps should be taken to exclude it from the horizontal field of view, and by default, exclude that imagery from the network bus and the server’s hard drive.
Guidance For Horizontal Coverage
So what is the answer to the universe, at least how it relates to field of view? How much image, if any, should we allocate to the area above the horizon line or the back security gate? As a guideline, 10% of the scene is more than enough. This percentage typically provides a sufficient frame of reference to the human viewing the scene. It also ensures enough room for any vertical detection zones, whereby the camera may be monitoring intrusions that come over a fence or a building.
What if a “great photographer” set up your security cameras, ensuring a perfect rule of thirds in all views? Should you go back and realign all those cameras? In most cases the answer is no. The cost will likely be prohibitive. However, it is worth noting each camera that needs adjustment, and during routine maintenance, or when replacing failed equipment, be prepared to properly align these cameras at that time.
Often times we take the setting of the field of view of a camera for granted. The concept of field of view is straightforward and we have been pointing cameras for well over a century, so the action does not come with a lot of forethought or planning. However, something that looks good to the human eye, may not make sense to your security design, and in many cases can be detrimental. So don’t forget how the rule of thirds works when framing an image, just remember when to use it and when not to use it.