> photons can leave from any point in space at any angle Not really. They can leave from any point on a surface at any angle. Once they're on their way, they d

> photons can leave from any point in space at any angle Not really. They can leave from any point on a surface at any angle. Once they're on their way, they don't change direction. If a surface is an infinite distance away, then by the time they reach your face, the photons from that surface are all travelling parallel to each other. That's what collimation is simulating: looking at objects an infinite distance away. Why? As the article says: "But try this experiment. If you can find a fixed test pattern, put that on the HMD's screen, then pull the HMD away from your face a little, and move it side to side by an inch or so (don't rotate it, just translate it). The image stays fixed in space! It’s a really strange thing – you’re moving the physical box, but the image inside it doesn’t move. That’s the power of collimated-light images, and it's still very unintuitive to me even though I understand the physics and maths very well." Just how, when you look at a surface a great distance away, its appearance doesn't change if you move your head a few millimetres from side to side. "Of course real VR lenses are not perfectly collimated for various reasons. But they're close enough that getting the IAD (optical axis separation) adjustment perfectly right is not as important as it appears at first." Collimation means that the device is robust to the exact distance between the user's eyes. That is very useful.