There is a general and popular misunderstanding about the nomenclature of the lenses in VR headset, or lenses in general.
The lenses, in general, are described by the shape of the curved surfaces they are formed from:
- Spherical - the surface is a part of a sphere,
- Aspherical - the surface is more complex.
Both of those types are usually considered to be symmetrical, in a sense that the shape is some kind of a rotational shape, or in other words, the shape is symmetrical along its optical axis.
More generic form (i.e. asymmetrical) is then anything else, with its most complex variant known as free form shape, which is usually a custom design for particular application. You may know those type of lenses from prescription glasses, where they are referred sometimes as “digitally surfaced”.
The difference between different categories is usually defined by the mathematical complexity of the defining formula. The “basic” aspherical (and even asymmetrical) lenses are defined by relatively simple formulas, while the true “free form” lenses are basically spline meshes.
So far I have not mentioned Fresnel lenses, because Fresnel principle is just a way of realization (or implementation) of the aforementioned types in a different way. It is a (very smart) way to compress otherwise thick body of the conventional design, while maintaining the same optical property. Well, not exactly the same as VR users are already aware of god rays, stray light, loss of acuity, etc., but in general they are just an equivalent of the corresponding traditional design with certain advantages and trade-offs.
So, if you would want to classify the lenses in some headsets correctly, you would say, for example for:
OG Vive - it has symmetrical, most likely aspherical, Fresnel lenses
Varjo - has probably the same, but conventional (non-Fresnel).
Pimax has most likely asymmetrical (which implies aspherical as well) Fresnels.
The final part is what is then considered to be a hybrid design? This is when two (or more) different implementations are used in one piece. E.g. you have one part of the lens of the Fresnel type (typically the center) and other other one (typically the outer rim) of the conventional design.