Mathematical Topics from 3D Graphics - 3D Math Primer for Graphics and Game Development - page 366

Figure 10.4
Horizontal field of view
twice as big on screen as it would if we were using a 90 o field of view. So
larger zoom values cause the image on screen to become larger (“zoom in”),
and smaller values for zoom cause the images on screen to become smaller
(“zoom out”).
Zoom can be interpreted geometrically as shown in Figure 10.5. Using
some basic trig, we can derive the conversion between zoom and field of
Figure 10.5
Geometric interpretation of zoom