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This question already has an answer here:

Most people that wear glasses or contacts can squint to reduce the blurriness of their vision.

How can this be explained, and has it ever been used in optics to enhance the focus or clarity of an image?

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marked as duplicate by John Rennie, docscience, sammy gerbil, Diracology, user36790 Jul 26 '16 at 4:04

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ youtube.com/watch?v=OydqR_7_DjI $\endgroup$ – Vishnu JK Jul 25 '16 at 17:44
  • $\begingroup$ The answer I gave you is incorrect, as far as the eye question is concerned. Please see the duplicate above for the correct answer. The telescope section, as far as I know, is correct. You might consider removing your acceptance of my answer in light of this. $\endgroup$ – user108787 Jul 25 '16 at 19:50
  • $\begingroup$ I reverted the title to correctly identify the intent of my question. $\endgroup$ – Burhan Khalid Jul 26 '16 at 6:55
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As far as the second part of your question is concerned you can directly see the image improvement with squinting. If you have a DSLR with aperture settings you keep the camera slightly defocused and now reduce the aperture you will see that the image is becoming sharper. However at the same time the image will become darker because you are collecting less and less light.

The sharpness of image comes from the increase in so called depth of focus of the lens. Due to reduction in the collection efficiency this method is not well suited for the practical applications.

With the reduction in aperture you will also reduce various aberrations of the lens which will again improve the image quality.

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EDIT Please see the comments following my answer, regarding the paragraph below , as it is incorrect.

Strabismus is the clinical name for squinting. The squint is simply compression of the eye muscles to compensate for problems with focusing and / or astigmatism.

END EDIT

In a somewhat similiar way, telescopes using adaptive optics can distort the primary mirror to allow for distortion due to atmospheric turbulence. Computer controlled jacks distort the mirror to achieve the best possible image, as illustrated in the diagram at the bottom of this answer.

enter image description here

A laser beam is used as a guide to the amount of turbulence in the upper atmosphere.

The efficiency of the system is shown below.

enter image description here

From Wikipedia Adaptive Optics

An adaptive optics system tries to correct these distortions, using a wavefront sensorwhich takes some of the astronomical light, a deformable mirror that lies in the optical path, and a computer that receives input from the detector. The wavefront sensor measures the distortions the atmosphere has introduced on the timescale of a few milliseconds; the computer calculates the optimal mirror shape to correct the distortions and the surface of the deformable mirror is reshaped accordingly. For example, an 8–10 m telescope (like the VLT or Keck) can produce AO-corrected images with an angular resolution of 30–60 milliarcsecond (mas) resolution at infrared wavelengths, while the resolution without correction is of the order of 1 arcsecond.

enter image description here

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  • $\begingroup$ Can you provide a reference to this explanation? The more common explanation is that squinting reduces the size of the aperture, thereby reducing the effect of the aberrations. Telescopes, of course, would not employ this mechanism. $\endgroup$ – garyp Jul 25 '16 at 18:39
  • $\begingroup$ Strabismus isn't the same thing as squinting in this sense at all: strabismus is a condition where your eyes won't point at the same thing: it's often called being 'cross-eyed' or 'wall-eyed' (I think depending on whether the eyes point towards or away from each other). This is also called 'a squint' in British English at least, but it's a completely different thing. $\endgroup$ – tfb Jul 25 '16 at 18:39
  • $\begingroup$ @garyp As someone who is short-sighted I am reasonably sure it's the aperture thing, yes. As a child I used to play around with squashing my eye (don't try this!) and although you get interesting optical effects, you don't get better focus (or I did not). $\endgroup$ – tfb Jul 25 '16 at 18:42
  • $\begingroup$ I think the OP should consider removing his acceptance of this answer until this answer can be justified. $\endgroup$ – garyp Jul 25 '16 at 18:46
  • $\begingroup$ @garyp I will ask the OP directly to remove acceptance, as the answer is wrong regarding the eye as listed above and edit the answer to reflect the points above. $\endgroup$ – user108787 Jul 25 '16 at 19:43

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