week 9

what is the primary clinical application of ophthalmic prisms within optometric practice

diagnose and treat various binocular vision anomalies

when light passes through a prism in which physical directions are the actual light rays deviated towards and perceived image shifted

deviated toward base of the prism and image displaced toward the apex

how does a patient’s eye physically behave when a prism is places in front of it to capture a shifted image

patient’s eye is forced to rotate toward the direction of the apex to align the fovea and capture displaced image

how is a prism dioptre physically defined in visual optics

linear displacement of an image in cm from an object located 1m away

what does the term “ray height” (h) represent when calculating prismatic effect through a spectacle lens

measure of decentration which is the exact distance or region of lens where a light ray passes through relative to the optical centre

what are the two main realworld scenarios that cause a patient’s eye to experience an unintended prismatic effect from a spectacle lens

eye moves away from the optical center of the lens and when the optical center is decentered in the lens

if a patient looks down below the optical center of a plus lens or if the lens is decentered upwards what prism base direction do they experience

base up

if a patient looks down below the optical centre of a minus lens or if the lens is decentered upwards, what prism base direction do they experience

base down

what is the simple physical shortcut to figure out the prism base direction for any lens movement

for a plus lens, what prism base direction is induced when the OC is decentered up or eyes look down

base up

for a plus lens, what prism base direction is induced when OC is decentered down (or eyes look up)

base down

for a minus lens, what prism base direction is induced when OC is decentered up or eyes look down

base down

for a minus lens, what prism base direction is induced when OC is decentered down or eyes look up

base up

for a plus lens, what prism base direction is induced when OC is decentered in/nasally or eyes look out/temporally

base in

for a plus lens, what prism base direction is induced when OC is decentered out/temporally or eyes look in/nasally

base out

for a minus lens, what prism base direction is induced when OC is decentered in/nasally or eyes look out/temporally

base out

for a minus lens, what prism base direction is induced when OC is decentered out/temporally or eyes look in/nasally

base in

what is convergence insufficiency (example with focusing on near target)

reduced ability to converge the eyes and maintain binocular fusion when focusing on a near target

what are the three classic clinical signs that usually accompany convergence insufficiency

reduced npc and convergence amplitudes and marked exophoria at near

what common symptoms do patient with convergence insufficiency complain of during near work

eyestrain when reading, closing or covering one eye when reading and blurred vision after short periods of near work

why is the physical convergence demand reduced for a myope reading through their minus lens

looking nasally through a minus lens induces a base in effect where the image would shift towards the apex so the medial rectus do not have to pull the eye in as much to converge

why is the physical convergence demand increased for a hyperope reading through their plus lens

looking nasally through a plus lens induces a base out effect where the image would shift towards the apex so the medial rectus have to pull the eye in further nasally increasing convergence demand

what is prismatic imbalance in spectacle lenses and how is it determined

net difference in prismatic effects between the right and left lenses when a patient looks through a specific pair of spectacles

in the context of prismatic effects what is the difference between a conjugate and disjunctive eye movement

conjugate is when both eyes rotate in the same direction and disjunctive is when they rotate in opposite direction

when calculating horizontal prismatic imbalance between right and left eyes, when do you add or substract prism values together

add if base directions are opposite to create disjunctive demand and subtract if base directions are same to create conjugate demand

when caculating vertical prismatic imbalance between right and left eyes, when do you add or subtract prism values together

add if base directions are same and subtract if base directions are opposite

when do horizontal prisms cause a disjunctive eye movement and why

occurs when base directions are the same in both eyes. happens because light is deviated nasal/nasal or temporal/temporal retina in each eye forcing the eyes to move in opposite directions to maintain fusion

what specific binocular eye movements are induced by the following horizontal prism combinations BO+BO and BI+BI

bo induces convergence and bi induces divergence

when do horizontal prisms cause conjugate eye movement and why

when base directions are opposite between eyes. happens becuase light is deviated to nasal retina in one eye and temporal retinal in the other forcing both eyes to track together in same direction

when vertical or horizontal prisms are added to find the net prismatic imbalance, how do you report the final base direction and eye

recorded with respect to either eye using the original base direction of the chosen eye

when prisms are subtracted to find the net prismatic imbalance, how do you determine which eye and base direction to report

recorded with respect to the eye with the larger initial prism value keeping that eyes original base direction

clinically, how does a patient’s physiological tolerance for a vertical prismatic imbalance compare to a horizontal one

smaller tolerance for vertical prismatic imbalance

why are standard progressive lenses physically much thinner at the bottom than the top before any modification is made

because the near viewing zone requires smaller, reduced radius of curvature to create the reading power

what physical modification do manufacturers perform to fix the thickness imbalance in progressive lenses and what type of prism does it introduce

bu material is physically removed from the lens blanks which introduces an equal vertical bd prism in both eyes known as yoked thinning prism

what are the three primary clinical and cosmetic benefits of incorporating yoked base down thinning prism into a progressive lens

reduces overall thickness of lens, weight and improves cosmetic appearance

how does the introduced base down thinning prism optically affect the patient’s view through a progressive lens without changing the lens surface curves

shifts the perceived image upward toward the reading zone making it easier for the patient to access the near vision area without manufacturer having to alter the radius of the lens curves any further

