the opposite is true actually...... in the rift your eye is completely natural, as it totally fools you that you are looking into infinity.
OTOH using a monitor your eye is at a fixed focal point. (unless you look away)
Sorry, I might not have explained what I meant properly, maybe I lack the vocabulary. I'll try again in more detail:
When you look at something in the real world, there are two things your eyes need to do to focus on the object properly : first, your two eyes move in order to have the object at the center of your field of view. If the object is close, you'll "cross your eyes" (sorry if that's not the correct way to say it), and if the object is very far, the lines of sight of your two eyes will be parallel (what you say when you talk about looking into infinity). The different angles from which you view the object with each eye is what will give you the sense of 3D, the scale and the distance of the object from you. This effect, called stereopsis, is what the Rift manages to recreate very convincingly, which is why you can see things in 3D like in real life, and sense the scale of ships and stations in Elite. Indeed, when you look at a planet, your eyes aren't crossing, they're looking into infinity.
However, the second thing your eye does when you look at a real object, is what the Rift is not capable of recreating : it's the accomodation. It's basically the way your eye focuses so that you can see the object clearly and not blurry (just like when a camera lens adjusts it focus so the picture is clear). If you hold your finger in front of you and look at it, the computer monitor behind it will appear in double (due to you being cross-eyed), but also blurry, due to your eyes accomodation. However, while using the Rift, no matter what the stereoscopic separation is between the images each eye receives, your eyes will always retain the exact same level of accomodation, which is the one where you see the pixels most clearly, and which is determined by the Rift's screen distance from your eyes, the shape of the lenses, and their position between your eyes and the screen. There is a simple test you can do to experience this issue in the Rift : just like you looked at your finger in real life, in the game try to look at seomthing very close to you, say the dirt on your ship's canopy. While you focus on it, the stars in the background will appear double (as they should), however they'll appear just as clear as the canopy, when in real life they would appear blurry. This is because your eyes constantly have the exact same level of accomodation when you're in the Rift.
This is what I meant when I said your focus was always the same with the Rift. And this constant accomodation at what appears to be a relatively short distance, can cause fatigue, since in real life your eyes constantly accomodate to a different level when you look at things at varying distances. When you play on a monitor, sometimes you'll look at your hands, your keyboard, or the wall behind the monitor, and your eyes will accomodate accordingly ; it's unlikely you'll keep your eyes focused on your monitor for hours without any interruption, which is what happens with the Rift. I hope that makes things a little clearer
