Vewing without Spectckles in Hypermetropia

After 40 years of age it is physiological condition ( hypermetropia) that glasses are necessary for most people during their near vision. Suppose you incidentally left your glass at home. You have to read a telephone number in a visiting card. What will you do now? You will seek help from anybody nearby. He may be busy or illiterate. But don't be worry. You can do it yourself and its' easy. First curl your index finger on itself against thumb and so that a small hole is made. Now look through the hole with one eye in bright light to read anything that you can't read with naked eye ( the other eye must be closed). Smaller the size of the hole better will be the resolution of the image. Alternatively if time allowed make a small hole on a paper card ( e.g, visiting card) by a needle ( James clip or pin ) or tooth pic. Look through the hole and you can now read easily the smaller letters on a visiting card or news paper or on computer monitor for a considerable time.

How does it work:

Veiwing through fingers

Vewing through hole in card


Explanations:

1. Depth of focus of visual field and the size of aperture has a reciprocal relation in photography. In a manual camera it is possible to control this system. But our eye is like an automatic camera. Its size of aperture ( pupil) is controlled by the illumination of visual area. In hypermetropic people focusing power of eye is somewhat is impaired even in presence of high illumination of visual field. In this case if an additional diaphragm is used in front of iris with an aperture there will be increase in the depth of focus of the visual field. So in this way visual acuity is increased by increasing depth of focus by passing the light from a bright visual field through double apertures.


2. The air trapped in the hole in the card or between fingers behalves like a convex lens ( power approximately 0.75+). One can test it by a standard test for a convex lens by moving the card with the hole when the image behind it will move in opposite direction. It may be due effect of surface tension that may condense the trapped air in the hole which become a small convex lens.