Figure 3.3. The Humphrey field analyser.!

Measurement of the visual acuity might not be valid unless the patient is wearing the correct spectacles. Some patients, when asked to read a Snellen chart, will put on their reading glasses. As these glasses are designed for close work, the chart might be largely obscured and the uninitiated doctor might be surprised at the poor level of visual acuity (Figure 3.5). If the

Figure 3.5. The uninitiated might be surprised at the poor level of visual acuity. D3

glasses have been left at home, long sight or short sight can be largely overcome by asking the patient to view the chart through a pinhole. Similarly, an appropriate spectacle correction (near) must be worn when testing visual fields and colour vision. In an ophthalmic department, a check of the spectacle prescription is a routine part of the initial examination. Figure 3.6 shows how the converging power of the optical media and the length of the eye are mismatched to produce the need to wear spectacles (the dotted lines indicate the paths for rays of light without any corrective lens).

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