How to Select the Focal Length and FoV Parameters for an IP Security Camera

In our previous article, <Current State of Security IP Camera Industrial Technology and How to Choose an IP Camera for Outdoor Use>, we discussed the development status of network security cameras and some characteristics to consider when choosing a camera. Today, we will focus on two important performance parameters often encountered when selecting cameras—camera focal length and field of view which is called FOV for short. This article will help us choose the most suitable camera for different usage scenarios to achieve the desired surveillance or filming effects.

1. What are Focal Length and Field of View, and How are They Defined? Modern Security IP Cameras typically use fixed focal length optical lenses, which adhere to the basic physical principles of convex lens imaging. Of course, there are also high-end products that use variable focal length optical lenses, which is another subfield within the security camera category, with more robust structures and functions. I will also provide a complete introduction to this in a separate article.

   Focal Length: In the description of physical optics, the focal length refers to the distance from the optical center of the lens to the focus point where parallel light rays converge after passing through the convex lens.

   Field of View: The angle formed by the two edge rays of light that define the maximum range through which the lens can observe the target object, taking the outermost tip of the convex lens as the vertex, is called the field of view(FOV for short). See the diagram below:

   

2. The Impact of Different Focal Lengths and Fields of View on Usage Scenarios.

   Typically, the focal lengths of commercial security monitoring cameras are industrial parameters such as 2.8mm, 3.6/4mm, 6mm, 8mm, 12mm, etc. Where is the logical relationship between these different focal lengths and fields of view, and what impact do they have on our choice of camera parameters? Generally, the larger the focal length, the farther the visual scene can be seen, and the relatively smaller the range of the field of view. For example, a 2.8mm focal length camera can achieve a field of view of up to 90°, even exceeding 105°. Our so-called wide-angle or ultra-wide-angle cameras require the use of optical lenses with smaller focal lengths, even micro-focal length optical lenses, which we call micro-focus lenses. Conversely, the smaller the focal length is, the more the visual scene is, and the larger the range of the FOV is. It can be seen that the relationship of focal length and FOV basically follows an inverse correlation. See the diagram below:

   

3. In Practice, How Do We Choose the Appropriate Camera Focal Length and FOV for Our Needs?

   Having learned the concepts and relationships of camera focal length and field of view, how do we choose these two important optical parameters for cameras in daily use? We can refer to the following dimensions:

   Firstly, we need to determine the range we need to monitor. If a wide field of view is required at a close distance, such as in a living room, baby crib, elderly care room, or small store, we need to choose a small focal length like 3.6/4mm, with a field of view ranging from 75° to 95°, and an effective visual range that can reach up to 5 meters; even a 2.8mm camera can provide a field of view greater than 90°. If these do not meet your needs, then we may need a camera with a dual-axis gimbal. I will also dedicate space to introduce such products with dual or triple-axis gimbals.

   If the distance of our monitoring range is quite far, especially for outdoor monitoring, such as courtyards, parking lots, factory areas, and other outdoor scenes, we need to choose cameras with longer focal lengths like 8mm or 12mm.

   Secondly, we must confirm our actual needs. Through the introduction above, we are always balancing our monitoring distance and angle between the focal length and field of view. In real life, one common premise when choosing a camera is to effectively see objects and people clearly. Other considerations in practical applications can also affect our choice of focal length and field of view. For instance, sometimes we do not need to see objects or people clearly; as long as the range is broad enough, we need to choose some ultra-short focal lengths, such as a 2.8mm focal length. Or a fisheye lens, where the field of view can reach or exceed 150°.

   Thirdly, sometimes, we only need to monitor a narrow area far away. For example, a 5-meter-wide area 30 meters away, where a small focal length cannot see clearly, and a large focal length cannot cover. In this case, our designers provide a better solution for everyone, which is a camera with dual or triple lenses and variable focal lengths. I will also introduce this content later.

How about that? Through the above explanation, we will choose the right camera according to our application environment. Learn a little every day, and you will become an expert too. You are the best!