Ametropia

Question 1

A 20/80 standard Snellen visual acuity letter viewed at a distance of 32 feet will subtend the same visual angle at the patient’s eye as which of the following standard Snellen letters viewed at a distance of 20 feet?

20/20
20/60
20/50
20/70
20/30
20/40

Answer 1

20/50

Explanation
Since the Snellen system of visual acuity notation is linear and based on visual angles subtended at the eye, equivalent visual acuities with different letter sizes and viewing distances can be determined with the use of ratios. In this case, the equivalent visual acuity at 20 feet for the given acuity of 20/80 at 32 feet (32/80) is determined by the ratios of 32/80 = 20/X. Thus solving for the value of X = 20*80/32 or 50. The equivalent acuity is then 20/50.

Question 2

A thick lens system has a center thickness of 1cm, index of 1.50, plane front surface, and back surface power of +20 diopters. What is the equivalent power of the lens?

+17 diopters
+23 diopters
+20 diopters
+21 diopters

Answer 2

+20 diopters

Explanation
De = D1 + D2 - (t/n) x D1D2
De = equivalent power, D1 = front surface power, D2 = back surface power
t = thickness of lens system, n = index between the 2 surfaces

In the above question, t = 1cm (0.01 m), n=1.5, D1 = 0 and D2 = +20

De = 0 + 20 -(0.01/1.5) x (0 x 20) 
De = 0 + 20 - 0 
De = +20 diopters 

The equivalent power of this thick lens system is +20 diopters, which is also equivalent to the back vertex power and back surface power of the lens. Therefore, to get the most accurate reading of this lens, it should be placed in a lensometer with the back surface against the stop in order to measure the back surface power.

Question 3

A 6-foot tall man wishes to buy a plane mirror in which he can visualize his whole length at the same time. How tall must the mirror measure in order for the above to occur?

3 feet tall
6 feet tall
2.3 feet tall
5.2 feet tall
4.5 feet tall

Answer 3

3 feet tall

Explanation
In order for a person to see their entire reflection, a plane mirror must be half as tall as the person. This holds true regardless of the position of the person. For the above example, 6/2= 3 feet.

Question 4

In retinoscopy, which of the following reflexes is indicative of a high myopic prescription?

A narrow, slow against motion reflex
A narrow, slow with motion reflex
A narrow, fast with motion reflex
A broad, fast with motion reflex
A broad, fast against motion reflex
A narrow, fast against motion reflex

Answer 4

A narrow, slow against motion reflex

Explanation
The location of the far point of the eye will change the intensity, speed and width of the reflex. As one nears neutrality, the reflex becomes wider/broader, brighter and faster-moving. Therefore, an uncorrected high myope will have a narrower, slower against motion reflex than a mild uncorrected myope.

Question 5

Using the Feinbloom chart at a distance of 5 feet, your patient is able to read the number ‘7’ (20/700 at a testing distance of 20 feet) on the first page. What would be the corresponding acuity at 20 feet?

20/2800
20/1000
20/700
20/1600
20/1400

Answer 5

20/2800

Explanation
The Feinbloom chart is useful for low vision patients and offers good portability. Although the chart is calibrated for a 20-foot distance, it can be used at any distance as long as the resulting acuity is altered accordingly. At a distance of 20 feet the initial optotype is 20/700; therefore, if the testing distance is moved to 5 feet (the clinician would record 5/700), the numerator and the denominator should be multiplied by four as the testing distance is four times closer. A benefit of using the Feinbloom chart is that it offers a good range in visual acuity along with high contrast optotypes. Drawbacks of this chart include only one optotype for several acuity ranges along with non-standard numbers, rendering it difficult to correlate the acuity measurement of this chart with that of others.

Question 6

How does uncorrected myopia affect a near phoria?

It results in greater hypophoria
It results in greater exophoria or less esophoria
It results in greater hyperphoria
It results in greater esophoria or less exophoria
It will not change the degree of phoria

Answer 6

It results in greater exophoria or less esophoria

Explanation
If myopia is not corrected the stimulus for accommodation is decreased resulting in less “accommodative” convergence; this will likely lead to less esophoria, or more exophoria. The opposite is true for an uncorrected hyperope. Uncorrected hyperopia will cause an increased accommodative convergence response leading to greater convergence and therefore less exophoria or more esophoria.

Question 7

The cornea can be simplified to a spherical surface that separates aqueous (index 1.33) from air. If the power of the cornea is 45 D, where is the second focal length located (in centimeters)?

2. 22 cm to the left of the cornea
3. 95 cm to the left of the cornea
4. 95 cm to the right of the cornea
5. 22 cm to the right of the cornea

Answer 7

2.95 cm to the right of the cornea

Explanation
The second focal length (also known as f’) is the distance between the second focal point (F’) and the refracting surface. The primary focal length (f) is the distance between the first focal point (F) and the refracting surface. The second focal length can be determined via the equation f’=n’/D. The primary focal length can be solved via f= -n/D. Solving for f’=1.33/45D, f’= 2.95 cm to the right of the cornea (because the answer is positive). Remember that for a surface with a positive power the focal points f and f’ are real and for a negatively powered surface, f and f’ are virtual (therefore f’ will be located to the left of the surface and f will be located to the right).