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Herman D’Hondt
Sydney, Australia
Remember that the eye is like a camera, the simplest form of which is the pinhole camera. To make one, take a box. Inside it, put film on one wall and make a hole in the opposite wall. That’s it.
The photos from a pinhole camera aren’t very exciting, though, because it suffers from an annoying trade-off. To let in enough light to take a photo quickly, the hole has to be large. However, to get a sharp image, the hole should be as small as possible.
When light rays from each point on the object can reach the film by taking many different paths, they blur the image. That’s what happens with a large aperture. Conversely, a small aperture leaves very few ways to reach the film, so you get a sharper, focused image. But, as fewer rays get through, less light reaches the film.
It’s for the same reason that you squint to get sharper vision: you reduce the aperture and hence improve the focus. You can get the same effect by making a tiny hole in a piece of cardboard and looking through that. As long as there is still enough light to see with, your vision will be improved.
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The iris regulates how much light enters the eye by dilating the size of the pupil in dim light and contracting it in bright light
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David Muir
Edinburgh, UK
Along with about 1.5 billion of the world’s population, I am blessed with myopia (short-sightedness). This makes distant objects appear blurred, as the eye focuses their images in front of the light-sensitive retina inside the eye, instead of on it. This type of refractive error, as with any optical system, can be increased or diminished by altering the aperture size.
In cameras, a small aperture increases the depth of field of clear focus. With the eye, a big pupil exaggerates refractive errors. A small pupil minimises them, as light passes through the centre of the eye’s lens, but not through the edges where most refraction, and thus most refractive error, occurs. On sunny days, when my pupils are less big, I have no problem identifying small birds in my garden, but on dull days, I have difficulty differentiating between a wood pigeon and a grey squirrel.
The iris regulates how much light enters the eye by adjusting the size of the pupil, which dilates in dim light and contracts in bright light. Stand in a brightly lit room, right in front of a mirror, and close your eyes for a minute to habituate them to darkness. Open your eyes and immediately observe your iris. If you are lucky, you will see the iris increase in size and the diameter of the pupil decrease, all in a second, as your eye adjusts to the brighter light to protect its internal parts, mainly the retina.
By scrunching up their eyes, a myope (short-sighted person) is diminishing the vertical aspect of their refractive error, allowing greater clarity. A hyperope (long-sighted person) can normally see distant objects clearly, but close objects are blurry. They can improve their near-vision by scrunching up their eyes, allowing them to maybe just make out writing that would otherwise be indecipherable.
Gerard Buzolic
Christchurch, New Zealand
Scrunching up the eyes helps with focusing. As we get older, our ability to focus on near objects declines. Ciliary muscles squeeze and thin fibres holding the lens of the eye in its capsule relax and stop pulling the lens into the weak-lens, thin shape needed for distance vision. However, the lens, when not stretched at all, fails to relax into the strong-lens, fat shape that it once could.
When I have forgotten my reading glasses and try to read small print, I scrunch my eyes and read through a grille of eyelashes. A pinhole needs no lens to focus an image on a screen. Looking through the thin, vertical slits made by the hairs in eyelashes has an effect similar to a pinhole in focusing the image on the retina.
If humans had eyelashes coming in from the sides as well as top to bottom, we would really have a grid of pinholes. The vertical grille is the best we can do.
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