Training: Using Lens CalculatorsBy John Honovich, Published Sep 27, 2010, 12:00am EDT
In video surveillance applications, lenses generally take a 'back-seat' to their sexier camera counterparts. However, selecting a proper lens for an application helps ensure the highest quality image. Specify a lens too narrow for your application and one will miss out on capturing the entire area of interest. By contrast, a lens that is too wide will result in poor video quality of the subjects monitored.
A lens calculator can determine the appropriate lens focal length required for a desired field of view and therefore optimize system performance.
Before using any lens calculator tool it is important to understand a few key concepts:
- Sensor Size or Format - Cameras have an imaging sensor that comes in a variety of sizes, such as, 1/4", 1/3", 1/2.5", and 1/2". Calculator tools usually provide a means to select one of these values for the calculation. This sensor size or format is easily determined by looking at the specifications sheet of the camera being considered. Note that whether the sensor is a CCD or CMOS has essentially no bearing on calculations.
- Focal Length (mm) - Lenses generally fall into two categories: fixed focal length and vari-focal length lens types. The focal length is essentially the distance between the lens and the imaging sensor. This distance determines how wide or narrow the scene captured will be. Shorter focal lengths are able to capture relatively wider scenes, while longer focal lengths provide tighter viewing angles for capturing more distant objects. Vari-focal lenses have the advantage of providing a range of focal-lengths for added flexibility and control.
- Horizontal Field of View (FoV) - The horizontal field of view (HFoV) is probably the most practical and well recognized property of the camera/lens system. It represents the width in units of length (feet or meters) of the camera scene at a specified distance from the lens. This is a measurement typically obtained first-hand, perhaps during a site survey. It can also be theoretically calculated given a 'distance from lens' parameter and a 'lens angle' parameter.
Learn how to use it below:
In this video, we guide you through the basic usage of a representative free online calculator tool. We take a quick tour of the lens calculator interface and highlight some key usage points. Then, we walk-through a few basic examples to better illustrate usage of the calculator tool. You'll see a unique feature of the tool that dynamically updates a reference image based on the input from the user. This gives the user a visual sense of the camera/lens system being considered.
In this video, we provide a conceptual explanation on how field of views (FoVs) are derived. You'll learn that FoVs are dependent on properties of the camera (sensor size), properties of the lens (focal length) and the distance from the lens being considered. We use a diagram to clearly illustrate why a larger sensor size results in a wider FoV for any given lens focal length.
Pixels/Foot Concepts and Application
In this video, we demonstrate some examples of how a lens calculator tool might be used in a more thoughtful video surveillance design scenario. We'll revisit the earlier examples provided in the 'Basics' video and delve deeper into the tool's ability to predict image resolution or pixels/foot for a given CCD/CMOS resolution and horizontal FoV.
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