Angle of view, the angle formed by the line of sight and the vertical direction of the display, etc. , and when observing an object, the angle formed by the light emitted from both ends of the object (up, down or left and right) at the center of the human eye.

∠ADO=90

According to Pythagorean theorem, OD is obtained.

Solving cos included angle according to cosine theorem

Then the angle can be obtained by using the inverse trigonometric function.

Finally, compare with 1'