Katz M, Kruger PB. The human eye as an optical system. In: Tasman W, Jaeger EA, eds. Duane’s Clinical Ophthalmology [CDROM]. Vol 1. Philadelphia: Lippincott Williams & Wilkins; 2006:chap 33.
Important Axes of the Eye
Following are important definitions of terms used to describe the axes of the eye. The principal line of vision is the line passing through the fixation target, perpendicular to the corneal plane. The pupillary axis is the imaginary line perpendicular to the corneal surface and passing through the midpoint of the entrance pupil. The visual axis is the line connecting the fixation target and the fovea. The optical axis is the line that best approximates the line passing through the optical centers of the cornea, lens, and center of the fovea. Because the lens is usually decentered with respect to the cornea and the visual axis, no single line can precisely pass through each of these points. However, because the amount of decentration is small, the optical axis is considered the best approximation of this line.
The angle alpha (α) is the angle between the visual axis and the optical axis. This angle is considered positive when the visual axis in object space lies on the nasal side of the optical axis. The angle kappa (κ) is the angle between the pupillary axis and the visual axis (Fig 2-3).
Figure 2-3 Angle kappa (κ). The pupillary axis (red line) is represented schematically as the line perpendicular to the corneal surface and passing through the midpoint of the entrance pupil (E). The visual axis (green line) is defined as the line connecting the fixation target (O) and the fovea (F). If all the optical elements of the human eye were in perfect alignment, these 2 lines would overlap. However, the fovea is normally displaced from its expected position. The angle between the pupillary axis and the visual axis is called angle kappa (κ) and is considered positive when the fovea is located temporally, as is the usual case. Conditions that cause temporal dragging of the retina, such as retinopathy of prematurity, can lead to a large positive angle kappa. Clinically, this will present as pseudoexotropia. A large positive angle kappa may also mask a small-angle esotropia, which can be detected by the cover-uncover test. Angle alpha (α) is the angle between the optical axis and the visual axis of the eye and is considered positive when the visual axis in object space lies on the nasal side of the optical axis, as is normally the case. (Courtesy of Neal H. Ateb ara, MD. Redrawn b y C. H.
Wooley.)
Pupil Size and Its Effect on Visual Resolution
The size of the blur circle on the retina generally increases as the size of the pupil increases. If a
pinhole aperture is placed immediately in front of an eye, it acts as an artificial pupil, and the size of the blur circle is reduced correspondingly (Fig 2-4; Clinical Problems 2-1).
Figure 2-4 Light rays from each point on an object (upright arrow) form a blur circle on the retina of a myopic eye. The retinal image is the composite of all blur circles, the size of each being proportional to the diameter of the pupil (A) and the amount of defocus. If a pinhole is held in front of the eye, the size of each blur circle is decreased; as a result, the overall
retinal image is sharpened (B). (Courtesy of Neal H. Ateb ara, MD. Redrawn b y C. H. Wooley.)
The pinhole is used clinically to measure pinhole visual acuity. If visual acuity improves when measured through a pinhole aperture, a refractive error is usually present. The most useful pinhole diameter for general clinical purposes (refractive errors between –5.00 D and +5.00 D) is 1.2 mm. If the pinhole aperture is made smaller, the blurring effects of diffraction around the edges of the aperture overwhelm the image-sharpening effects of the small pupil. For refractive errors greater than 5.00 D, the clinician needs to use a lens that corrects most of the refractive error in addition to the pinhole.
After the best refractive correction has been determined, the pinhole can also be used with a dilated pupil. If visual acuity improves, optical irregularities such as corneal and lenticular light scattering or irregular astigmatism are likely to be present, given that the pinhole serves to restrict light to a relatively normal area of the eye’s optics. (This technique also can be used to identify optical causes of monocular diplopia.) If visual acuity worsens, macular disease must be considered, as a diseased macula is often unable to adapt to the reduced amount of light entering through the pinhole.
CLINICAL PROBLEMS 2-1
Why do persons with uncorrected myopia squint?
To obtain a pinhole effect (or rather a stenopeic slit effect). Better visual acuity results from smaller blur circles (or even smaller blur “slits”).
Does pupil size affect the measured near point of accommodation?
Yes. With smaller pupil size, the eye’s depth of focus increases, and objects closer than the actual near point of the eye remain in better focus.
Why are patients less likely to need their glasses in bright light?
One reason is that the bright light causes the pupil to constrict, allowing the defocused image to be less blurred on the retina. Another reason is that bright light increases contrast.
Because of the refractive effects of the cornea, the image of the pupil when viewed by the clinician is about 13%–15% larger than the actual pupil; this enlarged image is called the entrance pupil.
Visual Acuity
Clinicians often think of visual acuity primarily in terms of Snellen acuity, but visual perception is a far more complex process than is implied by this simple measuring system. Indeed, there are a multitude of ways to measure visual function. The following are definitions of terms used in the measurement of visual function:
The minimum legible threshold refers to 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.
The minimum visible threshold is the minimum brightness of a target at which the patient can distinguish the target from the background.
The minimum separable threshold refers to the smallest visual angle formed by the eye and 2 separate objects at which a patient can discriminate them individually.
Vernier acuity is defined as the smallest detectable amount of misalignment of 2 line segments.
Snellen visual acuity is measured with test letters (optotypes) constructed such that each letter as a whole subtends an angle of 5 minutes of arc (arcmin), whereas each stroke of the letter subtends 1 arcmin. Letters of different sizes are designated by the distance at which the letter subtends an angle of 5 arcmin (Fig 2-5). 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 2-2 lists conversions of visual acuity measurements for the various methods in use—the Snellen fraction, decimal notation (Visus), visual angle minute of arc, and base-10 logarithm of the minimum angle of resolution (logMAR). LogMAR is useful for determining the mean of Snellen visual acuity in a series.
Figure 2-5 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 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. Ateb ara, MD. Redrawn b y C. H. Wooley.)
Table 2-2
Though widely accepted, the standard Snellen eye chart is not perfect. The letters on different Snellen lines are not related to one another by size in any geometric or logarithmic sense. For example, the increase in letter size from the 20/20 line to the 20/25 line differs from the increase from the 20/25 line to the 20/30 line. In addition, certain letters (such as C, D, O, and G) are inherently harder to recognize 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 clinical trials (eg, the Early Treatment Diabetic Retinopathy Study [ETDRS] or Bailey-Lovie charts) (Fig 2-6). 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 2-6 Modified Early Treatment Diabetic Retinopathy Study (ETDRS) 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.
(Courtesy of Kevin M. Miller, MD.)
Westheimer G. Visual acuity. In: Kaufman PL, Alm A, eds. Adler’s Physiology of the Eye. 10th ed. St Louis: Mosby; 2003.