6 - Corrective Curve Lens Design - Exam 2 Flashcards
What dioptric power does the back surface of a periscopic lens have?
-1.25 D
What dioptric power does the front surface of a meniscus lens have?
+6.00 D
Which aberration were the first corrective curve lenses designed to minimize?
Radial astigmatism
What is true about base curves that correct or minimize one type of aberration?
They often make another type of aberration worse
Would a patient with high sensitivity to chromatic aberrations be more receptive to a high or low abbe value material?
High abbe value
Chromatic aberrations minimized
What are the 5 types of Seidel (third-order) aberrations?
- Spherical aberration
- Coma
- Radial astigmatism (AKA oblique astigmatism)
- Curvature of field
- Distortion
Which 2 types of third-order aberrations are minimized by the pupil?
- Spherical aberrations
2. Coma
Which 2 types of third-order aberrations are minimized by selecting the appropriate base curve?
- Radial astigmatism
2. Curvature of field
Which type of third-order aberration can not be eliminated, but also is the least problematic?
Distortion
What are the 2 most important types of off-axis aberrations to control?
- Radial astigmatism
2. Curvature of field
Where is a hyperope’s far point located?
Behind the eye
What is the far point sphere?
The locus of far points for all possible viewing angles
For radial astigmatism, will the tangential focus or the sagittal focus be closer to the lens?
Tangential focus
Effective tangential power > Effective sagittal power
How many mm should you drop the optical center for 2 degrees of pantoscopic tilt?
1 mm
If you drop the optical center 1 mm, how many degrees of pantoscopic tilt are you accounting for?
2 degrees
In what type of eyewear are the effects of radial astigmatism and induced prism especially apparent?
Wrap-around prescription eyewear
What are 2 other names for curvature of field error?
- Petzval surface
2. Power error
How does the curvature of the Petzval surface compare to the curvature of the far point sphere
Petzval surface is usually flatter
What type of lens corrects radial astigmatism completely but leaves the curvature of field uncorrected?
Point focal lens
What type of lens minimizes the power error but leaves residual radial astigmatism?
Percival form lens
What type of lens partially corrects both curvature of field error and radial astigmatism?
Minimum tangential error form lens
Which type of corrective curve (or best form) lens is most common?
Minimum tangential error form lens
What are 4 important parameters to consider with lens design?
- Vertex distance
- Lens thickness
- Refractive index
- Front and back lens surface powers
What is the purpose of a Tscherning ellipse?
Tell what base curves eliminate radial astigmatism for a given lens power
Where does the Tscherning ellipse not cover, and what does this mean?
Does not extend into high plus power range
Radial astigmatism can not be eliminated for aphakes or high hyperopes
How is Tscherning’s ellipse used in the real world?
Divided into sets of prescriptions for a given base curve (+10 BC for high plus, +2 BC for high minus, etc.)
What base curve does a plano lens usually have?
+6.00 D
How does the back surface of a lens change as you increase minus power?
Back surface gets steeper with more minus
How does the back surface of a lens change as you increase plus power?
Back surface gets flatter with more plus
What is Vogel’s formula for a plus lens?
BC = Spherical Equivalent + 6.00 D
What is Vogel’s formula for a minus lens?
BC = Half of the spherical Equivalent + 6.00 D
For different powered left and right lenses, which base curve should you choose if both lenses are plus?
The steeper base curve
For different powered left and right lenses, which base curve should you choose if both lenses are minus?
The flatter base curve
For different powered left and right lenses, which base curve should you choose if one lens is plus and the other is minus?
The base curve with the higher numerical value (steeper)
What base curve are most frames designed to be used with?
+6.00 D
How may plus lenses need to be altered to fit in metal frames?
They may need to be flattened
How may minus lenses need to be altered to fit in metal frames?
They may need to be steepened
What is the effect of prism on magnification of a lens?
Increases magnification
Due to an increase in lens thickness
What may be used to reduce prismatic effects?
A flatter base curve
