Field of View defines the width of your view as an angle with its apex at your viewpoint and the ends at the sides of the view. The effect of changing FOV is similar to changing the lens on a camera. As the FOV gets larger you see more of your scene and the perspective becomes distorted, similar to using a wide-angle lens. As the FOV gets smaller you see less of your scene and the perspective flattens, similar to using a telephoto lens.
Above: Narrow field of view
Below: Wide field of view
A Perspective view uses an imaginary camera with only one setting, FOV. The FOV angle for the active Perspective view is displayed in the Rendering Methods panel of the Viewport Configuration dialog. You can type a value in the FOV field of the dialog to precisely set FOV for the active Perspective view.
Use Field of View (FOV) to change the amount of the scene visible and the amount of perspective flare applied to a Perspective or Camera view. The Field of View button appears in the viewport navigation control panel when a Perspective or Camera view is active.