It is seen that in the early year of the amplitude of accommodation is about 14 D and that the near point is situated at 7 cm distance. Thereafter it gradually and uninterruptedly recedes at the age of 36 it has reached 14 cm, when the amplitude of accommodation has become halved and is now 7 D instead of original 14 D. At the age of 45 it has reached 25 cm, and the amplitude of accommodation is only 4 D; at the age of 60 only about 1 D of accommodation remains.
In most of the cases near work is done at an average distance of 28 to 30 cm away from the eyes, and therefore in the emmetrope the actual limit of clear vision is reached at 45 years when an amplitude of 3.5 to 4 diopters of accommodation remains. This however would entail working at the near point continuously and thus exercising the whole of the accommodation to obtain useful vision a condition of strain that can rarely be tolerated over any length of time. Comfort demands that about one-third of the4 accommodation be kept in reserve, so that when this limit has been reached and the near point is at a distance of 22 cm presbyopia may be said to have set in. In the emmetrope this occurs between 40 and 45 years of age, although there is some geographical, perhaps racial variation: an earlier age of onset may occur in those living in the tropics. Thereafter the accommodation must be supplemented by a convex lens if near work is to be done without strain.
A hypermetrope starts life with this near point considerably further away than that of an emmetrope so that the symptoms of presbyopis will come on earlier. Thus a hypermetrope with an error of +3 D will require exercise 7 D of accommodation to give himself an amplitude of 4 D; he may therefore show presbyopic symptoms at about 25 years of age. In a myope, on the other hand, the opposite conditions hold, and if he has an error of – 4 D presbyipia will never occur.
Presbyopia is thus a relative term, depending not only on the age but also on the refraction. It also varies with the individual and with his habits. A person who has the habit of reading with his book on his knees complains of discomfort later than one who is used to reading more closely; and the carpenter or the book-keeper or the musician will be comfortable at his work at 30 or 35 cm or over, while the seamstress or the compositor or the engraver of the same age and with the same refractive state will have been forced to use spectacles in order to see at his working distance of 20cm. Or again, the ciliary muscle may fail in states of debility or disease and the physiological accommodation be at fault. There is thus no fixed presbyopic point, and there can be no rational rule-of-thumb treatment.
Reference:- Duke-elder's Practice of refraction tenth edition
Reference:- Duke-elder's Practice of refraction tenth edition
