Krefman R A
Am J Optom Physiol Opt. 1980 Nov;57(11):835-8. doi: 10.1097/00006324-198011000-00009.
Low vision aids used in the spectacle plane may be either plus lenses or telescopes focused for near. The plus lens has a shorter working distance than a near point telescope of the same equivalent power. A relationship between the anterior focal length (fv) of the near point telescope and the equivalent focal length (fe) of a thin lens of equal power can be derived from the formula fv = (mag) (fe). This formula provides a basis for determining what equivalent powers are equal and interchangeable between the two types of aids. Furthermore, the advantages of the increased working distance afforded by the near point telescope can be defined.
用于眼镜平面的低视力辅助器具可以是正透镜或聚焦于近用的望远镜。正透镜的工作距离比相同等效屈光度的近点望远镜短。近点望远镜的前焦距（fv）与等屈光度薄透镜的等效焦距（fe）之间的关系可以从公式fv = (mag) (fe)推导得出。该公式为确定两种辅助器具之间哪些等效屈光度相等且可互换提供了基础。此外，还可以明确近点望远镜所提供的增加工作距离的优势。
