Reading is an important goal for low vision patients, and simple low vision aids including optical magnifiers are often the first line of management for practitioners aiding a visually impaired person in achieving their goals. To streamline the low vision assessment, predicting whether a patient will be successful reading with low vision aids is important. To make such predictions, it must first be considered what is meant by “success.”
One important outcome is the physical print size that can be read with assistance, especially for spot or survival reading tasks.1,2 The achievement of many activities of daily living depends on whether specific text sizes can be read, including tasks such as reading price tags, utility bills, and medicine labels.3 Physical print size represents the size of the type used in the required task, which is invariant of working distance, and is given here in Sloan M units. Print of physical size 1M is around the size of typical newsprint4–6 and text messages on smartphones.7 Physical print size differs from angular print size, which represents the size of the retinal image and takes into account both print size and working distance. Angular print size is a clinical measure of acuity used to represent the resolving power of the eye and is usually given in logMAR.
Reading speed is also an important outcome measure of successful reading,8–10 particularly with respect to the fluency of reading and the ability to undertake sustained reading tasks, where it is considered that a reading speed of 80 words per minute (wpm) or more is required for reading not to be frustratingly slow.1 Research on the effect of print size on reading speed has led to an understanding of the importance of not only reading acuity (the smallest print size that can be read) but also the critical print size (the smallest print size that supports reading at or near-maximum reading speed).1,8
The relationship between critical print size and reading acuity is the acuity reserve for near-maximum reading speed, henceforth described as the optimum acuity reserve. This is determined, in physical print sizes such as Sloan M, by the ratio of the critical print size to the acuity threshold size. The optimum acuity reserve therefore indicates how much bigger print size must be to be read comfortably, as opposed to at the threshold size. It also represents the amount of magnification required to move a person from just being able to see a print size to being comfortably able to do so. The value of the optimum acuity reserve is used when determining the amount of magnification required in a low vision aid, as part of a two-step process. First, the magnification required to bridge the gap between a patient's current reading acuity and the goal print size is determined, which will be individual to each person's vision and their goals. Additional magnification to provide the “comfort margin” as determined by the optimum acuity reserve is then taken into account in predicting the overall magnification required for a task. There is no optimum value for the optimum acuity reserve: larger values simply mean that more magnification is required to allow a patient to read at near their maximum reading speed. What is useful to know is how consistent optimum acuity reserves are: if values are consistent across observers, then a fixed value can be used in prescribing magnification.11,12 If values vary across observers, then individual assessment of optimum acuity reserve may be recommended in the low vision assessment.10,13
Guidelines do exist regarding the visual function necessary for spot (40 wpm) and fluent (>80 wpm) reading. The acuity reserve required for spot (1.3×) and fluent (2×) reading tasks has been outlined.1,13,14 Contrast reserve guidelines of 3× for spot reading and 10× for fluent reading have also been determined.1,15 Field of view1 and scotoma size1,10 have also been identified as factors influencing reading success.
However, guidelines are not available to predict how a patient will respond to low vision aids in terms of the physical print sizes that can be accessed. In the authors' experience, even when using the magnification guidelines of Whittaker and Lovie-Kitchin,1 some patients respond far better to magnification than others and we wanted to explore why this might be.
In addition, as optimum acuity reserve has been shown to vary markedly between patients,1,10 it has been debated whether acuity reserve should be considered at a fixed value (2:111,12) or assessed on an individual basis.13,16 The former strategy has been suggested to be acceptable for people with age-related macular disease (AMD),13 but it is not clear whether this recommendation also holds for people with low vision from other causes.
Furthermore, while reading speed has been shown to be strongly related to reading acuity in people with AMD both with11 and without10 low vision aids, and guidelines predicting whether fluent reading will be achieved by AMD patients have been suggested,11 these findings have not been confirmed for people with other causes of low vision.
The purpose of this study was to compare the clinical visual function and habitual aided reading performance of a range of participants with vision impairment from mixed pathologies. The aim of the study was to provide guidelines for practitioners to quickly assess the likelihood of their low vision patients being able to read 1M print with a low vision aid, what acuity reserve is likely to be necessary to achieve near-maximum reading speed, and whether the patient will be able to read fluently (>80 wpm).
