Wolffsohn James S, Eperjesi Frank
Neurosciences Research Institute, Aston University, Birmingham, UK.
Clin Exp Optom. 2005 Mar;88(2):97-102. doi: 10.1111/j.1444-0938.2005.tb06674.x.
Prescribing magnification is typically based on distance or near visual acuity. This presumes a constant minimum angle of visual resolution with working distance and therefore enlargement of an object moved to a shorter working distance (relative distance enlargement). This study examines this premise in a visually impaired population.
Distance letter visual acuity was measured prospectively for 380 low vision patients (distance visual acuity between 0.3 and 2.1 logMAR) over the age of 57 years, along with near word visual acuity at an appropriate distance for near lens additions from +4 D to +20 D. Demographic information, the disease causing low vision, contrast sensitivity, visual field and psychological status were also recorded.
Distance letter acuity was significantly related to (r = 0.84) but on average 0.1 +/- 0.2 logMAR better (1 +/- 2 lines on a logMAR chart) than near word acuity at 25 cm with a +4 D lens addition. In 39.8 per cent of patients, near word acuity was more than 0.1 logMAR worse than distance letter acuity. In 11.0 per cent of subjects, near visual acuity was more than 0.1 logMAR better than distance letter acuity. The group with near word acuity worse than distance letter acuity also had lower contrast sensitivity. The group with near word acuity better than distance letter acuity was less likely to have age-related macular degeneration. Smaller print size could be read by reducing working distance (achieved by using higher near lens additions) in 86.1 per cent, although not by as much as predicted by geometric progression in 14.5 per cent.
Although distance letter and near word acuity are highly related, they are on average 1 logMAR line different and this varies significantly between individuals. Near word acuity did not increase linearly with relative distance enlargement in approximately one in seven visually impaired, suggesting that the measurement of visual resolution over a range of working distances will assist appropriate prescribing of magnification aids.
处方放大倍数通常基于远视力或近视力。这假定视觉分辨的最小角度在工作距离上保持恒定，因此当物体移至更近的工作距离时会放大（相对距离放大）。本研究在视力受损人群中检验这一前提。
前瞻性地测量了380名年龄超过57岁的低视力患者（远视力在0.3至2.1 logMAR之间）的远字母视力，以及在添加+4 D至+20 D近用镜片时适当距离下的近单词视力。还记录了人口统计学信息、导致低视力的疾病、对比敏感度、视野和心理状态。
远字母视力与（r = 0.84）显著相关，但平均比添加+4 D镜片时25 cm处的近单词视力好0.1 +/- 0.2 logMAR（在logMAR图表上相差1 +/- 2行）。在39.8%的患者中，近单词视力比远字母视力差超过0.1 logMAR。在11.0%的受试者中，近视力比远视力好超过0.1 logMAR。近单词视力比远字母视力差的组对比敏感度也较低。近单词视力比远字母视力好的组患年龄相关性黄斑变性的可能性较小。86.1%的患者通过缩短工作距离（通过使用更高度数的近用镜片实现）可以阅读更小字号的字体，尽管在14.5%的患者中缩小程度未达到几何级数预测的程度。
尽管远字母视力和近单词视力高度相关，但它们平均相差1 logMAR行，且个体之间差异显著。在大约七分之一的视力受损者中，近单词视力并未随着相对距离放大而线性增加，这表明在一系列工作距离上测量视觉分辨力将有助于合适地开具放大辅助器具的处方。
