Clinicians often think of visual acuity primarily in terms of Snellen acuity, but visual perception is a far more complex process than is encapsulated by this single measurement. Indeed, there are many ways to measure visual function. The following are definitions of terms used in the measurement of visual function:
Minimum legible threshold: the point at which a patient’s visual ability cannot further distinguish progressively smaller letters or forms from one another; Snellen visual acuity is the most common method of determining this threshold
Minimum visible threshold: the minimum contrast of a target at which the patient can distinguish the target from the background
Minimum separable threshold: the smallest visual angle formed by the eye and 2 separate objects at which a patient can discriminate them individually
Vernier acuity: the smallest detectable amount of misalignment of 2 line segments
Snellen visual acuity is measured with test letters or similar targets (optotypes) constructed such that each optotype as a whole is 5 times larger than the individual strokes or gaps that make up the optotype (eg, the horizontal lines, and the spaces between them, of the letter E, or the gap that differentiates a circle from the letter C). Letters of different sizes are designated by the distance at which the letter subtends an angle of 5 arcmin (Fig 3-5). The Snellen chart is designed to measure visual acuity in angular terms. However, the accepted convention does not specify visual acuity in angular measure; instead, it uses a notation in which the numerator is the testing distance (in feet or meters) and the denominator is the distance at which a letter subtends the standard visual angle of 5 arcmin. Thus, on the 20/20 line (6/6 in meters), the letters subtend an angle of 5 arcmin when viewed at 20 ft. In examination rooms with shorter distances than 20 ft (6 m), mirrors can be used to increase the viewing distance. On the 20/40 (6/12) line, the letters subtend an angle of 10 arcmin when viewed at 20 ft, or 5 arcmin when viewed at 40 ft. The “40” in the 20/40 letter (or the “12” in the 6/12 letter) refers to the viewing distance at which this letter subtends the “normal” visual angle of 5 arcmin. Table 3-2 lists conversions of visual acuity measurements for the various methods in use—the Snellen fraction, decimal notation (Visus), visual angle in minutes of arc, and base-10 logarithm of the minimum angle of resolution (logMAR). LogMAR is useful for averaging the results of multiple measurements of Snellen visual acuity.
Figure 3-5 Snellen letters are constructed such that they subtend an angle of 5 arcmin when located at the distance specified by the denominator. For example, if a Snellen E is about 26 mm in height, it subtends 5 arcmin at 60 ft. Correspondingly, a 26-mm letter occupies the 20/60 line of the Snellen chart at the standard testing distance of 20 ft.
(Courtesy of Neal H. Atebara, MD. Revised illustration based on a drawing by C. H. Wooley.)
Table 3-2 Visual Acuity Conversion Chart
Though widely accepted, common Snellen eye charts are not perfect. The letters on different Snellen lines are not necessarily related to one another by size in any regular geometric or logarithmic sense. For example, the increase in letter size from the 20/20 line to the 20/25 line (an increase of 25%) differs from the increase from the 20/25 line to the 20/30 line (an increase of 20%). In addition, certain letters (such as C, D, O, and G) are inherently harder to recognize or to distinguish than others (such as A and J), partly because there are more letters of the alphabet with which they can be confused. For these reasons, alternative visual acuity charts have been developed and popularized in high-quality clinical trials (eg, the Early Treatment Diabetic Retinopathy Study [ETDRS] or Bailey-Lovie charts) (Fig 3-6). These charts feature careful choices of optotypes, uniform proportional (logarithmic) progression of optotype size from line to line, and the same number of optotypes on each line. Computer-based acuity devices that display optotypes on a monitor screen have also become popular because they allow presentation of a random assortment of optotypes and scrambling of letters, thereby eliminating problems associated with memorization by patients.
Figure 3-6 Modified Early Treatment Diabetic Retinopathy Study (ETDRS)-type eyecharts with alternative optotypes. A, Visual acuity chart produced by the Lighthouse. The chart is intended for use at 20 ft (6 m) but can also be used at 10 ft (3 m) or 5 ft (1.5 m) with appropriate scaling. The optoptypes are Sloan letters. B, Tumbling E optotypes. C, Landolt C optotypes. D, HOTV optotypes. E, Lea symbols.
(Part A courtesy of Kevin M. Miller, MD; parts B–E courtesy of Precision Vision.)
The original Snellen eye chart used ordinary typography font letters of various sizes. However, it is preferable to choose optotypes from simpler fonts that better present identifying features of 20% of the overall letter height, and to restrict the choice of letters to a subset of the alphabet that is as uniform in distinguishability as possible. A standard set of letters in a simple, sans-serif font, known as the Sloan Letters (C, D, H, K, N, O, R, S, V, and Z) is commonly used. (Note that the letter E is not a Sloan letter.) Alternative optotypes are available for patients with special needs (Fig 3-6 B–E). The tumbling E and Landolt C optotypes are suitable for preliterate patients. The 4-letter subset HOTV can be used with a reference sample to allow patients to point to the optotypes which they see, even if they cannot name the letters. Picture charts are often useful for small children. The carefully calibrated Lea symbols (circle, square, house, apple) are greatly preferable to the older traditional picture set (hand, horse, telephone, bird, birthday cake), because the former blur into indistinguishable circles with decreasing acuity, whereas the latter can often be recognized even after the smallest details can no longer be resolved.
Levi DM. Visual acuity. In: Levin LA, Nilsson SFE, Ver Hoeve J, Wu SM, eds. Adler’s Physiology of the Eye. 11th ed. New York: Elsevier; 2011.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.