The angle of view, often referred to as AOV, describes how “wide” or “narrow” the scene that is imaged by a camera. Confusingly, we often refer to focal lengths as being “wide” or “narrow” but, in fact, we are actually referring to the image we see (the field of view or angle of view) through the lens at that focal length, rather than the focal length itself.
The AOV can be expressed in 3 ways; the diagonal, vertical or horizontal (dAOV, vAOV or hAOV). This is because a sensor of film frame is rectangular and can be measured along it’s length, height or diagonally from opposite corners (like TV’s are measured).
For example, the diagram below shows a representation of a Nikon DX sensor (with a 1.5x crop).
Now, before everyone runs off in a fit of panic, this tutorial does require some basic trigonometry. Remember Pythagoras? The sum of the squares of the 2 sides is equal to the square of the hypotenuse? Good. You can use Pythagoras’ theorem to work out the diagonal size of a sensor.