Focal stack
A focal stack is when two or more flat display layers are stacked together at different focus depths, which alleviates VAC if driven using the right software. It can be done in a desktop display or a head-mounted display.
One of the optical engineering challenges is luminance loss, due to the fact that light has to pass through multiple layers, which can dim the image or cause internal reflections.
Displaying digital objects[edit]
If a digital object is meant to be at 1.0 meter, the system renders it on both screens simultaneously, but with specific transparency and brightness levels.
The user's eye naturally integrates the two light sources. Because the light is physically coming from two different depths, the eye's ciliary muscles actually adjust to find the "sweet spot" of focus in between the two physical screens.
An advantage of a 2-deep stack is that it handles retinal blur correctly.