I am short-sighted. However, if I remove my glasses and peer through a tiny hole made between my fingers, or a pinhole in a piece of card, I can read signs from an even greater distance than with my glasses on. Why is that?
鈥� The same effect is produced when a photographer 鈥渟tops down鈥� a camera lens 鈥� that is, reduces its aperture. Stopping down decreases the degree of blur caused when an object is out of focus. The smaller the aperture, the sharper an object appears.
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An object is blurry if the light from it is not focused to a precise point on the focal plane, be that the retina in your eye or some film. Instead, the focal point converges in front of or behind this plane, which is what happens in short and long-sightedness 鈥� known as myopia and hyperopia, respectively. In these cases, where the light cone from the lens intersects this plane it forms a disc, not a point (see upper Diagram). This disc of light is called a 鈥渃ircle of confusion鈥� (CoC) and thousands of such overlapping discs on the focal plane make for a blurry sight.FIG-mg19526232.500-1_896
As the diameter of a lens aperture reduces, so does the diameter of the CoCs, because blocking light from the outer parts of the lens makes the cone of light narrower (see lower Diagram).FIG-mg19526232.500-2_893
Smaller CoCs give a sharper image. Out-of-focus objects viewed through a pinhole therefore contain greater clarity of detail and appear sharper at the focal plane. This occurs naturally too. Many slightly short-sighted people will tell you they can see well in bright sunlight, but in twilight their vision is blurred. This is because the eye鈥檚 iris contracts in bright light to make the pupil diameter small. This shrinks the CoCs, sharpening objects usually out of focus. In twilight, the iris dilates and blurring increases.
鈥淚t is the myriad overlapping discs of light on the focal plane that make things look blurred鈥�
Interestingly, the questioner says that his vision is better with his unaided but 鈥渟topped down鈥� eyes than it is with his spectacles. I suspect his prescription is out of date. Another possibility is that the periphery of his cornea or lens may be causing localised distortions or light scatter that cannot be corrected with spectacles. Stopping down would remove these distortions, leaving only the clearer (but dimmer) image from the central retina.
Mark Shuttleworth, Newton Abbott, Devon, UK