The conventional formula for the increase in size of the retinal image provided by a simple magnifier is M=D/4. This formula is based on arbitrary assumptions as to the accommodative powers exerted when the observer views a near object with and without the magnifier. Since these assumptions do not apply in a majority of cases, the conventional ratings of magnification are of limited use in the prescribing of reading aids for patients with subnormal vision. More realistic formulas are proposed which are applicable to many special cases, and which take account of the fact that the increase in size of the retinal image may depend upon the way in which the magnifier is used.
It is shown that the dioptric power of the magnifier required by a given patient is determined primarily by (a) his visual acuity and (b) his near point of distinct vision. A simple test of near-acuity for meaningful text is described which gives direct and valid information as to the increase in size of the retinal image required for useful reading vision. Further studies are needed to determine the best method of measuring the accommodative powers of patients with subnormal vision.
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