Why do you see better through a small hole?

Answer 1

Pinhole spectacles consist of a standard spectacle frame in which are mounted opaque plastic sheets in place of lenses. These sheets have a pattern of small holes cut in them. Advertisements for "pinhole glasses" have appeared in the press and elsewhere regularly. These advertisements often make extravagant claims concerning the supposed therapeutic benefits of pinhole spectacles, including claims that they "realign light rays", "repair" the eyes, "exercise" the eyes, improve blood circulation, "teach your eyes to focus again", reduce refractive errors, or otherwise permanently improve vision. There is no objective evidence to support any of these claims. In order to see an object clearly, light passing through all parts of the pupil of the eye must be focussed to converge on a single point on the retina. If the light is not accurately focussed, it will fall on a circular region on the retina, rather than a single point, and the object will appear blurred. The size of this "blur circle", and thus the amount of blur perceived, is proportional to the degree of refractive error and the size of the pupil of the eye. Pinhole spectacles substitute a pinhole with a diameter of 1 mm or less for the eye's normal pupil, which typically has a diameter of 3 to 4 mm. This does not alter the focus of the eye, but reduces the size of the blur circles to the extent that reasonably clear vision is obtained. Pinhole spectacles have been used by optometrists and ophthalmologists for hundreds of years. Today they are mainly used as a diagnostic tool and are rarely prescribed, as in almost all cases better vision can be obtained through the use of other forms of correction. While pinhole spectacles will not directly cause any damage to the eyes, they do pose some hazards. As they rely on blocking out most of the light rays which would otherwise reach the eyes, they can reduce the size of the visual field and cause difficulties in seeing in dim lighting. Because of this they are effectively useless at night, and could cause considerable danger if they were used in these conditions. The multiple pinholes can also cause the wearer to see multiple images. There is also a risk that the use of a self-prescribed symptomatic treatment, such as using pinhole spectacles to correct blurred vision, may mask a more serious underlying condition. It is analogous to treating recurring headaches with aspirin, without ever trying to find the cause of the headaches. Self-prescription of vision correction is particularly dangerous in the case of elderly people as they have a high incidence of serious eye conditions, such as glaucoma. Pinhole glasses may alleviate their blurred vision, but in doing so may deter them from having their eyes examined properly, thus delaying the diagnosis and treatment of any underlying conditions. There is no substitute for a full vision and eye health assessment by an appropriately trained and qualified eye care practitioner. Self-diagnosis and prescription of any treatment, whether pinhole spectacles or any other form of correction, poses serious risks to ocular health and vision. Optometrists Association Australia recommends that anyone who is suffering from poor vision should seek advice from an optometrist or ophthalmologist.

Source: Optometrists Association Australia