The field of view is determined by the angle of view from the lens out to the scene and can be measured horizontally or vertically. Because the aspect ratio differs between formats, the more universal picture angle, measured along the diagonal of the scene is often used. A shorter focal length (such as a 28mm wide angle) produces a wider picture angle, while a longer focal length (such as a 200mm tele) produces a narrower picture angle. In 35mm photography, a 50mm lens is called a normal lens because it produces roughly the same picture angle as the human eye (about 46°).

The picture angle is measured diagonally

The example below shows the difference between two focal lengths, 30mm and 100mm.

30mm wide angle	100mm tele has more narrow field of view (indicated in red in the wide angle image)