What is the reason you don’t want to change your IPD (interpupillary distance)?
The reason is that we have wide diameter lenses, who 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 optical flexible IPD as when the IPD is a few mm wider or narrower, the lenses still remain in focus.
In a 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 only if you could do the both it would have the intended effect. Or you can buy the smartphone size based on your IPD.
Graph from Cambridge University: Our fixed IPD is 63 mm, which is the top of the graph, meaning the most common IPD. Taking the flexible viewpoint in mind 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 people mostly don’t set their IPD correct, or won’t set it at all meaning they use the IPD setting of the last person who used the product. This will get people to squint their eyes and get headaches very quickly when using a wrongly set IPD.