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CLE 270.1 – Ophthalmic Optics
Notetaker: Fatima Ibrahim
Date: 08/13/2013
Page1
The Big Three: Equations to know and use for everything
1. Δ = d ˙ F
The Prentice equation is used for prisms. “d” is measured in air and is in centimeters.
2. F = (n’- n) / r
This equation for surface power illustrates the inverse relationship between the power and the radius.
Retinoscopy determines this.
3. Fx = F / (1 - cF)
This equation for effective power can be used to determine the power of a contact lens from a glasses
prescription and vice versa.
Myopic patients need divergent lenses to compensate. Glasses rest at a certain vertex distance from the
eyes which adds convergent power. Contact lenses are right on the eye and eliminate excess convergence.
Near Sighted:
• High convergence power
• Focus at shorter distances
• Needs minus to compensate
• Example: +65 D (if normal is +60 D)
Far Sighted:
• Low convergence power
• Focus at longer distances
• Needs plus to compensate
• Example: +56 D (if normal is +60 D).
Recall: Doing Retinoscopy you see against motion - person has two much convergence
“Shaketometry”: A way that can be used to determine whether an individual is wearing plus or minus
corrective glasses. Look at a line and move lenses back and forth and see that the line either moves with
or against the motion of your hand.
• If you see with the motion in glasses, the lenses are divergent. These glasses would minify. These
glasses correct a near sighted person.
CLE 270.1 – Ophthalmic Optics
Notetaker: Fatima Ibrahim
Date: 08/13/2013
Page2
• If you see against the motion in glasses, the lenses are convergent. These glasses would magnify.
These glasses correct a far sighted person.
Individuals that push their lenses up against their eyes → they want more divergent power
Individuals that push their lenses away from their eyes → they want more convergent power
Note: A myopic individual who needs -2 D can be corrected with a -10 D lens if its positioned very far
away from his/her eyes. This is how auto refractors work - they assess a variety of distances.
Variable “c” is used for a change in medium. Light slows down in denser medium.
Variable “d” is used when there is no change in the medium
Variables c and d are “the same”
Individuals that have anisometropia can wear contact lenses (CLs) since these would help prevent prism
distortions. With glasses, when the eyes move around and away from the center, there are prismatic
effects. If the eyes have similar refractive errors, the prisms cancel out. But when a person has very
different refractive error in each eye, what results is prism distortions.
Optometrists prescribe:
• CLs
• Spectacles
• Refractive surgery
• Low vision magnifiers
• Intraocular aides
• Vision therapy to relax the accommodative system
• Or, do nothing
Right angle geometry: Trigonometry with sin, cos and tan to discover values
Telescope: It magnifies. When turned around, it minifies. Parallel light enters the telescope through the
objective lens. Parallel light leaves through the ocular lens. The equivalent power is 0.00 D. There is no
vergence change. Angular mag can be used to change image size.
CLE 270.1 – Ophthalmic Optics
Notetaker: Fatima Ibrahim
Date: 08/13/2013
Page3
Microscope: A type of telescope. The object is finite instead of at infinity.
Types of Magnification:
• Relative distance
• Relative size
• Projection
• Angular
Determining Magnification:
• Relative distance: If an object is at 50 cm and moves to 25 cm, it becomes bigger: 50/25 = 2 x. (Old
distance divided by new distance)
• Relative size: If an object is 1 cm table and becomes 2 cm tall, it is magnified 2/1 = 2x
• If a 1 cm object is at 50 cm, moves to 25 cm and is now 2 cm tall, it is magnified: (2)(2) = 4 x
• Projection: Back in the day you placed slide into projector machine between the APF and 2 APF and
projected it
• Object at infinity: Image at PPF
• Object at 2 APF: Image at 2 PPF
• Object at APF: Image at infinity
• Object between infinity and 2 APF: Image between PPF and 2 PPF
• Angular magnification is relative distance, relative size, and projection
• M = L / L’
• M = image size / object size
Entrance pupil: The iris displacement about 3mm before the eye.
Larger angle → large image
Magnification of any lenses can be found with M = 1 + zF
The variable “z” is the vertex distance plus 3mm deep into the eye
Question: You examine a patient and find that this patient is a refractive myope with an eye power of 8.00D at a 12.0mm vertex distance. You decide to fit this patient with corneal contact lenses. What
changes in angular magnification would you expect? Why?
CLE 270.1 – Ophthalmic Optics
Notetaker: Fatima Ibrahim
Date: 08/13/2013
Page4
Answer: No calculation is needed. With CLs, there will be less divergent power needed to see the same.
The decrease in divergent power means there will be an increase in image size. So there is an increase in
magnification.