Field of View
The field of view of a photograph or camera is a measure of the proportion of a scene included in the image. A typical fixed lens camera might have a field of view of 50�, a fisheye lens can have a field of view greater than 180� and a full equirectangular or cylindrical panorama would have a field of view of 360�.
Field of view is often abbreviated as FoV, it is also known as angle of coverage or angle of view. Usually field of view refers to horizontal field of view (HFoV), some applications make use of the vertical field of view (VFoV) which can be calculated from the Aspect Ratio of the image:
Aspect Ratio = sin (HFoV / 2) / sin (VFoV / 2)
Conversion from focal length
The other standard measure of the width or narrowness of a lens is Focal Length.
Assuming a 35mm negative width and a rectilinear lens, the field of view can be calculated like so:
Field of View = 2 x atan ( 35 / ( 2 x Focal Length ) )