Selection of Glasses

From Ophthalmology Book
Correction through the use of the glasses is one of methods of refraction defect treatment. 

In low farsightedness, when far vision is normal and work at a close distance causes no discomfort, glasses are not needed.  If hypermetropia causes asthenopia, it, as well as moderate and severe hypermetropia, should be corrected through the use of glasses.  In this case, glasses should be worn all the time. 

Myopia is corrected by divergent lenses.  When choosing lenses, optometrists usually take as basis the degree of myopia corrected by the weakest divergent lens providing the highest visual acuity.  In order to prevent glasses mismatching in the so-called "pseudomyopia" in childhood and adolescence, refraction should be measured with both narrow and wide pupil.  That is why full ophthalmologic examination is recommended for children.

As a rule, in low myopia, full correction equal to myopia degree is recommended.  In this case, glasses are required only when necessary.  In medium and especially in high myopia, full correction at short-distance work results in ciliary muscle overstraining causing reading discomfort.  In such cases, two pairs of glasses are recommended: distance glasses and glasses for short-distance work (reading, computer).

Astigmatism reducing visual acuity makes short-distance work difficult, which makes glasses necessary.  For its correction, cylindrical or astigmatic lenses are used.

Age-related defects of accommodation - diminished ability to focus on near objects developing after 40 - can be regarded as a separate condition called presbyopia requiring glasses for short-distance work (reading, computer).