# Training: Using Lens Calculators

by Antony Look, posted on Sep 27, 2010

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.

Key Concepts

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.
• Field of View (FoV) - The horizontal field of view (H.FoV) 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.
• Lens Angle (degrees) - Lens angle is a property determined by a combination of the camera's sensor size and the lens' focal length. This angle is used to calculate the actual H.FoV. Note that there is both a horizontal and vertical lens angle. Each is used to obtain their respective horizontal and vertical field of views. However, horizontal FoV is the more commonly sought after information when designing video surveillance. So, assume horizontal in cases where it is not clearly specified.

Inside this report, we provide three (3) video tutorials on using lens calculators for video surveillance applications. We'll use the Theia Lens Calculator tool to demonstrate lens calculations including a valuable feature that shows a sample image for the calculated FoV.

Here are a number of online lens calculator tools you can consider:

Earlier we performed an in-depth study covering topics, such as lenses, FoVs, and pixels/ft. We encourage you to view the report: Testing How Many Pixels are Needed for High Quality Surveillance Video to enhance your understanding. Also, you may want to view this training: Resolution of IP Cameras.

Inside the Pro Section

Pro members may access our 3 video screencasts teaching you how to use a lens calculator and select lenses. We show the basics plus go further in-depth on pixels per foot and adjusting field of views.

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