Naked Science Forum
General Science => General Science => Topic started by: Graham on 27/05/2009 17:30:02
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Graham asked the Naked Scientists:
Dear Drs,
I very much enjoy listening to the podcasts (http://www.thenakedscientists.com/HTML/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?
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Graham asked the Naked Scientists:
What do you think?
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
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So the question is " how does squinting your eyes help them focus
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.
The same factors that determine depth of field also determine depth of focus...
Both depth of field and depth of focus increase with smaller apertures.
http://en.wikipedia.org/wiki/Depth_of_focus
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).
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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.
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Listen to the answer to this question on our podcast. (http://www.thenakedscientists.com/HTML/podcasts/show/2009.08.02/)
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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?