Comfortable and Critical Print Sizes are Similar in Patients With Low Vision

Large Type on Screen
Large type on closed circuit TV for visually impaired woman. National Eye Institute, Bethseda, MD.
Using patient identification of comfortable reading size may provide better use of clinical time than a full analysis of reading speed, a study suggests.

Comfortable print size for reading in people with visual impairment appears to be similar to the critical print size (CPS)—the minimum size to achieve maximum reading speed—and could serve as a time-efficient clinical method to estimate magnification requirements, according to research published in Ophthalmic and Physiological Optics.

The researchers aimed to determine what print size a person with visual impairment considers comfortable for reading and to assess whether this parameter mirrors any of the 3 parameters currently used to identify the print size required for sustained reading tasks: CPS, minimum size for functional reading at 80 words per minute (wpm), or a size that is double the reading acuity. They assessed participants using MNREAD charts.

Investigators evaluated 47 participants (mean age 77±13, 62% women) entering low vision rehabilitation.  The most common causes of vision loss reported by participants were macular degeneration (n=24), glaucoma (n=7), diabetic retinopathy/maculopathy (n=3), and retinal vascular occlusions (n=3). 

The researchers found that comfortable print size and CPS were highly correlated (R=0.81; P <.001). While mean CPS was slightly larger than comfortable print size, the team determined the difference was not statistically significantly (mean difference, 0.05±0.18 logMAR; P =.08), and did not appear to differ across the functional range (limits of agreement, ±0.35 logMAR). 

Among participants, a subset of 41 achieved a functional reading speed of at least 80 words per minute. Researchers determined that functional reading size and comfortable print size were highly correlated (R=0.80; P <.001) and not significantly different (mean difference, 0.01±0.20 logMAR; P =.70). However, differences between comfortable and functional print sizes varied across the functional range (difference =0.25 –(0.35 x mean); R=−0.47; P <.01), indicating comfortable print size was progressively larger than functional print size at smaller mean sizes. 

Finally, investigators determined that comfortable print size and double sized reading acuity values were highly correlated (R=0.82; P <.001). However, comfortable print size was smaller than double the reading acuity size resulting in a statistically significant difference equivalent to approximately 1 logMAR line (mean difference, 0.11±0.17 logMAR; P <.001).

The researchers question, “In a low vision assessment, can we ask people to identify a comfortable print size as a value to aim for with magnification for sustained reading?” They argue that “doing so could provide an efficient use of clinical time, with identification of comfortable print size being much quicker than a full analysis of reading speed and a size plot, even when a full reading curve assessment can be semi-automated by use of an app.”

Limitations of the study include a small sample size, reliance on oral, instead of silent reading to identify comfortable print size, and failure to investigate intra-participant variability of comfortable print size.


Latham K, Macnaughton J. Is patient identification of ‘comfortable’ print size a useful clinical parameter for low vision reading assessment? Published online February 14, 2022. Ophthalmic Physiol Opt. doi:10.1111/opo.12946