As you examine object points progressively further from the top of the object you will eventually find one such that a ray from it to the interface will refract along the same path as the limiting ray in air. The angle 'i' this ray (dashed line) makes with the normal to the interface must be consistent with **Snell's Law**, namely

sin(i) = [n(a)/n(w)]*sin(r)

where n(a) and n(w) are the **indices of refraction**, respectively, of air and water.

Since the ratio n(a)/n(w) is approximately 0.75, angle 'i' is always smaller than angle 'r', and the lens' effective field of view is smaller. If the lens is very close to the interface, its **effective angular field of view** is '2i'. Some typical values are shown below (angles in degrees).

in-air angular field of view: 180 140 100 60 30 in-water angular field of view: 97 90 70 44 22

For example, the underwater field of view of a 180-degree 'fisheye' lens is only 97 degrees behind a plane port.