Critical subjective measurement of amplitude of accommodation.

The use of fixed print size to measure amplitude of accommodation by the push-up method will result in a range of angular sizes of the print at the nearpoint for patients with different amplitudes. We investigated the effect of this on measured amplitude of accommodation in 60 subjects aged 25 to 45 years. We designed a near-vision chart, based on the Bailey-Lovie near-vision charts, but for which the letter sizes on adjacent lines are varied so that the difference between the inverse of letter sizes is constant (dioptric scale) rather than the geometric ratio between letters on adjacent lines being constant (logarithmic scale). Using this new chart, we compared the amplitudes obtained using N5 print (N5 Blur method) and with two critical methods for which the print of interest was always close to threshold acuity. This was achieved by having patients' attention drawn to a smaller line of letters every time the chart was moved closer in half-diopter steps. The N5 Blur method gave considerably higher amplitude measures than the two critical methods, but the mean differences decreased markedly as age increased: 1.8 to 2.2 D for a 25- to 29-year-old group to 0.7 to 0.8 D for a 40- to 45-year-old group. We believe that the use of fixed size print for measuring amplitude of accommodation by the push-up method gives overestimations that are more marked the higher the amplitude. This occurs because smaller measuring distances that accompany the higher amplitudes will increase angular size and consequently depth-of-focus (in dioptric terms).(ABSTRACT TRUNCATED AT 250 WORDS)