I'm a computer programmer trying to work on a personal hobby project that uses lenses to focus a very close display for the eye (designed to fit into a headset similar to VR).

I am under the assumption that part of why working on a computer or phone strains the human eye is due to keeping it focused on a near object for long durations. To avoid this, I'm trying to understand what it is about how the light enters the eye that causes the eye to contract the lens (to prevent the eye from doing this)

I've seen many diagrams showing diverging vs parallel "beams" of light hitting the eye's lens, is this part of it?

  • 2
    Welcome to the site - this I think would be better on the Biology Stack - you can request migration there. Note that both sites require evidence of prior research - so what did you find about the eye and how light enters.
    – bob1
    Commented Jun 24, 2023 at 8:48
  • Thanks for the welcome! I was unsure as to whether med sci or biology would be better suited, so I will definitely look at migrating this there. As for research, I had learned about this topic in highschool, but one of the sources I used to brush up my understanding was the following and this one features the diagram that shows the two light "beam" states Commented Jun 25, 2023 at 9:02
  • I find your question puzzling. If your eye doesn't focus on the headset screen then that screen will be blurry and out of focus. Why would you want that? You can't achieve focus without changing the shape of the lens. There's no way to "trick" the eye into focusing without having it actually do so.
    – Carey Gregory
    Commented Jun 28, 2023 at 0:02
  • @CareyGregoryisonstrike the goal is to make sure the image provided by the screen is in focus using lenses to the human eye when it is relaxed and not constricting the pupil. This way the user does not suffer from eye strain. Chris nailed what I was looking for below. Commented Jul 1, 2023 at 23:52
  • I see. So you're looking to adjust focus with physical lenses rather than letting the eye do it's thing.
    – Carey Gregory
    Commented Jul 2, 2023 at 3:02

2 Answers 2


To form a clear image, light must be focused on the retina, which detects the light and sends signals through the optic nerve to the visual centre of the brain. To achieve the focusing, most refraction of light happens at the cornea, with the ciliary muscles adjusting the lens for fine-tuning.

Eye anatomy

When the lens is relaxed, the eye is focused at infinity - i.e. parallel light rays from distant objects will be in focus.

For a near object, the ciliary muscles have to work harder the closer the object is, to adjust the lens to manage diverging rays. Most people have a near point of about 30cm, but it gets further away as we age, due to natural longsightedness, known as presbyopia.

This image shows the rays being refracted coming from a far object (parallel rays) and a near object (divergent rays).

Eye focusing far and near objects

Adjusting the lens (and constricting the pupil) in response to a near object is called accommodation - the lens needs to converge the rays more strongly for a near object.

This is a reflex coordinated by the second cranial nerve (optic nerve) as the afferent arc (sending a sensory signal to the brain) and the third cranial nerve (oculomotor nerve) as the efferent arc (receiving signals back from the brain and enacting a motor response). This is shown in the diagram below.

Accomodation reflex arc

For an object closer than the near point, the eye will need the help of lenses to focus the divergent light rays sufficiently. Any sort of prolonged focus closer than infinity will be tiring, but more so the closer the object is.

That is why convex lenses are used for near-eye displays, like in a virtual reality headset. The same type of lenses are used for long-sightedness, when the eye cannot focus diverging light rays from near objects sufficiently to form a focused image on the retina.

This image shows a convex lens focusing parallel rays to a point, and also a diverging or concave lens (which is less relevant here, but is used to correct short-sightedness).

Convex and concave lenses

Ideally, you would have the eyes completely relaxed and use lenses to provide an image in focus in this state. This is not practically possible, but the virtual image that the person sees can appear further in front of them, well beyond the natural near point.

The image below shows one example of this.

Lens arrangement in a near-eye headset

The image taken from an interesting paper by Xia et al describing a novel approach for a mixed reality headset that is adjustable for those with refractive errors (i.e. who wear glasses or contact lenses).

The middle lens is the main focusing lens, with the one nearer the eye being adjustable. At the far right you can see the virtual image, which is where the wearer will experience the display being; comfortably beyond their near point.

So in summary, the eye cannot be tricked into relaxing unless you’re displaying an image it can perceive to be far away, or at least as far away as possible. While in the example of the VR headset above, this creates a virtual image further away from the eye, it is optics rather than trickery!

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    I agree this might be better on biology SE, but thought I’d do an answer.
    – Chris
    Commented Jun 30, 2023 at 20:31

Should better be considered a comment rather than answer - but what you are asking is covering a range of topics. involving optics, neural reflexes & Neural reflexes involving the image formation at the retina & psychic Level.

Basically in simplest terms the feedback system at Psychic level is regulating the Lens curvature for optimum formation of image at psychic level. e.g., when "attention" focuses in a sea of awareness from a distant object to some object of reference then eyes move (reflexes), lens widens through ciliary muscle relaxation causing the new clear image of object of interest to form at psychic level.

I would refer you to neuro-opthalmology texts for further research.

  • Would it be reasonable to assume then that provided an image appears "clear" to the human eye, that the brain would perceive it so and not trigger the response to try and focus the eye? Also do you have any recommendations for neuro-opthamology texts? I appreciate the response! Commented Jun 25, 2023 at 9:04
  • you can read if interested in basic's (misnomer because it is really advanced in terms of questions at the heart) of eye then adler's or if interested in more clinical then yanoff's ophthalmology but more that you should focus on specific topic of visual perception which although would be covered in some basic formats in above texts i could not find s reference for your query. although A MIT opencourseware on visual system would be a start. you may dislike it for shear irrelevance but trust me, you would find yourself in uncharted waters which would answer your query.
    – Sage
    Commented Jun 26, 2023 at 5:30
  • Regarding the assumption, if something is already clear then what would be need to re-focus, Hope u appreciate the anomaly in question, Also the soul of question is really at Psychic dimension which as i mentioned depends on attention in sea of awareness=> it may be that someone has eyes wide open but is not focussing on anything as when someone is "Dazed". Whether brain would perceive it or not would depend on what is region of interest or object of attention. Another phenomenon which could answer your query is "Cognitive Blindness"
    – Sage
    Commented Jun 26, 2023 at 5:35

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