7. CL VII - CL Optics I Flashcards
Why do contact lenses and spectacles have visual differences despite both producing in-focus retinal images?
Due to the CL being placed directly onto the eye rather than in front of the eye.
What are the visual differences between contact lenses and spectacles? (5 points)
- Magnification
- Binocular vision (accommodation and convergence)
- Field of view
- Oblique astigmatism
- Cosmesis
How do you convert between spectacle BVP and contact lens BVP and what must you be weary of?
You use the effectivity equation
Fcl = Fspec/(1-vd x Fspec)
Where:
Fcl = BVP of contact lens
Fspec = BVP of spectacles
vd = vertex distance
You must do it for each meridian individually.
What is the general rule for adjusting spectacle BVP to contact lens BVP?
Plano to +-4D = no change
+-4D to +-6D = +0.25D
+-6D to +-8D = +0.50D
>+-8D = use effectivity equation
How does magnification change between spectacles and contact lenses?
Myopes:
* objects look ... with contacts
* can potentially achieve a ...
Hyperopes:
* objects look ... with contacts
* may result in ... appearing to be ...
How does magnification change between spectacles and contact lenses?
Myopes:
* objects look larger with contacts
* can potentially achieve a better VA
Hyperopes:
* objects look smaller with contacts
* may result in fine print appearing to be even finer
How does accommodative demand change between spectacles and contact lenses?
Accommodative demand ... with contacts for myopes and ... for hyperopes. This is clinically significant when Rx > ...D.
How does accommodative demand change between spectacles and contact lenses?
Accommodative demand increases with contacts for myopes and decreases for hyperopes. This is clinically significant when Rx > +-4D.
How do spectacles affect convergence demand and how does this change when wearing contacts?
At near, myopes have induced ... due to their spectacles which ... convergence demand. Hyperopes have induced ... due to their spectacles which ... convergence demand.
When wearing contacts, ... is removed as lens stays .... This results in ... convergence demand for myopes and ... convergence demand for hyperopes.
How do spectacles affect convergence demand and how does this change when wearing contacts?
At near, myopes have induced BI prism due to their spectacles which reduces convergence demand. Hyperopes have induced BO prism due to their spectacles which increase convergence demand.
When wearing contacts, prismatic effect is removed as lens stays centred over the cornea. This results in increased convergence demand for myopes and decreased convergence demand for hyperopes.
How do spectacles affect phoria and how does this change when wearing contacts?
E.g. At near, high myope is 7XP. When their near PD is 2mm/eye less than distance, the spectacles induce 3.2BI prism. Therefore, near phoria becomes ..., so convergence demand is ... and patient is more .... Contacts have ... and as such, this beneficial effect is ..., and so the patient is more ....
The opposite holds true for hyperopes.
How do spectacles affect phoria and how does this change when wearing contacts?
E.g. At near, high myope is 7XP. When their near PD is 2mm/eye less than distance, the spectacles induce 3.2BI prism. Therefore, near phoria becomes 3.8BI, so convergence demand is reduced and patient is more esophoric. Contacts have no prism and as such, this beneficial effect is lost, and so the patient is more exophoric.
The opposite holds true for hyperopes.
Can you incorporate horizontal prism into contact lenses?
Prism base always rotates ... due to ... and .... So it is ... to incorporate horizontal prism.
Can you incorporate horizontal prism into contact lenses?
Prism base always rotates downwards due to eyelid interactions and gravity. So it is impossible to incorporate horizontal prism.
How does field of view change between spectacle and contact lenses?
Spectacle limit FOV based on ..., ..., and .... Hyperopic lenses have ... due to ....
When wearing contact lenses, FOV is only limited by ....
How does field of view change between spectacle and contact lenses?
Spectacle limit FOV based on size of lens, vertex distance, and BVP. Hyperopic lenses have 'ring scotoma' due to spectacle magnification.
When wearing contact lenses, FOV is only limited by eye movements.
How do oblique aberrations change between spectacle and contact lenses?
In spectacles, as the lens is far from the aperture stop, may result in significant ..., ..., ..., ..., and ....
In contact lenses, these effects are ... as the lens is ... and the eye is ....
How do oblique aberrations change between spectacle and contact lenses?
In spectacles, as the lens is far from the aperture stop, may result in significant oblique astigmatism, coma, distortion, transverse chromatic aberration, and field curvature.
In contact lenses, these effects are mitigated as the lens is closer to the aperture stop and the eye is always looking through the lens optical centre.
How does cosmesis/apparent eye size change between spectacle and contact lenses?
In spectacles, negative prescriptions make the eye appear ... and positive prescriptions make the eye appear ....
This effect is ... when wearing contact lenses.
How does cosmesis/apparent eye size change between spectacle and contact lenses?
In spectacles, negative prescriptions make the eye appear smaller and positive prescriptions make the eye appear larger.
This effect is absent when wearing contact lenses.