what is the purpose of the distance reference point on a progressive lens and what is its prismatic status

location on lens where distance prescription is measured and no prismatic effect

what is the function of prism reference point on progressive lens

point used to identify prescribed or unwanted prism in lenses which enables accurate prism measurement and compliance with optical standards

why are the drp and prp critical for lens verification and what is the clinical benefit for the patient

ensures precise lens fabrication and verification during manufacturing process and improves visual comfort and reduces adaptation issues for patient

what is the clinical purpose of nrp on progressive lens

location on lens where near vision power is measured

what does the fitting cross on a progressive lens align with and what is its primary function

aligns with patient’s pupillary center/line of sight for distance vision it serves as the reference point for fitting and positioning lens accurately within frame

what is the conceptual definition of the effective power of a lens or optical surface

power that a lens would need to have if it were moved to a different location in order to still form an image in the exact same focal position

what are the two primary clinical and optical applications where effective power calculations are commonly used

contact lens power and effect of lens thickness

when moving a plus prescription from the spectacle plane closer to the eye like converting to a contact lens, does the required lens power increase or decrease

increases

when moving a minus prescription from the spectacle plane closer to the eye like converting to a contact lens, does the required lens power increase or decrease

decreases

how is the back vertex power defined when considering the thickness of a lens

kv’ formula

how is the front vertex power defined when considering the thickness of a lens

kv formula

what is the fundamental difference between between calculating total power for a thin lens versus a thick lens

in thin lens you can add two surface powers together but in thick lens you have to account for lens thickness

at which surface is a patient’s spectacle lens power always measured and verified in clinical practice

back vertex

why are effective power calculations significantly more impactful for thick plus lenses than for minus lenses

plus lenses are thickest in the middle given them greater central thickness (d) causing effectivity shift between the front and back surface more pronounced

what is the definition of the far point and where is it located for an emmetropic eye

farthest distance at which an object can be clearly focused on the retina without any accommodation, located at infinity

where is the far point of a myopic eye located and what does it mean to sat it is the optical conjugate of the retina

finite distance in front of the eye. means that any object placed exactly at this far point will be clearly imaged on the retina without any accommodation

how does a minus lens or contact lens optically correct a myopic eye for distance vision

diverges incoming parallel light rays so they form a virtual image of the distance object at the myopic far point. light rays appear to originate from the far point and re-imaged onto the retina by the refractive system of the eye

if a myope slides their minus lens further down their nose while reading a near object, what happens to their accommodative demand and why

by moving the lens further down their nose, the vertex distance increases which causes the lens to lose its effective minus power. for a near object, the image formed by the correcting lens is closer to the eye than the far point (image is within the far point) so the eye must accommodate to increase its power to handle the extra divergence. when the spectacle lens is pushed away, its effective power decreases so the image is formed closer to the far point so less accommodation is needed

conceptually how does changing vertex distance of a spectacle lens alter the physical accommodative demand for a patient viewing a near target

vertex distance increases which alters effective power of the lens

where is the far point of a hyperopic eye located and what kind of point is it

finite distance behind the eye and a virtual far point

how does a plus lens or contact lens optically correct a hyperopic eye for distance vision

converges incoming light rays, shifting the distant object from the eye’s virtual far point to infinity so the real image forms on the retina

if a hyperope slides their plus lenses further down their nose while reading a near object, what happens to their accommodative demand and why

by moving the lens further down their nose, the vertex distance increases which causes the lens to gain effective plus power. for a near object, the image formed by the plus lens is further behind the eyes than its virtual far point so the eye must accommodate to increase its power to pull the image forward. when the spectacle lens is pushed away, its effective power increases which moves the image closer to the virtual far point so less accommodation is needed

in theory, how should the accommodative demand for a near object compare between an emmetrope, a corrected myope, and a corrected hyperope? What happens in reality?

near object should induce the exact same amount of accommodation in all three eyes if they are fully corrected for distance. in reality, effectivity calculations show that a spectacle-corrected hyperope must accommodate more than an emmetrope whereas a spectacle corrected myopes accommodates less

why does a spectacle-corrected hyperope have to exert more accommodative effort than an emmetrope to see near objects clearly

due to lens effectivity where the image formed by the plus spectacle lens falls further behind the retina than it would for an emmetrope so the hyperope must accommodate more to pull that image forward onto the retina

why does a spectacle corrected myopes get to exert less accommodative effort than an emmetrope when reading at near

due to lens effectivity where the image formed through the minus spectacle lens sits closer to the retina than it would for an emmetrope. because the image is already closer to the retina, the myope doesn’t need to accommodate as hard to focus it

why can you not simply add the front and back surface powers together to find the total power of a thick lens

because the physical thickness of the lens material between the two refracting surface changes the path of the light rays

a thick lens system has six cardinal points used to map its optical behaviour

front and back focal, principal and nodal points

what is the front and back focal points

point where parallel light rays from either directions are brought to a focus

what is the front and back principal points

the two planes where the refraction is assumed to take place in a thick lens when the light ray enter and exit the lens

what is the front and back nodal points

the two points in a thick lens from where the light rays entering and exiting the lens remain undeviated (or parallel to one another)

what is the formula for calculating the front and back focal points

fv=1/kv

what is the formula for calculating the front and back principal points

same as equivalent focal length fe=1/ke