This simple lens calculator helps you select the correct lens for a Phantom camera based on some details about your shot.
Select the camera model you are using. Type in the horizontal and vertical resolution. Select a measurement unit. Then, enter the distance from the subject and the length of the horizontal field-of-view you need in the image as well as the height of the vertical field-of-view you need.
Click "Calculate" for the required lens focal length.
If the two numbers differ significantly, then you can either select a focal length that is an "average" of the two, or bias the lens toward either the horizontal or vertical field-of-view, depending upon your subject and needs.