Science Questions

Why does squinting help you to see more clearly? How does it work

Sun, 2nd Aug 2009

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Graham asked:

Why does squinting help you to see more clearly? How does it work


Dave -   Okay.  The way your eye works in the first place is it has a lens at the front - a lens in the cornea. They act together to focus light onto the back of your eye.  Basically, a lens is a curved piece of transparent material. The light goes slightly slower in the eye irislens than in the air so when light hits it at an angle it will slow down and go on the corner.  A perfect lens is shaped so as it focuses all of the light from one point to outside your eye to one point on the back of your retina.  The problem is as things get closer and further away, you need to change the focus. 

You need to change the distance.  The way your eye does that is by changing the shape of your lens. The problem comes if youíre short-sighted or long-sighted. Then your lens isnít the right shape for the length of your eye.  And you canít adjust enough to help to see things so things look fuzzy.  But the smaller the iris, the smaller an area of lens which light can get through, the less it can go wrong so the less fuzzy, the less the errors are so the less fuzzy the image is.  So, basically, the main thing that squinting does is it reduces the area that light can get in through in the same way if you go out on a sunny day things look less fuzzy because your iris closes down.  So if you squint you should close down your eye.  You let light in through less of the lens and things look less fuzzy


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Graham asked the Naked Scientists: Dear Drs, I very much enjoy listening to the podcasts and always find your show very interesting and informative. For the first time I have thought of a question to ask. On bank holiday Monday my wife and I were watching a very colourful butterfly in our garden. We have no expertise in this subject but wondered whether it was a painted lady or a peacock etc. When trying to look it up in a nature book I realised that I didn't have my glasses on but persisted anyway through the index by squinting my eyes. Although the letters still appeared blurred they were just about readable. So the question is " how does squinting your eyes help them focus "Does it exert pressure on the eyeball and alter its' shape ? Do the eyelid hairs alter the incoming light in some manner ? Best wishes, Graham Watts. PS We think the butterfly was a painted lady! What do you think? Graham, Wed, 27th May 2009

As we grow older, the lense in our eye becomes less pliable. This results in a greater effort when the eye muscle attempts to focus the lense. I believe the squinting is a subconsious reflex which senses the greater stiffness within the lense and is trying to overcome it.

..................Ethos Ethos, Wed, 27th May 2009

When you squint your eyelids partially cover your pupils reducing their size.
This is analogous to reducing the aperture on a camera lens which increases the depth of focus and depth of field.

You could confirm my explanation is correct by looking at your book through a 2mm hole in a piece of card held close to your eye,
this will improve the sharpness of the text without squinting, (because the hole in the card is smaller than your pupil). RD, Fri, 29th May 2009

I remember, as a lad, finding this out for myself. I was diagnosed as short sighted when I was about 9 but, previously, I used to look at things at a distance, on a sunny day, through a hole I made between my fingers. Stopping down is a great way to increase the focal depth. lyner, Sun, 31st May 2009

I always thought that it put pressure on the sides of an eye with astigmatism making it less like a football and more round thus helping the eye to focus as a normal shaped eye would had it not had the astigmatism? Karen W., Wed, 5th Aug 2009

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