Vergence refers to the dioptric power of lenses. It is equal to the reciprocal of the second focal length and is measured in diopters. The Gullstrand schematic eye model provides cardinal data on the optical elements and structure of the human eye, including refractive indices, radii of curvature, and positions. Listing's reduced eye model simplifies the eye into a single refracting surface to allow for easier calculations of refraction. Visual acuity refers to spatial visual discrimination and is measured as the reciprocal of the minimum resolvable visual angle. It depends on factors like illumination, contrast and the physical and physiological components of the visual system.

1. UNIT ONE

2. VERGENCE
• It is a dioptric power of lenses (vergence).
• The vergence power of lens (positive for convex converging lens and negative for a concave diverging lens) .
• It is reciprocal of the second focal length expressed in meters, i.e. D=1/f2, (f2 is second focal lengh)
• The unit of power is diopter (D).
• one diopter is the power of a lens with second focal length as 1m.
• The vergence of a ray of light from an object is equal to 1/u, where u is the distance of the object along the optic axis of
the lens.
• The vergence of ray of light is directly related to the index of refraction of the optic medium in which it is travelling and
equals n/u.
• The index of refraction (n) of air or vaccum equals 1, vergence in these media equals 1/u.

3. SCHEMATIC EYE
• Listing and gauss(Two scientist) , while studying refraction by lens combinations, concluded that for a
homocentric lenses system, there exist three pairs of cardinal points, which are two principal focus ,two principal
points and two nodal points, all situated on the principal axis of the system.
• Cardinal data of the gullstrand’s schematic eye
• These are as follows:
1. Principal foci F1 and F2 lie 15.7mm in front of and 24.4mm behind the cornea, respectively.
2. Principal points P1 and P2 lie in the anterior chamber 1.35mm and 1.60mm behind the anterior surface of cornea,
respectively.
3. Nodal point N1 and N2 lie in the posterior part of the lens 7.08mm and 7.33mm behind the anterior surface of
cornea, respectively.

4. Refractive index of component of eye
Gullstrand's data on refractive index of the components of the optical system of eye are as below:
• Cornea 1.376
• Aqueous humor 1.336
• Lens cortex 1.386
• Lens core 1.406
• Vitreous humor 1.336

5. Radii of curvature of refractive surface of eye
Gullstrand's data for radii of curvature of the refractive surfaces are as below:
• Anterior surface of cornea 7.70 mm
• Posterior surface of cornea 6.70 mm
• Anterior surface of lens cortex 10.00 mm
• Anterior surface of lens core 7.91 mm
• Posterior surface of lens core and Posterior surface of lens cortex 6.00 mm

6. Position of optical elements in the eye
Position of optical elements in the eye is:
• Anterior surface of cornea 0.3 mm
• Posterior surface of cornea 0.5 mm
• Anterior surface of lens 3.6 mm
• Posterior surface of lens 7.2 mm

7. THE REDUCED EYE
• Listing’s reduced eye
• Single principal point and single nodal point lying midway between two principal points and two nodal points, respectively. This is
called listing’s reduced eye.
1. Principal point (P) lies 1.5mm behind the anterior surface of cornea and represents the vertex of a single refracting surface whose
radius of curvature is 5.7mm.
2. Nodal point (N) is situated 7.2mm behind the anterior surface of cornea, i.e. at the center of curvature of single refracting surface
(1.5+5.7=7.2).
3. Anterior focal point is 15.7mm in front of the anterior surface of cornea.
4. Posterior focal point (on the retina) is 24.4 mm behind the anterior surface of cornea.
5. Anterior focal length is 17.2mm (15.7+1.5) and the posterior focal length's 22.9mm (24.4-1.5).
6. Uniform refractive index is 1.336.
7. Total dioptric power is +58.20.

8. Diagram of reduced eye

9. VISUAL ACUITY
Visual acuity is considered a measure of form sense
• so it refers to the spatial limit of visual discrimination.
• In terms of visual angle, the visual acuity is defined as the reciprocal of the minimum resolvable visual angle
measured in minutes of arc for a standard test pattern.

10. Components of visual acuity
Measurement of the threshold of discrimination of two spatially separated targets (a function of the fovea centralis) is
termed visual acuity is a highly complex function that consists of the following components:
Minimum visible
• The ability to determine whether or not an object is present in an otherwise empty visual field is termed visibility or
detection. This type of task is referred to as the minimum visible or minimum detectable function. The limit of
visibility reflects the absolute threshold of vision e.g., a black dot against a white background can be detected if its
diameter is of the order of 30 seconds of arc or more.
•
Resolution or ordinary visual acuity
• Discrimination of two spatially separated targets is termed resolution. The minimum separation between the two
points, which can be discriminated as two, is known as minimum resolvable. Measurement of the threshold of
discrimination is essentially an assessment of the function of the fovea centralis and is termed as ordinary visual
acuity.
• The distance between the two targets is specified by the angle subtended at the nodal point of the eye. The normal
angular threshold of discrimination for resolution measures approximately 30 to 60 sec. arc; it is uaually called the
minimum angle of resolution (MAR).
•

