The FOV of the virtual cameras must match the visible display area. In general, Oculus recommends not changing the default FOV.
Field of view can refer to different things that we will first disambiguate. If we use the term display field of view (dFOV), we are referring to the part of the user’s physical visual field occupied by VR content. It is a physical characteristic of the hardware and optics. The other type of FOV is camera field of view (cFOV), which refers to the range of the virtual world that is seen by the rendering cameras at any given moment. All FOVs are defined by an angular measurement of vertical, horizontal, and/or diagonal dimensions.
In ordinary screen-based computer graphics, you usually have the freedom to set the camera’s cFOV to anything you want: from fisheye (wide angle) all the way to telephoto (narrow angle). Although people can experience some visually-induced motion sickness from a game on a screen, this typically has little effect on many users because the image is limited to an object inside the observer’s total view of the environment. Computer users' peripheral vision can see the room that their display sits in, and the monitor typically does not respond to their head movements. While the image may be immersive, the brain is not usually fooled into thinking it is actually real, and differences between cFOV and dFOV do not cause problems for the majority of people.
In virtual reality, there is no view of the external room, and the virtual world fills much of your peripheral vision. It is therefore very important that the cFOV and the dFOV match exactly. The ratio between these two values is referred to as the scale, and in virtual reality the scale should always be exactly 1.0.
In the Rift, the maximum dFOV is determined by the screen, the lenses, and how close the user puts the lenses to their eyes (in general, the closer the eyes are to the lens, the wider the dFOV). The configuration utility measures the maximum dFOV that users can see, and this information is stored inside their profile. The SDK will recommend a cFOV that matches the dFOV based on this information.
Deviations between dFOV and cFOV have been found to be discomforting (though some research on this topic has been mixed). If scale deviates from 1.0, the distortion correction values will cause the rendered scene to warp. Manipulating the camera FOV can also induce simulator sickness and can even lead to a maladaptation in the vestibular-ocular reflex, which allows the eyes to maintain stable fixation on an object during head movements. The maladaptation can make the user feel uncomfortable during the VR experience, as well as impact visual-motor functioning after removing the Rift.
The SDK will allow manipulation of the cFOV and dFOV without changing the scale, and it does so by adding black borders around the visible image. Using a smaller visible image can help increase rendering performance or serve special effects. Just be aware that if you select a 40° visible image, most of the screen will be black—that is entirely intentional and not a bug. Also note that reducing the size of the visible image will require users to look around using head movements more than they would if the visible image were larger; this can lead to muscle fatigue and simulator sickness.
Some games require a “zoom” mode for binoculars or sniper scopes. This is extremely tricky in VR, and must be done with a lot of caution, as a naive implementation of zoom causes disparity between head motion and apparent optical motion of the world, and can cause a lot of discomfort. Look for future blog posts and demos on this.
 Stoffregen, T.A., Faugloire, E., Yoshida, K., Flanagan, M.B., & Merhi, O. (2008). Motion sickness and postural sway in console video games. Human Factors, 50, 322-331.
 Draper, M.H., Viire, E.S., Furness, T.A., Gawron, V.J. (2001). Effects of image scale and system time delay on simulator sickness with head-coupled virtual environments. Human Factors, 43(1), 129-146.
 Moss, J. D., & Muth, E. R. (2011). Characteristics of Head-Mounted Displays and Their Effects on Simulator Sickness. Human Factors: The Journal of the Human Factors and Ergonomics Society, 53(3), 308–319.