One hundred people participated in the study, which was carried out at Essex County and Clacton and District Hospitals and Anglia Ruskin University Eye Clinic. Observers at each location were approached having attended low vision support and rehabilitation sessions and were included if they had experienced vision loss for more than 6 months which they felt caused restriction in their daily life. Those who were younger than 18 years, unable to perform verbal evaluations in English, had no perception of light in both eyes, or were cognitively impaired (as determined by a score of <22 out of a possible 28 on a reduced version of the Mini-Mental State Examination,17 which excluded two visual items from administration) were excluded. Ethical approval was granted by Anglia Ruskin University Research Ethics Committee and NHS Essex Ethics Committee. The tenets of the Declaration of Helsinki were followed, and all observers gave informed consent after the nature and possible consequences of the study were explained.
All study interviews and assessments were carried out by the same examiner, a qualified optometrist (DRT). Participation was at least 2 weeks after patients' routine low vision assessments, by which point patients had received any newly prescribed aids. Low vision assessments were not carried out by DRT but by six different optometrists working within the hospital eye service. The prescribing methods of the optometrists were not defined but involved prescribing levels of magnification to achieve access to patients' goal print sizes.
A structured face-to-face interview elicited key demographics including age, gender, primary cause of visual impairment, and the length of time since ocular diagnosis for each eye. Details of any optical low vision aids habitually used for reading were recorded, which included the use of reduced working distances (relative distance magnification) and additional light. Habitually used aids included those prescribed at recent low vision assessments.
Clinical Visual Function
Distance visual acuity (VA) was measured binocularly with habitual spectacle prescription using an externally illuminated Bailey-Lovie chart18 at 3 m (chart luminance 95–100 cd/m2). Acuity was measured on a per letter basis until no letters on a line could be correctly identified.19,20
Contrast sensitivity was measured binocularly using a Pelli-Robson chart21 at 1 m (chart luminance 95–110 cd/m2). Observers wore any habitual distance spectacle prescription with a +1.00 D addition where necessary. Contrast sensitivity was measured per letter until no letters in a triplet could be correctly identified.22
Static threshold binocular central visual fields were conducted with a Humphrey Field Analyzer using a Central 30-2 SITA-Fast strategy.23 The mean threshold24,25 of the central 5, 10, and 30° were calculated and used for analysis.
Near reading performance was measured binocularly with a MNRead Chart.26 Reading performance was assessed binocularly at 40 cm using a static reading stand with observers wearing any habitual distance spectacle prescription and a corresponding +2.50 D addition where necessary. If the largest sized texts at this distance could not be seen, the chart was moved closer in a log unit progression (minimum 8 cm) until the observer was able to read the largest sized texts (maximum +2.0 logMAR) and the reading addition was altered appropriately. The chart luminance was between 110 and 150 cd/m2. Text size was expressed in terms of its angular size (logMAR).
Aided Reading Performance
A further assessment of reading function with low vision aids was made using a different version of the MNRead chart from that used for clinical near vision assessment. Any habitual spectacle correction and/or low vision aid commonly used for reading tasks was utilized, with participants using the aid monocularly or binocularly as required to best reflect habitual function. Chart luminance varied throughout the evaluation (e.g., with the use of illuminated low vision aids and variable working distances) depending on observer preference. Because the working distance varied during the assessment, logMAR notation was not used, as its calculation would be dependent on working distance measurement. The physical print size accessed was instead noted in terms of Sloan M. See Appendix (available at http://links.lww.com/OPX/A92) for comments on conversion between Sloan M and N point sizes.
For both aided and clinical reading measures, near text acuity, maximum reading speed, and critical print size were determined. Clinically assessed reading acuity was determined following the MNRead manufacturer's instructions27 according to the below equation:
Where reading function was assessed at shorter working distances, the value of 1.4 in the above equation was replaced by the appropriate figure (36 cm: 1.45; 32 cm: 1.5; 28 cm: 1.55; 24 cm: 1.62; 20 cm: 1.7; 8 cm: 2.1).27
Maximum reading speed was determined as the mean of the fastest three readings obtained28 and expressed in words per minute. Critical print size was determined as the smallest print size at which reading was achieved at 80% of the maximum reading speed.28
Data were analyzed using SPSS version 16. Where stepwise multiple regressions have been used, these are linear regressions by the forward method, with probability of F to enter of 0.05 and for removal 0.10. In all regressions, fewer than 5% of cases had standardized residuals >±2, as would be expected in an accurate model. Unless stated, goodness-of-fit statistics showed that Cook's distances were <0.8, Mahalanobis distances <8, and leverage <0.1, indicating that no individual cases unduly influenced the models.
Table 1 summarizes the descriptive characteristics of the study population. Observers were predominantly older adults with established visual impairment and pathologies reflecting the mix on the UK register of sight impaired.29 As many of the subjects had macular dysfunction likely to predominantly affect central vision, the smaller number of subjects likely to have predominantly peripheral loss (glaucoma, n = 9; retinitis pigmentosa, n = 2) are highlighted in the subsequent figures to indicate how these subjects compare to the sample as a whole.
Table 2 provides details of the aids used by observers for the assessment of aided reading function. Eighty-eight percent of observers reported using these low vision aids on a regular (>4 days per week) or daily basis, with 13% reporting use on 3 days or fewer per week.
Details of the clinical visual function and aided reading measures within the sample are given in Table 3. In the clinical assessment of reading function, 22 of the participants had visual impairment that was severe enough that even with the largest print sizes available (up to +2.0 logMAR), it was not possible to obtain three readings reflecting a maximum reading speed plateau. In these cases, a single reading was used to determine the clinical maximum reading speed and subsequently the size of the clinically assessed critical print size. This was the best possible action under the circumstances but may represent a ceiling effect in that higher maximum reading speeds and critical print sizes might have been obtained had larger sizes been available. However, when assessing aided reading function, the maximum reading speed for all subjects was determined from three readings on the maximum reading speed plateau. For subjects whose maximum reading speeds were derived from three readings in both conditions, the mean clinically assessed maximum reading speed was 118 wpm, improving to 133 wpm with low vision aids (dF 77, t = −5.81, p < 0.001). The equivalent comparison for the group with less robust maximum reading speeds in the clinical assessment shows that maximum reading speed increased from 43 wpm when assessed clinically to 45 wpm with low vision aids (dF 21, t = −0.55, p = 0.68), suggesting that the maximum reading speed in the clinically assessed condition is unlikely to have been markedly underrepresented.
Table 4 indicates the relationships between the functions assessed. All visual functions were significantly related to one another. Multicolinearity of >0.90 was observed between distance VA and clinical reading acuity (r = 0.92), between clinical reading acuity and critical print size (r = 0.94), and between clinical and aided maximum reading speed (r = 0.94). As the primary objective was to examine reading function, clinical reading acuity was retained in the regression analyses in preference to distance VA. Clinical reading acuity was also retained in preference to the less easily established clinical critical print size. Where relevant, only one measure of maximum reading speed was used in each regression.
Aided Reading Acuity
To determine the clinical visual function assessments best reflecting aided reading acuity, stepwise multiple regression was carried out, with results as shown in Table 5. The best test of clinical visual function for predicting the smallest physical print size that can be accessed with low vision aids is the clinically determined angular reading acuity (logMAR at specified working distance). Those reading smaller print sizes with low vision aids have better clinical reading acuity, and the relationship is shown in Fig. 1. Contrast sensitivity also added significance to the model.
In Fig. 1, the horizontal line indicates a print size of 1M. Observers falling below this line are able to access print of this size or smaller with their low vision aids. The vertical line represents a clinically assessed reading acuity of 0.85 logMAR. Splitting the data at this point maximizes the positive predictive value (98%) of the logMAR acuity predicting the ability to be able to access 1M print. The positive predictive value is calculated from TP/(TP + FP),30 where the true positive (TP) represents those for whom the clinical reading acuity is better than 0.85 logMAR and are able to read 1M with a magnifier, while the false positive (FP) represents those for whom the clinical reading acuity is better than 0.85 logMAR but are not able to read 1M print with a magnifier. Those with clinical reading acuity of 0.85 logMAR or better are very likely to be able to read 1M, whereas those with acuity worse than 0.85 logMAR may or may not be able to access 1M print with low vision aids.
A stepwise multiple regression was performed with the subset of observers who had clinically assessed reading acuity of poorer than 0.85 logMAR. The best predictor of the physical print size that they could access was contrast sensitivity (Table 6), with those responding better to magnification having better contrast sensitivity. Splitting the data at a cutoff point of 1.05 logCS maximizes the positive predictive value (77%) of the contrast sensitivity predicting the ability to be able to access 1M print in this group of observers with poorer clinical reading acuity.
The two factors of clinical reading acuity and contrast sensitivity can be used to predict the ability of an observer to access print of 1M or smaller with low vision aids, as summarized in the flowchart in Fig. 2. This two-step guideline predicts with 87% accuracy the ability of an observer in this sample to read print of 1M or smaller with low vision aids.
Distance and Reading Acuity
It has been suggested13 that larger differences between distance and near VA (>0.1 logMAR) may be related to scotoma size and position in people with macular dysfunction. To assess this hypothesis, the difference between distance VA and clinical near reading acuity was calculated. The mean difference was +0.03 ± 0.19 logMAR (with positive values indicating that the near reading acuity was better than the distance VA), with a range from −0.66 logMAR to +0.50 logMAR. Correlation analysis (Pearson two-tailed test) indicates that as the difference between distance and near acuity becomes more positive, there is a statistical association with higher clinical maximum reading speed (r = +0.49, p < 0.01), better status of the central 5° field (r = +0.40, p < 0.01), and better contrast sensitivity (r = +0.27, p < 0.05). These figures provide some support for the hypothesis, although the relationships account for little of the variance in the difference between distance and reading acuities (R2 = 0.08–0.24).
Optimum Acuity Reserve
The optimum acuity reserve specifies how much bigger print for comfortable reading needs to be than print that can just be read at threshold and, in Sloan M terms, is calculated by the critical print size divided by the reading acuity. The optimum acuity reserve therefore indicates how much more magnification an observer needs to read comfortably as opposed to at their acuity limit.
The median optimum acuity reserve for aided reading was 2.00 (interquartile range 1.58–2.50). Stepwise multiple regression was used to determine the best predictors of aided optimum acuity reserve. Using only the clinically assessed variables (detailed in Table 5), none were selected as predicting optimum acuity reserve. Repeating the analysis with the inclusion of parameters assessing aided reading gives the result shown in Table 7. Though a relatively weak correlation, the best predictor of aided optimum acuity reserve was the aided reading acuity, shown in Fig. 3, with higher acuity reserves tending to be associated with better aided reading acuity. This regression also showed some evidence of undue influence from two cases with standard residuals >±3. Repeating the regression in the absence of these cases did not alter the parameters selected.
From Fig. 3, observers who responded poorly to magnification and could read only larger print sizes with low vision aids tended to be in the lowest quartile of acuity reserve requirements (up to 1.58× more magnification required to achieve near maximum reading speed than acuity threshold). Half of the sample required text that was around twice as big (1.58–2.50× bigger) to be able to read comfortably as opposed to at their acuity limit. The 25% of observers who required a greater optimum acuity reserve than 2.5× tended to have better acuity with a magnifier (at least 1.2M).
Maximum reading speed for the short MNRead sentences as assessed clinically and with low vision aids were highly correlated (Spearman two-tailed test r = 0.94; Table 4). Reading speed with low vision aids (median 119 wpm) was slightly faster than the reading speed clinically assessed at a specified working distance (median 112 wpm) (t = −5.46, p < 0.001). Therefore, the best predictor of maximum reading speed with low vision aids is clinically assessed maximum reading speed. Of the 100 subjects, only 10 who did not read fluently (<80 wpm) in the clinical assessment were able to read fluently with their low vision aid. The remaining 90 subjects fell in the same group (fluent or non-fluent readers) both with and without low vision aids.
Stepwise multiple regression was performed, excluding clinically assessed reading speed as a variable due to multicolinearity. The regression shows that the best predictor of aided maximum reading speed, other than clinically assessed maximum reading speed, is clinically assessed reading acuity, as shown in Table 8 and Fig. 4.
In Fig. 4, the horizontal line represents a reading speed of 80 wpm, consistent with the ability to read fluently.1 The vertical line represents a cutoff value for clinical reading acuity of 1.0 logMAR. Those with acuity better than 1.0 logMAR are generally able to read fluently with a low vision aid, while those with worse acuity are generally not, with this prediction holding true for 85% of observers in the sample.
While the group of participants had mixed causes of visual impairment, the primary diagnosis of the largest group was macular dysfunction. It is possible that people with central visual loss have different requirements for reading than people with peripheral visual loss. In a group with mixed visual impairment, it can be difficult to distinguish between people with mainly central and mainly peripheral visual loss. For example, diabetic retinopathy can affect central and/or peripheral vision, and people may have secondary causes of visual loss. However, subjects who could reasonably be expected to have predominantly peripheral visual loss (nine with glaucoma and two with retinitis pigmentosa) are identified within the figures presented and fit in with the pattern of the data as a whole. There is no evidence for the guidelines presented being dependent on the cause of visual loss.
Accessing Physical Print Sizes
Predicting whether a patient will be able to access 1M print with a low vision aid can be achieved by assessing near acuity at a specified working distance and contrast sensitivity (Fig. 2). If a patient reads 0.85 logMAR or better on an MNRead chart under standardized conditions (or potentially any other log-scaled reading chart such as the Bailey-Lovie word chart31), the prospects are good for achieving at least spot reading of 1M print. If a patient reads worse than 0.85 logMAR, contrast sensitivity should be assessed. If a patient's threshold on a Pelli-Robson chart lies on the bottom four lines of the chart (better than 1.05 logCS; normal or noticeable loss), then 1M print is likely to be accessible with a low vision aid. If the Pelli-Robson threshold lies in the top four lines of the chart (1.05 logCS or worse; significant or severe loss), 1M print is less likely to be accessible, even with low vision aids.
Clinical decision making on the basis of these findings should be related to the patient's goals. If the goal is to access 1M print, and this appears achievable from the clinical results, continue toward prescribing optical low vision aids as a first-line management option. If the goal does not appear likely to be achieved, optical low vision aids should still be tried because the model is not completely predictive, but consider at an early stage alternative options such as sensory substitution methods and electronic magnifiers with the opportunity to provide enhanced object contrast.
The findings highlight the importance of assessing contrast sensitivity in the low vision patient, particularly those with poorer acuity. Similar to our findings, contrast sensitivity measurement has previously been found to relate to observed difficulty with reading tasks32–36 and to perceived difficulty in reading.23,37 Lovie-Kitchin14 has proposed that low-contrast VA or low- luminance and low-contrast SKILL card38 assessment is more useful in predicting function than contrast sensitivity. Although we have not assessed SKILL or low contrast acuity in the current study, and so cannot compare these functions, we would strongly suggest that some assessment of low contrast visual function is vital to include in routine low vision assessment.
All participants had recently been assessed for low vision aids and 88% reported regular use of their aids for reading. However, it is possible that the clinicians prescribing the participants' low vision aids did not prescribe magnifiers that optimized the critical print size achieved or allowed maximum reading speed at their goal print size. Also, while maximum reading speed could be determined on the MNRead chart with low vision aids in all cases, there were some ceiling effects in determining maximum reading speed and critical print size in the clinically assessed condition resulting from the maximum available print size of +2.0 logMAR, such that these may have been underestimated for some subjects with poor acuity. However, the regression analyses show no significant evidence of having been influenced by outliers, and the cutoff values determined would be uninfluenced by these ceiling effects. Any ceiling effects would also affect clinicians using the charts but would not affect their use of these guidelines in practice.
Optimum Acuity Reserve
We find an average (median) aided optimum acuity reserve of 2.0:1 (interquartile range 1.58–2.50; minimum 1.0:1, maximum 9.1:1). Finding a wide range of acuity reserves is consistent with other authors,13 including Cacho et al.10 who noted a mean acuity reserve of 2.5:1, rising as high as 11.6:1. The optimum acuity reserve required is therefore considered here in three groups: those with below average reserve needs (<1.58:1), with average reserve needs (1.58–2.50:1), and with high acuity reserve needs (>2.50:1).
Low Optimum Acuity Reserve
Those observers with poor magnifier-aided reading acuity tended to have low optimum acuity reserves (Fig. 3), although some observers with good acuity also had low acuity reserves. A truncated range of available print sizes may have contributed to low optimum acuity reserve values in some cases with poor aided reading acuity. A significant relationship between poorer aided reading acuity and lower optimum acuity reserve has also previously been observed in children with low vision.39
Average Optimum Acuity Reserve
Fifty percent of the observers in this study would be adequately served by an acuity reserve of 2:1 to achieve close to their maximum reading speed. This finding is consistent with previous suggestions12,13 that using a fixed acuity reserve of 2:1 (three lines on a logMAR chart) is adequate for most observers with macular disease. We extend this finding to being applicable to all low vision observers and not just those with macular disease.
The clinical implication of this finding is that having measured clinical reading acuity and compared it to the patient's goal print size, the likely magnification required for fluent reading will be that which makes print half the size of the goal print size just readable to the patient. The average patient will also require different levels of magnification in the low vision aids that they use for spot and for fluent reading tasks.
High Optimum Acuity Reserve
An optimum acuity reserve above 2.5:1 is required by 25% of the low vision observers. Fig. 3 shows that these observers tend to be those responding better to magnification who are able to read 1.2M or smaller with a low vision aids, of whom 31% required more than 2.5:1 acuity reserve to achieve near maximum reading speed.
Therefore, we agree with Cheong et al.12 that while using a fixed optimum acuity reserve of 2:1 is adequate for most low vision observers, some people require greater magnification for which an individualized assessment of acuity reserve would be relevant. However, it is difficult to predict who these people might be, other than that they are likely to have good acuity with a low vision aid. Neither we nor other authors10,11,13 have been able to identify factors that will reliably predict which patients will have high acuity reserve needs. However, clinicians do need to bear in mind that one-third of patients with good acuity with a low vision aid will need more magnification than expected to achieve their best reading speed. If a patient does not respond as expected to a 2:1 acuity reserve, it is suggested to present magnification representing a greater acuity reserve, such as 3:1 or 4:1, and explore whether improvement can be made in reading speed. Alternatively, undertaking a full MNRead chart assessment to directly assess the critical print size may be valuable. Clinically, those with a large acuity reserve will benefit from large jumps in magnification to achieve their maximum reading speed. They may particularly benefit from the greater magnifications provided by electronic low vision aids for sustained reading.
Maximum reading speed for sentences is no slower with low vision aids than when assessed clinically, similar to previous findings.12,40,41 Note however that short MNRead sentences rather than longer passages were used to assess reading speed. Reading speed might be expected to drop with longer passages,12,42 and the influence of factors such as the ability to manipulate the magnifier efficiently might be stronger.43
In fact, maximum reading speed for short sentences with a magnifier was well predicted by clinically assessed reading speed, assessed with large print and an appropriate add. Correlation between the two reading speeds was high (r = 0.94), with aided reading speeds only marginally higher than clinically assessed speeds. Clinicians can predict that those reading slowly on large print with an appropriate add will also read slowly with low vision aids.
Because reading speed is not always formally assessed in low vision examinations, it is also useful to consider other factors that predict reading speed with a low vision aid. Other than clinically assessed reading speed, we find that aided maximum reading speed is best predicted by clinically assessed reading acuity, with significant secondary factors of central field status and contrast sensitivity. That reading speed is best predicted by clinical reading acuity and a measure of central field status has also been shown by Cacho et al.10 in AMD subjects, with a similar amount of variance accounted for (R2 = 60% in their study and 65% in present study). The similarities demonstrated are despite differences in the assessment of central field status, as Cacho et al. used a kinetic technique (modified Bjerrum screen), and the present study used mean threshold of the central 5° field assessed with static perimetry. Techniques such as landmark-driven fundus perimetry, or microperimetry, using instruments such as the Nidek MP-1 are becoming more accepted44,45 and may be preferable for assessing central scotomas, but the purpose of this study was to use clinical methods that are within the reach of the majority of low vision practitioners. Validated methods of assessing scotomas in the clinic are needed13 and might make predictions of function based on scotoma size and position stronger and more clinically relevant.
A cutoff value for clinically assessed reading acuity of better than 1.0 logMAR predicted fluency of reading in low vision observers with 85% accuracy. This cutoff is in agreement with Lovie-Kitchin et al.11 who found the best predictor of maximum rauding rate (reading for understanding) with a magnifier was near acuity and determined that the same value of 1.0 logMAR predicted fluent reading in a smaller group (n = 22) of subjects with macular disease.
CONCLUSIONS AND CLINICAL RECOMMENDATIONS
In conclusion, practitioners can use the clinical guidelines outlined in Table 9 to predict low vision patients' performance with low vision aids from reading acuity and contrast sensitivity. In addition, maximum reading speed assessed with large print at a specified working distance is likely to be similar to maximum reading speed with low vision aids.
Department of Vision & Hearing Sciences
Anglia Ruskin University
Cambridge CB1 1PT United Kingdom
DRT was supported by a College of Optometrists' Research Scholarship.
This paper was presented in part at Vision 2011, Kuala Lumpur, Malaysia, February 2011.
The appendix, with comments on conversion between Sloan M and N point sizes, is available online at http://links.lww.com/OPX/A92.
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