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Reduced eye model
-interior eye
-cornea to anterior focal point
-cornea to posterior focal point
-total eye power

n = 1.33

16.67mm (axial length)




Far point

aka punctum remotum (PR)

Where the eye is looking without accommodation to have light perfectly focused on the retina

i.e. the point conjugate to the axial retinal point
i.e. a point object object at the far point will result in the formation of a point image and the retina, and vice versa


Far point sphere

When the eye rotates, the far point is no longer just a point, but traces a spherical surface - known as fps

Center of curvature = center of rotation of the eye (~27mm behind typical spectacle plane)


How to find far point of ametropic eye

With correcting prescription (F):

Far point = abs(1 ÷ F)


Near point

aka punctum proximum (PP)

Same as far point, but with MAX ACCOMM


Reduced eye model

Incoming plane waves converge to a point on the retina
-retina located at Ƒ’ (secondary fp of lens), which must account for the IR used for the interior eye

For reduced eye model: power +60D, internal n=1.3
-retina located at 1.3/60m, or about 22mm from the lens

The far point for an emmetrope is optical infinity (same for corrected my/hyperopes)


Reduced eye model

Eye is too strong (>60D) -> light from incoming plane waves converge in front of the retina

The far point for a myope is between the eye and infinity (i.e. will sit in front of the retina)


Describe night myopia

Pt becomes more myopic under low light levels

Due to combo of:
-incr spherical aberrations
-light levels that are too low to fully relax accomm when viewing distance


Myopia trends

At birth ~25-50%
By 1yo, few children are myopic
By 6yo, -0.50 or more in only 2%
-incr in prevalence b/w ages 6 and 20, reaching 20% by 20yo
Incr somewhat in later years due to nuclear sclerotic lens changes


Reduced eye model

Eye is too weak (<60) -> light from plane waves converge to a point behind the retina

Far point located behind the retina, considered virtual


Optically speaking, a correcting lens should be place so that...

Its secondary focal point coincides with the far point of the ametropic eye



Accommodated/covered up with accomm

Found in subjective refraction

Amount that cannot be corrected in a hyperope whose rx is too large to be neutralized by accomm

Amount that can be neutralized by accomm


Hyperopia trends

Prevalence ~6% in children 6-15yo
-unlike myopia, prevalence doesn’t change with age for this range

A hyperopic child approx 5yo, will likely be:
-hyperopic at age 14 if orig >1.50 D
-emmetropic at 14 if orig 0.50 to 1.25 D
-myopic at age 14 if orig <0.50 D

Ages 20-40:
-expected to be relatively constant
-decr in accomm ability may functionally highlight otherwise non-burdensome hyperopia

Over age 45, hyperopes + emmetropes tend to show an incr with age


Measuring interpupillary distance
-PD ruler

1) dr sit at 40cm and closes OD
2) pt fixates on dr OS, PD measured (OS edge -> OD edge) = NEAR
3) with ruler in place dr closes OS/opens OD, pt fixates on dr OD
4) measure again = DISTANCE


Measuring interpupillary distance

1) dr sets ometer at 40cm/infinity
2) place ometer against bridge of nose/at spec plane
3) ometer light produces corneal reflex visible to dr
4) dr aligns vertical hairline with location of corneal reflexes
5) both binoc and monoc PD can be read directly from instrument


Measuring interpupillary distance
-using a pupilometer reduces...

Errors resulting from parallax


Measuring visual acuity
-resolution vs recognition acuity

Resolution: pt distinguishes a pattern from a uniform patch of equal luminance (e.g. Teller cards)
-typical young adult has cutoff of 40-60 cycles/degree (0.75 MAR)

Recognition: gives info about ability to resolve high frequencies
-not useful for pts who show probs at other frequencies (e.g. cataracts)


Measuring visual acuity
-minimum detectable acuity

Essentially asks what is the thinnest possible wire that’s visible
-e.g. vernier lines

Avg person’s MDA (~1 arcsec) is much better than resolution/recognition


Measuring visual acuity

Person’s ability to sense directional relationships
-e.g. whether 2 lines are parallel (ability to so is very good, threshold on order of several arcsecs)
-believed to be the result of higher cortical processing


Measuring visual acuity
-MAR and snellen

MAR = minimum angle of resolution
-typically measured in arcminutes (1/60 of a degree)

Snellen fraction = 1/MAR
-fraction is typically multiplied by 20/20 to obtain standard form

20/20 = 1 MAR = 0 logMAR


Measuring visual acuity
-snellen letter size

Each leter on 20/20 subtends 5 arcmin at 20 ft

Each distinct bar = 1 arcmin (think of letter E)

20/20 = distance/letter size


Measuring children’s visual acuity
-examples of recognition acuity
-examples of recognition acuity matching

Birthday cake, tumbling E, landolt C

STYCAR, HOTV, lea symbols


Measuring children’s visual acuity
-allen vision test

Ages 2+

Child names series of images on cards at close range -> pt closes/occludes one eye, dr determines longest dist at which child can resolve images

Acuity expressed as x/30
-where x is distance (ft) at which child can read


-corneal curvature
-corneal thickness

Curv: keratometer (only central 3mm) or topographer (whole k)

Thick: pachymeter
-old measured dist bw purkinje images from ant + post k
-modern methods use ultrasound


Optical principles of static retinoscopy

Streak of light reflected from pts cornea, refracted as it passes thru the cornea and focus, and focused to the FAR POINT of eye
-image = retinal reflex

Examiner observes both ret reflex + light reflected from exterior of pt’s eye (streak)

Myope = against motion (eye is too strong, neutralize with minus)
Hyperope = with motion (eye is too weak, neutralize with plus)


Retinoscopy prescription equation

RX = F - WD

WD assumed 1.5 D unless otherwise stated


-e.g. +3D vertical streak, +1D horizontal streak

Horizontal streaks = vertical meridian
Vertical streaks = horizontal meridian

+3D vertical streak, +1D horizontal streak = +0.50 -2.00 x 180


Dynamic retinoscopy

Measure accommodative accuracy/response


Dynamic retinoscopy
-types and descriptions (3)

MEM: target at reading dist/Harmon, lenses quickly placed in front of pt

Nott’s: retinoscope moves in/out (no lenses)

Mohindra: aka “near retinoscopy”, used for determining refractive state of children/infants
-performed in dark @ 50cm
-adjustment factor 1.25D must be subtracted from sphere component of lens powers


Optics of autorefractor

Lens in machine placed at a distance = focal length from spectacle plane
-target on other side of lens moves back-forth, continuously changing vergence of light at spec plane
-effectively same as keeping the target stationary and using a trial lens whose power can change continuously