Subjective Refraction

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When evaluating subjective refraction the examiner assesses the degree of ametropia. During the examination the patient is given a trial frame, specially adapted for every individual to ensure a comfortable fit.

Next, one eye (in this case the left) is occluded. The patient is asked to read the smallest line on the reading chart still clearly visible to her.

In the collection of lenses one finds spherical lenses, plus (or convex) lenses in black, and minus (or concave) lenses in red. Additionally, there are cylindrical lenses in plus and minus, although we usually use plus cylinders.

The examiner starts by using a plus 0.5 diopter spherical lens while asking the patient if the image worsens. The patient indicates that the image does worsen. This excludes hyperopia.

Now the patient is given a spherical lens of minus 0.5 diopter and asked whether her vision improves.

She affirms. This prompts the use of another minus 0.5 diopter spherical lens. Once again the patient indicates that her vision improves and that it is possible for her to read the bottom line of the reading chart. The examiner now changes the two minus 0.5 diopter lenses for a minus 1.0 diopter lens.

Now he adds another minus 0.25 diopter. This leads to a small improvement in vision, after which both lenses are replaced by a single minus 1.25 diopter lens.

When the examiner adds another minus 0.25 diopter lens, vision does not improve and the image actually gets smaller. Therefore this lens is discarded and the best spherical lens for this myopic patient is minus 1.25 diopter.

To confirm the presence of astigmatism, the subjective cross cylinder test is performed. The cross cylinder, or Jackson cylinder, is used to determine the axis and the power of the astigmatism. Because plus cylinders are conventionally used here, the axis along the two white dots is relevant.

The examiner holds the cross cylinder horizontal with the axis along the white dots. Subsequently, he turns the cross cylinder so that the axis is oriented vertically. The patient prefers the vertical axis at 90°.

The patient is then given a cylinder of plus 0.5 diopter along the axis at 90°. To compensate for the spherical effects of this lens, the examiner adds a spherical lens with half the power of the cylinder and opposite sign Thus, for a plus 0.5 diopter cylinder, a spherical lens of minus 0.25 diopter.

Source: EyeExam

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Oliver Findl - EyeExam