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Optometry
B2.2-2.4: Prismatic effect, effective power and thick lenses
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Terms in this set (88)
A base-out prismatic effect is induced on a spectacle lens when
a) Optic centre of a plus lens is decentred outwards
b) Eyes look inwards in plus lenses with respect to the optic centre
c) Eyes look outwards in minus lenses with respect to the optic centre
d) All of the above
d
A man wearing a spectacle lens of +3 D experiences a prismatic effect of 4.2 ∆ BD when looking through the lenses at a particular gaze. Is he looking up or down with reference to the optic centre of lens and by how much?
a) 12.6 cm up from the optic centre of the lens
b) 1.4 cm up from the optic centre of the lens
c) 1.4 cm down from the optic centre of the lens
d) 12.6 cm down from the optic centre of the lens
b
An individual has a prescription of -3.00 DS in the right eye and +2.00 DS in the left eye. What will be horizontal prismatic imbalance of the eye when the person looks 10 mm inside from the optic centre from each eye?
a) 1 ∆ BI right eye
b) 5 ∆ BI right eye
c) 1 ∆ BI left eye
d) 5 ∆ BO left eye
a - RE prismatic effect: 3∆ BI, LE prismatic effect: 2∆ BO. Both prisms will make conjugate movements so the prismatic imbalance: 1∆BI right eye.
The right lens of a pair of eyeglass lenses has 2.0 ∆ BU and the left lens has 3.0 ∆ BD. What is the net prismatic effect or prismatic imbalance between these two lenses?
a) 5 ∆ BD right eye
b) 5 ∆ BD left eye
c) 1 ∆ BU left eye
d) 1 ∆ BD right eye
b - These prisms will make disjunctive eye movements, so we add the prisms. This can also be written as 5 ∆ BU right eye
For a myopic eye that can be corrected with a -12.00 D spectacle lens mounted 12 mm in front of the cornea, what would be the required contact lens power?
Hint: K = K/1-dK
a) -9.64 D
b) -10.49 D
c) -12 D
d) -4.92 D
b
Front (r1) and back surface (r2) radius of curvature of a plano-convex ophthalmic lens in air (n=1) are 0 mm and -50 mm respectively. The lens has a refractive index of 1.500 and central thickness of 5 mm. What is the front vertex power of the lens?
a) +11.50 D
b) +14.99 D
c) +10.34 D
d) -9.68 D
c
T/F: An uncorrected -2 D myope would require more accommodation to clearly see near objects at 50 cm distance compared to an uncorrected hyperope of +2 D?
False
An individual reading a book from 50 cm with his spectacle lenses of +5.00 D has an incident vergence of +3.19 D at the cornea (vertex distance is 20 mm). Calculate the increase in corneal incident vergence if the individual slides down his glasses to a vertex distance of 40 mm?
a) +0. 17 D
b) +0. 34 D
c) +0.22 D
d) +0. 43 D
c
The front (K1) and back (K2) surface powers of an ophthalmic lens are +5.2 D and +10.40 D respectively. The lens has a thickness of 20 mm and n of 1.52. Answer the following questions
a) The equivalent lens power is?
b) The equivalent focal length is?
Hint: Ke = K1 + K2 - d/n K1K2
a) +14.89 D
b) 67.16 mm
Nodal point of the human eye is located at the:
a) Anterior chamber
b) Retina
c) Posterior pole of the lens
d) Cornea
c
What is the 'prismatic effect; of a lens?
in every lens, when a ray does not pass through the optical centre of the lens it is deviated. The deviation is the prismatic effect
How is prismatic effect measured in optometry?
In optometry, prismatic effect is measured by the displacement of the image 'h' as a matter of convenience in the consulting room
How is Prentice's 'prism dioptre' measured?
he said it should be measured by the displacement of an object, measured in cm, at a distance of 1 m
What is the symbol for prism dioptre?
What is Prentice's rule equation?
To produce a base UP prism a plus lens is decentred "?" and a minus lens decentred "?"
upwards, downwards
To produce a base DOWN prism a plus lens is decentred "?" and a minus lens is decentred "?"
downwards, upwards
Prisms: cancelling vs compounding
A patient wears -3.00 D lenses. When they look down 14mm to read what is the prismatic effect?
Base in prism occurs when the optical centres of the plus lens are decentred "?" or the optical centres of the minus lens are decentred "?"
in, out
Base out prism occurs when plus lenses are decentred "?" and minus lenses are decentred "?"
out, in
What is diplopia?
double vision, an image cannot be formed on the fovea of both eyes
What is the deviation of a covered eye called?
phoria
What test evaluates phoria?
the Maddox rod test
Convergence demand in myopia
a myope who wears glasses for distance vision converges to read, and the lines fo sight are then nasal to the optical centres of the lenses. They have a base IN effect
Convergence demand in hyperopia
A corrected hyperope needs to converge more than an emmetrope to read because of the base OUT prism included by their spectacle lenses. Convergence insufficiency is a greater problem in corrected hyperopes
A female patient wears spectacles of power RE +4.00 LE +4.00
To read she looks in 2 mm from the optical centres of each lens.
What is the prismatic effect at near?
When looking down through a plus lens the prismatic effect is base "?"
UP
When looking down through a minus lens the prismatic effect is base "?"
down
What is anisometropia?
when the lenses for each eye have different powers
T/F: the tolerance for vertical prism imbalance is low compared with horizontal imbalance
true
A male patient wears glasses with power: RE -2.00, LE -4.00.
He looks down 12 mm to read.
What is the vertical prism imbalance?
A patient complains of occasional vertical double vision when reading. What degree of anisometropia could cause this problem?
