Field of View: typically measured in degrees, the field of view defines what is the horizontal, vertical and diagonal extent that can be viewed at any given point. This is often specified as a monocular (single eye) field of view, but it is also customary to specify the binocular field of view and thus the binocular overlap
|Illustration of eye relief|
- If the optics are too close to the eye, they generate discomfort such as when the eyelashes touch the optics.
- If the eye relief is large enough, the system might be able to accommodate people wearing glasses without the need to provide a focusing mechanism to compensate for not having glasses
- The total depth of the optical system (distance from eye to screen) becomes larger and the overall system potentially more cumbersome.
- The minimal diameter first optical element is dictated by a combination of the desired field of view and eye relief. Larger eye relief requires the lens to be wider and thus likely heavier.
|Comparing optical quality at a distance away from the optimal eye position|
Material and type of lens: a lens is typically made from optical-grade plastic or from glass. There are hundreds of different optical-grade glass types but only about a dozen optical-grade plastic material. Different material provide different light bending properties (e.g. index of refraction) so it is quite common that multi-element optical systems are made with more than one material. Glass is typically heavier, more expensive to mold, but has greater variety, provides better surface quality and is often physically harder (e.g. more resistant to scratches). Plastic is cheaper and lighter. Additional lens types and non-linear optical elements such as Fresnel Lenses and polarizers are also available.
Distortion: optical distortion is one type of imperfection in an optical design. Distortion causes straight lines not being seen as straight lines when viewed through the optics. An example of this is shown below.