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.

两个基本限制限制了一次可以显示的文本中的字符数：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：视力正常的人在小型数字显示器上阅读文本，以及视力低下的人阅读放大的文本。在这里，我们描述了一个统一的框架，用于评估这些限制因素对阅读性能的共同影响。我们针对30位正常视力和10位弱视参与者测量了三种数字格式（笔记本电脑，平板电脑和手机）的阅读速度与打印尺寸的关系。我们的结果表明，要达到最大读取速度的80％的标准，每行字符数最少必须为：正常视力者为13个字符，低视力阅读者为8个字符。在字体和显示格式中，这个关键字符数几乎是恒定的。讨论了此所需字符数的可能原因。将这些字符数限制与对足够的打印尺寸的要求相结合，可以发现个人使用小型数字显示器或放大打印的需求会缩小或完全消除实现最大读取速度所必需的打印尺寸范围。