If you look yourself in the eyes, you’ll start to realize that your eyes and your head are different than anyone else’s. The spacing between your eyes, known as the interpupilary distance is about 65mm, but this varies from 50mm to about 75mm, depending on who’s eyes you’re looking through. Also the position of your eyes, relative to the shape of your head is unique; some people have eyes that are more inset, or perhaps bulging outward.
The designers of VR helmets have to deal with all this variation in the human phenome. Everyone has a sweet spot where the two lenses of a VR helmet are perfectly aligned with their eyes. Similarly, each of us want the lenses to be positioned as close as possible to our eyes (to achieve wide field of view), without discomfort. If you wear eyeglasses, you need to have room to fit your glasses between your eyes and the lenses.
So… let’s look at how one helmet design deals with these issues: