Frequently Asked Questions:
Visual Functions and Functional Vision
Why should I use a reading test?
How can I judge the imaging range of various presbyopia corrections?
What is the best print size notation for Reading tests?
What is the best layout for Reading tests?
At this point we need to make a distinction between Visual Functions and Functional Vision. Visual functions refer to how the eye functions; functional vision refers to how the person functions. Visual functions are measured one parameter at a time in an artificial environment; functional vision refers to actual functioning in a real life environment where multiple parameters can change and interact. Although the two aspects are closely related, they should not be confused. Letter chart testing at 20 ft is an artificial task, where we are interested in one parameter: the smallest detail seen. Reading tests are closer to regular activities of daily living (ADL). The following schematic diagram places these concepts in a broader frame work.
The oblique arrows indicate that there is not just a simple cause and effect relationship between the different boxes, but that each link can be influenced by additional factors and interventions. The ultimate goal of all interventions is to optimize not just specific activities, but the ultimate Quality of Life of the subject.
It is important to realize how assessing parameters of visual functions differs from the assessment of visual abilities and functional vision. When assessing parameters of Visual Function, we create an artificial environment where we vary only one parameter at a time. When we vary the letter size on a letter chart, we test visual acuity. When we vary the contrast, we test contrast sensitivity; when we vary the illumination, we test dark adaptation. To test all three parameters, we need to do three different tests. Each test provides us with a threshold value.
When assessing Functional Vision we want an environment that is close to real life. That means that several parameters can vary simultaneously and independently. We also are interested in sustainable performance, rather than threshold performance. For instance: standard acuity (20/20) allows us to read 1 M letters (average newsprint) at 100 cm; normal visual acuity (20/16) allows us to do that at 125 cm. Yet, most people prefer to read the newspaper at about 40 cm, which is 3x above threshold. This difference between threshold performance and comfortable, sustainable performance is also known as the magnification reserve. The magnitude of the chosen reserve is not the same for all people or for all tasks.
When assessing visual functions, we limit our attention to one strictly visual parameter at a time. When assessing functional vision, on the other hand, we must deal with multiple parameters, including some that are not strictly visual. While visual acuity is an important parameter of reading ability, it certainly is not the only relevant factor. Other visual factors include illumination and contrast. If language skills and letter recognition are not trivially easy, we must resort to pictures, as discussed earlier, and to matching rather than to naming.
As part of a routine eye exam, reading tests are primarily used to verify that the prescribed reading correction is appropriate. For this limited goal, almost any reading sample, whether a letter test, a word test or a reading test, can be used.
When dealing with patients with low vision (see section E), the emphasis is not on optics, but on functional performance. Accurate measurement of reading performance is important, since it is the basis for the prescription of magnification.
Reading tests require a much larger retinal area than do letter recognition tests. They thus give us a better assessment of the peri-foveal area. Furthermore, reading (not letter recognition) is the function most patients list as their primary objective.
Routine reading tests are often performed at 40 cm (16”). However, not all near vision tasks are performed at 40 cm. This is why the array of presbyopia corrections was expanded from bifocals to trifocals to progressive lenses. More recently, the range of accommodation or pseudo-accommodation solutions has been augmented with solutions such as mono vision, multifocal lenses and accommodating IOLs.
To compare these different solutions, it is important to objectively document visual acuity at various distances. Since not all activities of daily living involve high contrast objects, it is also relevant to document the effect on low contrast acuity. Until recently no cards were available to easily document and compare these parameters.
The Colenbrander Mixed Contrast Card Set fills that gap. The set has two cards (four surfaces) and allows measurement of both high-contrast and low-contrast (10%) acuity at three distances: 40 cm (16”), 63 cm (25”) and 100 cm (40”), which are at 2.5, 1.5 and 1.0 diopter from infinity. These distances were chosen because they are spaced by two line increments on a logarithmic scale. Cords attached to the cards make it easy to maintain the accurate viewing distance for each test. The fourth side provides high and low contrast reading sentences for 40 cm.
