10 - Spectacle Magnification (Exam 3) Flashcards
How are anisometropia and antimetropia different?
Anisometropia is a difference in lens power between the two lenses
Antimetropia is a difference in sign of lens power between two lenses
What challenge does a large difference in lens power between eyes bring?
Difference in spectacle magnification
What is the formula for spectacle magnification?
Retinal image size (corrected) divided by Retinal image size (uncorrected) for the same eye
How is spectacle magnification expressed?
As a ratio (1.05 is equal to 5% magnification)
How is the normal image size taken?
The image size for an emmetropic eye with +60D refractive power
What 2 factors contribute to magnification (or minification) of an image?
- Power of the lens
2. Shape of the lens
What 2 parameters are included in the power factor of a lens?
- Vertex distance
2. Back vertex power
What 3 parameters are included in the shape factor of a lens?
- Lens thickness
- Index of refraction
- Front base curve
What is the formula for shape factor?
1 divided by 1-((t/n)f)
What is the formula for power factor?
1 divided by 1-dP
What are the units for lens thickness in the shape factor equation?
Meters
What are the units for vertex distance in the power factor equation?
Meters
What is spectacle magnification equal to?
Product of shape factor and power factor
shape factor)(power factor
What is the formula for relative spectacle magnification?
Image size for corrected ametropic eye divided by image size for standard emmetropic eye
What does Knapp’s Law state?
Refractive error caused by axial ametropia can be corrected by thin lenses positioned 15mm in front of the eyes
Do spectacle lenses follow Knapp’s Law?
No
They are not thin, flat lenses
What is the best way to correct patients with refractive ametropia without changing retinal image size?
Contact lenses
What is the normal magnification of an uncorrected emmetropic eye?
1.00
What do plus lenses give in terms of magnification?
Gain in magnification (>1.00)
What do minus lenses give in terms of magnification?
Loss in magnification (<1.00)
Where is the entrance pupil for the eye located?
3 mm behind the cornea
How is the ‘d’ value found for the power factor of a lens?
vertex distance plus 3 mm
For a myope, will CLs or spectacles result in a smaller loss of magnification?
Contact lenses
For a hyperope, will CLs or spectacles result in a smaller gain of magnification?
Contact lenses
What other problems result in symptoms similar to aneiseikonia?
Refractive error and oculomotor imbalance
What are 4 indications of clinically significant aneiseikonia?
- High anisometropia/ high astigmatism
- Physical alterations to the eye (IOL, LASIK, etc.)
- Complaints of spatial distortion
- Monocular vision is more comfortable
What is aneiseikonia?
Dissimilar retinal image size
What are the 4 types of aneseikonia?
- Physiological aniseikonia
- Symmetrical aniseikonia
- Meridional aniseikonia
- Asymmetrical aniseikonia
What is significant about physiological aniseikonia?
Allows for the perception of depth
What is symmetrical aniseikonia?
Images equally larger or smaller in every meridian
What is meridional aniseikonia?
Symmetrical, but meridional size difference in the eyes
Can be along any meridian
What is noticed with asymmetrical aniseikonia?
Progressive increase or decrease across the visual field
What are the 2 types of ametropia?
- Axial ametropia
2. Refractive ametropia
What is the best way to correct ametropes with refractive ametropia to prevent aniseikonia?
Contact lenses
How can you tell clinically whether a patient’s ametropia is axial or refractive?
Keratometry findings
Significant differences signal refractive ametropia
What is the best way to correct anisometropes with high astigmatism?
Contact lenses
What are 4 methods of reducing aniseikonia with spectacles?
- Short vertex distance
- Small eyesize frame
- Aspheric lenses
- High index lenses
What may be necessary to correct with presbyopes that have aniseikonia?
Vertical imbalance
via Slab off
What is another term for iseikonic lens?
Size lens
When correcting aniseikonia, should aspheric lenses be used in just the right or left lens?
No
Aspheric lenses should be used OU
What is the minimum center thickness for finished poly/trivex lenses?
1.0 mm thick
What is the minimum center thickness for semi-finished poly/trivex lenses?
1.5 mm thick
What is the expected base curve for a minus lens?
Flatter than +6.00D
What is the expected base curve for a plus lens?
Steeper than +6.00D
What is the minimum vertex distance for spectacles, and what does a smaller distance result in?
11 mm
Lash crash
What is the industry standard for minimum center thickness of a lens?
2.0 mm
At what percent of retinal image size difference can patients reliably fuse 95% of the time?
3.90 to 3.99%
If you can’t get retinal image size difference to just below 4%, should you force it and risk poor cosmetics?
No
Patients are likely already monocular at this point
What 4 components of a lens can be altered in iseikonic lens design?
- Front base curve
- Index of refraction
- Vertex distance
- Center thickness
When dealing with aniseikonia, should you add magnification or minification?
Always add magnification to the eye that has less
What are 3 ways to increase magnification in any lens?
- Steepen the front base curve
- Increase center thickness
- Increase vertex distance