11. Recognition
•It is that faculty by virtue of which an individual not only discriminated the spatial characteristics of the test pattern but
also identifies the patterns with which he has had some experience. Recognition is thus a task involving cognitive
components in addition to spatial resolution.
For recognition, the individual should be familiar with the set of test figures employed in addition to being able to resolve
them. The most common example of recognition phenomenon is identification of faces. The average adult can recognize
thousands of faces
• Minimum discriminable or hyper acuity
Minimum discriminable refers to spatial distinction by an observer when the threshold is much lower than the ordinary
acuity the best example of minimum discriminable is vernier acuity.

12. •Line acuity
•The problem of visibility of a line is a function to some degree of the length of line. In
addition the thickness of the line, the general illumination level and contrats are important
variable. Most people can distinguish these images with a minimum visible angle 1’ and this is
considered to be a practical limit.
•Vernier acuityVernier acuity or aligning power refers to the ability to determine when two
parallel straight lines are exactly in the line. The vernier acuity is defined as minimum
detectable displacement from perfect alignment. The threshold is lower for longer lines.
Adaptation and illuminating are important variables of vernier acuity.

13. FACTORS AFFECTING VISUAL ACUITY
• Factors affecting visual acuity
• The factors that influence the spatial resolution can be classified into physical and physiological.
• Physical factors
• Include those which influence the light characteristics of the distribution and hence the influence the nature of retinal
image.
• Physiological factors
• It includes those which influence the processing of the stimulus and are thus mainly observer related. However, there
is some overlap between the physical and physiological groups.
• For example, the lens is a physical factor but the related accommodation process is physiological. Similarly, the size
of pupil which controls the amount of light entering the eye is a physical factor but the reflexes controlling its size
are physiological processes.

14. HOW TO MEASURE VISUAL ACUITY
• Measurement of visual acuity
• Position the patient, sitting or standing, at a distance of 6 metres from the chart. The patient can hold one end
of a cord or rope of 6 metres long to ensure that the distance is maintained
• Test the eyes one at a time, at first without any spectacles (if worn).
• Note: Some people prefer to always test the right eye first. Others prefer to test the ‘worse’ eye first (ask
the patient out of which eye they see best). This ensures that the minimum is read with the ‘worse’ eye, and
more will be read with the ‘good’ eye. This means that no letters are remembered, which could make the
second visual acuity appear better than it is.
• Ask the patient to cover one eye with a plain occluder, card or tissue. They should not press on the eye; this is
not good for an eye that has undergone surgery. It can also make any subsequent intraocular pressure reading
inaccurate and it will distort vision when the occluded eye is tested.

15. OCCLUSION OF EYE

16. VISUAL ACUITY MEASURMENT
• Ask the patient to read from the top of the chart and from left to right. If the patient cannot read the letters due
to language difficulties, use an E chart. The patient is asked to point in the direction the ‘legs’ of the E are
facing.
• Note: there is a one in four chance that the patient can guess the direction; therefore it is recommended that
the patient should correctly indicate the orientation of most letters of the same size, e.g. four out of five or
five out of six.
• The smallest line read is expressed as a fraction, e.g. 6/18. The upper number refers to the distance the chart is
from the patient (6 metres) and the lower number (usually written next to the line on the chart )is the distance
in metres at which a ‘normal’ eye is able to read that line of the chart.
• Incomplete lines can be added to the last complete line. e.g. 6/12+3, indicating that the patient read the ‘12’
line at 6 metres and gained three of the letters on the ‘9’ line.
• Record the VA for each eye in the patient‘s notes, stating whether it is with or without correction (spectacles).
For example: Right VA = 6/18 with correction, Left VA= 6/24 with correction.

17. • If the patient cannot read the largest (top) letter at 6 metres, move him/her closer, 1 metre at a time, until the top letter can
be seen – the VA will then be recorded as 5/60 or 4/60, etc.
• If the top letter cannot be read at 1 metre (1/60), hold up your fingers at varying distances of less than 1 metre and check
whether the patient can count them. This is recorded as counting fingers (CF): VA = CF
• If the patient cannot count fingers, wave your hand and check if he/she can see this. This is recorded as hand movements
(HM): VA = HM
• If the patient cannot see hand movements, shine a torch toward the eye and ask if they can see the light. If they can, record
‘perception of light’ (VA = PL). If they cannot, record ‘no perception of light’ (VA = NPL).
• After testing without any correction, test the patient while wearing any current distance spectacles and record the VA in
each eye separately, with correction.
• If 6/6 (normal vision) is not achieved, test one eye at a time at 6 metres using a pinhole occluder (plus any current
spectacles). The use of the pinhole reduces the need to focus light entering the eye.
• If the vision improves, it indicates the visual impairment is due to irregularities in the cornea, a problem in the lens, or
refractive error, which is correctable with spectacles or a new prescription.
• Repeat the whole procedure for the second eye
• Summarise the VA of both eyes in the patient's notes, for example:
• Right VA= 6/18 without specs, 6/6 with pinhole and Left VA= NPL.
•