Multifocal lenses and other presbyopia solutions involve a trade-off between ultimate clarity at one distance vs. reasonable clarity at a variety of distances. These trade-offs may be judged differently by different individuals. This has increased interest into how various interventions affect not just objectively measured visual acuity thresholds, but also the more subjective experience of satisfaction and ultimate quality of life. Assessing the goals and priorities of the patient requires the use of questionnaires, which is beyond the scope of this introduction.
Reading tests have been used for centuries and have become especially prevalent since Jaeger (1856) and Snellen (1862) each published their print samples.
Variants of Jaeger’s print samples are routinely used in US practice. Jaeger first published his print samples several years before Snellen introduced the concept of visual acuity measurement. Jaeger had to use the reference numbers of a print catalogue in Vienna in 1856 to identify his print samples. As others attempted to reproduce these samples with locally available fonts, deviations were bound to occur. This explains the wide variability in current Jaeger notations from card to card. All current Jaeger samples use larger print than Jaeger’s original and do not even cluster around average values, as indicated in Table 2.
Snellen, on the other hand, recognized the importance of a well-defined standard. His measurement unit, which would later become known as M-unit, has a physically defined size that can be reproduced or recalculated anywhere. 1 M-unit subtends 5 min of arc at 1 meter; it measures 1.454 mm or 1/7 cm (error -2%) or almost 1/16 inch (error 10%).
M-units can be applied to the capital letters on letter charts, as well as to the lower case letters in reading samples. They thus allow a comparison between letter acuity and reading acuity.
Table 2 – Letter Size Measurements on Various Jaeger Cards
The above diagram lists the Jaeger numbers that were found when print sizes were measured on 20 current cards. The black cells refer to Jaeger’s original print sizes, rounded to the nearest M-unit size on a logarithmic scale. The vertical gray columns indicate how often various letter sizes were found with the same J-designation. It shows that the print sizes labeled as J5, J6, J 7 and J8 each varied over a 3x range. 1 M print (average newsprint) was labeled as J2 on four of the 20 cards and as J8 on four others. It is clear that this makes J-numbers unfit to serve as a measurement standard.
Sometimes print samples are identified with printer’s points (N-notation in Britain). This notation originally referred to the slugs on which fonts were mounted. Most peoplle are familiar with this notation, since it is used on computers. Since the relation of letter size to slug size is not constant, the point notation may vary from type font to type font. Computer users may have noticed that 10 pt Arial is about as large as 12 pt Times Roman. As an approximation, 8 pt Arial Verdana or Courier equals about 1 M-unit (average newsprint).
Furthermore, point notation is not applicable to letter charts and to distance acuity.
In summary, Jaeger numbers and printer’s points do not allow the calculation of a near-vision acuity, since they do not have a defined numerical value and do not fit into Snellen’s acuity formula. The only notation that is standardized, can be used in calculations and is aplicable to distance charts and reading material alike, is the M-unit notation, as defined by Snellen. How to use the Modified Snellen Formula to calculate reading acuity using M-units will be discussed in the Low Vision section.
All Precision Vision reading cards carry M-unit notation, as used by Snellen.
Near vision letter tests are appropriate when vision is limited only by optical factors, such as insufficient refractive correction or by opacities. In these cases, retinal function is assumed to be normal and the blur of the area onto which each letter is projected predicts equal blur for surrounding retinal areas.
An increasingly important group of patients are those with retinal disorders, such as AMD. Here, the media are often normal and vision is limited by the retinal condition. Since the condition of the retinal area onto which a letter is projected cannot predict the condition of the surrounding retina, tests that involve a larger retinal area are desirable. For this purpose reading tests are better suited than letter tests.
Precision vision offers near-vision letter charts for use with patients whose retinal function is assumed to be normal.
Precision vision also offers near-vision reading cards for a more comprehensive assessment of reading performance.
Precision Vision offers the Colenbrander Mixed Contrast Card Set for the assessment of presbyopia corrections at various distances for both high contrast and low contrast.
Calculation of a visual acuity value requires consideration of both print size and reading distance. Note that the visual acuity labels on reading cards are valid only if the card is used at the distance for which it was designed.
Most of the Precision Vision near-vision tests have a cord attached to easily maintain the desired reading distance.
Additional reading tests will be discussed in the Low Vision section.