Assume:
a) a vertical prism imbalance of more than 1 might cause binocular vision problems
b) people typically look down 12 mm in their glasses to read
How far would +10.00 D lenses need to be decentred so that the wearer does not need to converge at all for binocular vision of an object in the focal plane?
Assume PD is 60 mm
Where is the apparent position of an object for myopes?
always positioned TOWARDS the optical axis of the lens
Where is the apparent position of an object for hyperopes?
always positioned AWAY from the optical axis of the lens
A golfer has a refractive error of -2.00 R and L.
He stands 2 metres from the ball and he looks down through his glasses 20 mm below the optical centre.
Where does the ball appear to be?
the golfer has a refractive error of -2.00 D so since he is a myope the image is displaced towards the optical axis of the lens. As he is looking below the optical axis, this is above the true position of the ball. The ball then appears to be 8 cm further away from him than its true position
A woman wears prescription lenses of power +1.50 L and R. To shoot a target 50m away she turns her head so she is looking through the lenses 2 mm to the right of the optical centres. Where does the target appear to be with respect to the actual position?
To reduce overall thickness of a lens, vertical base "?" prism is introduced into each lens. 'prism thinning'
down
A bifocal lens with a +2.50 D 28 mm Round Seg set 3.5mm below the distance optical centre. What is the prism jump at the Seg line? And what is the prism at near due to the seg?
What is the effective power of a surface or lens?
effective power is the power a lens/surface needs to have to form an image in the same place, if it was located in a different plane
Effective power equation
notes: since power is in dioptres - d is in metres
to find the effective power to the right of the lens - d is positive.
to find the effective power to the left of the lens - d is negative
A lens in air of power +2.00 D is moved 10cm to the right. What is the effective power in that position?
A lens in air of power -2.00 D is moved 10 cm to the right. What is its effective power in that position?
What is the vertex point?
it is the point where the optical axis intersects with the surface
What is the overall lens power in thin lenses?
the sum of the front and back surface powers - this is not the case when the lens thickness is taken into account
Back vertex power
BVP = 1/back focal length
Front vertex power
FVP = 1/front focal length
Vertex power equations
A lens has the surface powers K1=+10.00 D, K2 = -4.00 D and a centre thickness of 8mm. Its refractive index is 1.500. What are the front and back vertex powers?
FVP = +6.08 D
BVP = +6.56 D
The power of an ophthalmic lens is always specified as the "?" vertex power
back
A lens has surface powers K1=+4.00 D, K2= -10.00D, centre thickness of 2mm, refractive index of 1.500. what is the back vertex power?
Spectacle plane
the spectacle plane is remote from the eye and is used to measure refractive error
When refraction is referred to the anterior corneal plane it is known as what?
ocular refraction
Spectacle to contact lens power
Refractive error measured in spectacle plane is +5.00 D. The vertex distance is 12 mm. What is the ocular refraction?
Refractive error measured in spectacle plane is -5.00 D. The vertex distance is 15 mm. What is the ocular refraction?
Alternate formula to calculate contact lens power
How accurate does the vertex distance need to be in measuring refractive error and in fitting glasses? What change in vertex distance will cause an error of more than 0.25D?
For refractive errors below about +5.00 D it is not usually necessary to consider vertex distance as would need about 10mm vertex distance to make 0.25 D difference
Accommodative demand for a correct myope
Accommodative demand for a corrected hyperope
A person's refractive error is measured to be +10.00D at a vertex distance of 20mm. How much less accommodation (referred to the cornea) would they need to see an object at 200mm if they move their glasses to a vertex distance of 40mm?
They must accommodate by +6.25 D of effective power in the corneal plane to make up the difference
T/F: accommodation is reduced when vertex distance is increased
true
A myope who is wearing glasses for distance vision needs "?" accommodation than an emmetrope to see clearly at near
less
A hyperope who is wearing glasses for distance vision needs "?" accommodation than an emmetrope to see clearly at near
more
T/F: A corrected hyperope must always accommodate more than an emmetrope
True
T/F: A corrected myope must always accommodate more than an emmetrope
False - less
A -10.00 D myope wears glasses with a vertex distance of 15mm.
What is the difference between the accommodation they need to read at 25cm from the spectacle plane and that of an emmetrope?
The corrected myope needs 0.90 D less accommodation than an emmetrope
Thick lens optics
When the thickness of the lens is taken into account
T/F: the eye is a thick lens
True
A thick lens has 6 cardinal points
Back/front vertex power and focal length equations
Principal points and planes
Principal planes diagram
Principal plane example
Equivalent power of a thick lens equation
Where are the principal points?
The distance from the front and back vertex to the principal points (e and e' in the diagram) is the difference between the equivalent focal length and the vertex focal length.
The equivalent focal length can be found from the equivalent power and the vertex focal lengths can be found from the vertex powers
A lens has surface powers +10.00 D and -5.00 D, a thickness of 9mm and a refractive index of 1.5. Where are the principal points?
back principal point = 11.4mm in front of the back vertex
front principal point = 5.5 mm in front of the front vertex
Nodal points of a thick lens
Ray tracing through thick lens
Ray tracing with minus thick lens
What are the cardinal points?
The focal points, the principal points and the nodal points
Where is the back focal point in the eye?
For an emmetrope - on the retina
Where are the principal points in the eye?
both are in the anterior chamber
Where are the nodal points in the eye
On either side of the posterior lens surface, usually within 0.1mm of the surface
T/F: the eye is usually considered to have a single nodal point, located at the posterior pole of the lens or just slightly inside the lens
True - due to the nodal points being so close
T/F: any ray directed from an object towards the nodal point continues undeviated by the refracting system of the eye
True - this refers only to single rays
Ray tracing through the eye
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