18. VISUAL ACUITY TRANSCRIPTION TABLES

19. EYE TESTING CHARTS IN ADULTS
Snellen’s test types
• 1ST LINE—6/60
• 2ND LINE—6/36
• 3RD LINE-- -6/24
• 4TH LINE -–6/18
• 5TH LINE –6/12
• 6TH LINE –6/9
• 7TH LINE –6/6 (NORMAL VISION )
• IF 8TH LINE PRESENT-6/5
• IF 9TH LINE PRESENT –6/4

20. Landolt’s broken ring test

21. Test types used in children
1.Tumbling E test
2.Graded hand test
3.Sheriden gardiner test

22. EMMETROPIA
• Emmetropia is a state of refraction where a point at an infinite distance
from the eye is conjugate to the retina.
• Definition: Emmetropia is the term used to describe a person’s vision
when absolutely no refractive error or de-focus exists. Emmetropia
refers to an eye that has no visual defects. Images formed on an
emmetropic eye are perfectly focused, clear and precise.
• Eyes that have emmetropia do not require vision correction. When a
person has emmetropia in both eyes, the person is described as having
ideal vision. When an eye is emmetropic, light rays coming into the
eye from a distance come to perfect focus on the retina.

23. AMETROPIA
• Ametropia (a condition of refractive error ) is defined as a state of refraction ,when the parallel rays of light coming
from infinity (with accommodation at rest) ,are focused either in front or behind the retina in one or both meridians
• Ametropia is a state where refractive error is present, or when distant points are no longer focused properly to the
retina.
Examples of ametropia
• MYOPIA
• HYPERMETROPIA
• ASTIGMATISM
Other related conditions are:
1. Aphakia
2. Pseudophakia

24. MYOPIA
Myopia or shortsightednes is a type of refractive error in which parallel rays of light coming from infinity are
focused in front of retina when accommodation being at rest that is image is formed infront of retina
Myopia or near-sightedness (short-sightedness) is one form of ametropia where the eye is effectively too long or
has too high a power.
•Nearsightedness: Nearsightedness, or myopia, is a condition in which nearby objects are seen clearly, but
distant ones are blurred.
• Nearsightedness can be inherited and is often discovered during childhood.
• However, you can develop nearsightedness in early adulthood.
• People that develop myopia in early adulthood usually do not develop high amounts of nearsightedness.
Parallel rays of
light coming
from infinity
Retina
Image is formed infront of
retina

25. HYPERMETROPIA
Hypermetropia (Hyperopia ) or long sightedness is the refractive state of eye wherein parallel rays of light coming
from infinity are focused behind the retina with accommodation being at rest
• The posterior focal point in behind the retina ,which therefore receives a blurred image
• Hyperopia or far-sightedness is a form of ametropia where the eye’s power is too weak or the eyeball too short.
• In this case, a point at infinity focuses behind the retina.
• A point behind the eye is therefore conjugate to the retina.
Parallel rays of light
coming from
infinity
HYPERMETROPIA
Image is formed behind the
retina
Retina

26. •Farsightedness: Farsightedness, or hyperopia (also
referred to as hypermetropia),
• Usually causes distant objects to be seen clearly, but close
objects to appear blurred.
• Farsightedness often runs in families.
• When someone has higher levels of farsightedness, their
distance vision may become blurry in addition to their near
vision.
• Many people mistake farsightedness for presbyopia, the
refractive error that usually occurs over 40 years of age.

27. ASTIGMATISM
• Astigmatism is a type of refractive error wherein the refraction varies in different meridian
• The rays of light entering in the eye cannot converge to a point focus but form focal lines
•
• Astigmatism usually occurs when the cornea has an irregular curvature. The cornea is
curved more in one direction, causing blurry vision.
• Astigmatism can cause blurry vision at all distances, and it often occurs along with
farsightedness or nearsightedness.
• Most people have very small amounts of astigmatism.
• Larger amounts of astigmatism cause distortion in addition to blurry vision.
• Very high amounts of astigmatism sometimes have a difficult time achieving 20/20
vision.

28. PRESBYOPIA
Presbyopia (eyesight of old age ) is not an error of refraction but condition of physiological insufficiency
of accommodation ,leading to failing vision for near
• It is the loss of elasticity of the lens that occurs with aging, causing difficulty focusing at close ranges.
• Scientists also believe that in addition to the loss of elasticity of the lens, the muscle that makes the lens
change focus, called the ciliary body, also begins to not work as well.
• Presbyopia usually becomes significant after the age of 40-45 years of age but people between 35-40 may
exhibit early signs depending on their visual state, work, and lifestyle
• Presbyopia is the normal aging process of the lens of the eye.

29. THANK YOU