Two fundamental constraints limit the number of characters in text that can be displayed at one time—print size and display size. These dual constraints conflict in two important situations—when people with normal vision read text on small digital displays, and when people with low vision read magnified text. Here, we describe a unified framework for evaluating the joint impact of these constraints on reading performance. We measured reading speed as a function of print size for three digital formats (laptop, tablet, and cellphone) for 30 normally sighted and 10 low-vision participants. Our results showed that a minimum number of characters per line is required to achieve a criterion of 80% of maximum reading speed: 13 characters for normally sighted and eight characters for low-vision readers. This critical number of characters is nearly constant across font and display format. Possible reasons for this required number of characters are discussed. Combining these character count constraints with the requirements for adequate print size reveals that an individual’s use of a small digital display or the need for magnified print can shrink or entirely eliminate the range of print size necessary for achieving maximum reading speed.

两个基本约束限制了文本中可同时显示的字符数：打印尺寸和显示尺寸。这些双重约束在两种重要情况下发生冲突：当视力正常的人在小型数字显示器上阅读文本时，以及当视力不佳的人阅读放大的文本时。在这里，我们描述了一个统一的框架，用于评估这些限制对阅读性能的联合影响。我们测量了 30 名视力正常的参与者和 10 名弱视参与者在三种数字格式（笔记本电脑、平板电脑和手机）下的阅读速度与印刷尺寸的函数关系。我们的结果表明，要达到最大阅读速度 80% 的标准，每行需要最少字符数：视力正常的读者需要 13 个字符，弱视读者需要 8 个字符。这个临界字符数在字体和显示格式中几乎是恒定的。讨论了所需字符数的可能原因。将这些字符数限制与足够打印尺寸的要求相结合表明，个人使用小型数字显示器或放大打印的需要可能会缩小或完全消除实现最大阅读速度所需的打印尺寸范围。