FOCAL LENGTH: This is the distance measured in millimeters between the center of the lens and the camera's CCD sensor. The smaller the focal length, the greater the width of the target area will be. Lenses with a focal length of 2.8 to 4mm are considered to be "wide angle" lenses and those greater than 6mm are considered to be "telescopic." To determine the ideal focal length of a lens, you just need to know the distance between the camera and the objective, and the desired width of the target area. Instructions: Enter the distance from your camera to the object or area you are viewing. Enter the width of the target area, also know as "horizontal view" that you would like to have. Click the "Calculate" button to display focal length needed.