What is the reason you don’t want to change your IPD (interpupillary distance)?
The reason the distance is fixed is because we use wide diameter lenses that greatly improve the feeling of immersivity (as you’re not feeling like you’re looking through 2 holes in a box as with many other VR headsets). These wide lenses have a second advantage that they create an optically flexible IPD, therefore, even if the IPD is a few millimeters wider or narrower, the lenses still remain in focus.
Either way, changing the IPD does not make a lot of sense anyway as the 2 objects you are viewing, the two halves of your screen of your smartphone, won’t go further apart or closer together when you change the IPD, so unless you could change both, only then would it have the intended effect. However, you can buy a certain sized smartphone based on your IPD.
Graph from Cambridge University: Our fixed IPD is 63 mm, which is the top of the graph, meaning it is the most common IPD. Keeping in mind the flexible viewpoint of our wide diameter lenses, you can see that for around 95% of the population, we don’t need to change the IPD.
What is the disadvantage of having an adjustable IPD?
In addition to the fact that it does not make a lot of sense to change only the IPD of the lenses and not of the object you’re viewing, see above, the disadvantage we have learned from studying user behaviour is that most people don’t set their IPD correctly, or won’t set it at all meaning they use the IPD setting of the last person who used the product. This results in people squinting their eyes or getting headaches quickly from using the wrong IPD.