Single Vision Flashcards
What are single vision lenses?
This are lenses that either correct vision at distance or near.
Does dispensers rule apply to single vision lenses?
Only for distance
What is the dispensers rule?
For every 2° pantoscopic tilt, reduce heights by 1mm
4 possible choices of lens materials
Glass, Plastic, Polycarbonate, Trivex
Ideal properties of a perfect lens?
Scratchproof, indestructible, protection against UV and IR, perfect vision (at all distances- progressive), Invisibility of MAR coating or transitions. Thin and light weight.
Choice of frame depends on what 2 factors?
Size and shape
Anti-reflective coating lasts on average for how long?
Anti reflective coating on average lasts for 2 years. But with proper care it should last the life of the lens.
How does MAR coating increase lens durability?
It is an anti-scratch coating that helps prevent scratches, improves durability and helps prevent damage from dirt and water. It also reduces glare and reflections.
True or false: Anti-reflective coating is damaged by hot water?
True
What is BVD?
What is it normally?
BVD is the distance between the front surface of the cornea and the back surface of a lens. Normally: 12-14m
3 important measurements taken for high powered single vision lenses?
- Mono PD
- Heights - take pantoscopic tilt into account.
- BVD
What is important to note down when measuring heights?
Above or below HCL?
BVD is required when?
When power is +/- 5.00D or more
Effective power of the specs depends on?
BVD
How do hyperopes and myopes moves specs in relation to increasing power of the lens?
Hyperopes push specs down nose and myopes push specs up nose to increase the power.
Does the power increase or decrease when specs moved away from the eye?
Decrease (specs get more + when moved away from the eye).
How is pantoscopic tilt measured when using a trail frame?
Using a paper ruler to read off side of trail frame.
2 ways of measuring pantoscopic tilt during dispensing?
With frame rule and using callipers
Benefits of keeping BVD to a minimum?
*Maximises FOV.
*Reduces distortion.
*Reduces chromatic aberration (also known as colour distortion).
*+ve lenses – reduces retinal image with drop in VA.
*-ve lenses – increases retinal image with increase in VA
* +ve lenses – reduces spec. mag. of px’s eyes.
* -ve lenses – reduces spec. min. of px’s eyes.
What formula is used to compensate for BVD increase:
Is this the same formula for astigmatic lenses?
F(new)= F(old) / 1+ (dx F (old))
Yes, however compensation for each principle meridian and re-write sph-cyl form.
Except for using the formula, what are the other methods of compensating for change in BVD?
- Using focal length and focal power
- Using step-along
- Use conversion table
3 reasons why centration of the lens is important?
*Positioning of zone of lens where paraxial Rx is most effective should be in front of pupil.
*Reduces unwanted differential prismatic effects.
*Reducing the possible formation of ghost images
Why is mechanical consequences of decentration a bigger issue with high RXs?
Because both thickness and weight are affected.
How is decentration calculated?
Decentration= (BCD/2)- Each mono PD
When is decentration required
When BCD is not equal to PD
or used to produce prescribed prism
Effect of inward decentration of higher powered lenses?
-ve lenses: Increase temporal thickness
+ve lenses: Increase nasal thickness
2 lens forms avaiable?
Full aperture and reduced aperture
2 vital features important when selecting a frame for high myopic eyes?
Smallest eye size possible and wide sides to cover thick lens edges
What is the disadvantage of high index lenses?
Higher index lenses have a lower v-value which gives colour fringing and power rings
8 methods of reducing TCA?
*Use materials with highest v-values possible that gives tolerable amount of colour fringing.
*Apply correct horizontal and vertical centration and pantoscopic tilt (zero pantoscopic tilt reduces TCA).
*Best form design
*Sensible frame selection (correct shape and size)
*Fit frames with as small a BVD as possible
*Aspheric surfaces
*HMAR
*Lenses should give good off-axis performance in oblique gaze.
Reduced aperture lenses are custom made for?
Custom-made aspherised lenses for high myopes: aims to eliminate thick edges. Comprises aperture and margin.
For RX: -11D (Super lenti)
Benefits of using reduced aperture lenses for patients?
*Good VAs
*Good FOV
*Controlled peripheral aberration.
*Improved edge thickness and mass.
*Enables wider frame selection but using small horizontal and vertical eye sizes.
*Cosmesis ok
*Helps remove minification of face.
High hypermetropia lenses, what are the problems?
*Weight and thickness of lens.
*Oblique performance when viewing off axis.
*Magnification
*Lack of accommodation if aphakic (aphakic- means no lens inside in the eye).
*Restricted FOV
What are polynomial lenses?
Higher order aspheric surfaces which are obtained by deforming a conicoid to get a surface which flexes back on itself. Polynomial design, the surface power reduces from the centre of the lens to the edge, where the periphery is a focal.
Advantages of polynomial lenses?
*No visible dividing line
*Good mean oblique power when viewing off-axis.
*Reduced distortion
*Slightly thinner lenses
*Some sensitivity to fitting distance changes.
*Increased FOV
*Reduced “Jack-in-the-box” effect.
*Flatter
*Less magnification
*Less TCA
Importance of minimum size uncut (MSU)?
Gives smallest blank size possible- important for high powers.
Frame PD calculation?
Frame PD= Eye size +DBL
How is MSU calculated ?
MSU = [Difference between px’s PD and frame PD] + longest diameter + 2 for glazing
Oblique astigmatism is due to?
Due to obliquely incident light on the spectacle lens becoming astigmatic after refraction due to differing angles of incidence in the principal plane of refraction.
Oblique astigmatism is reduced by?
1.Changing lens form (e.g., to point focal lens such as Zeiss Pointal)
2. Altering stop (pupil) position but not size
Curvature of field?
*Describes the blur seen by patients when the look through the edge of their spectacles.
*Also known as mean oblique error(MOE).
What is far point sphere (FPS)?
Is the image plane of the eye
Curvature of field is reduced by?
- Using a combination of lenses
- Changing lens position and form
Distortion affects shape or image sharpness?
Shape of image
Why does distortion arise?
Arises due to fact that power of spherical surface increases towards periphery.
Difference in distortion of a +ve vs -ve lens?
+ve lenses give a pincushion shaped image and –ve lenses give a barrel shaped image.
Distortion reduced by?
Lens form
Size and position of stop (pupil)
What are best form lenses?
What are the 3 possible lenses that can be used?
This is defined as being one whose surface powers have been specially computed to eliminate, or at least minimize, some stated defects in its image properties.
1. Point focal lenses
2. Percival lenses
3. Minimal Tangential Error- most common
Point focal lenses eliminate?
OA
Percival lenses eliminate?
Field curvature
Definition of refractive index
RI is that it is the speed of light in a vacuum divided by the speed of light in a material.
Relation between RI and thickness of lens?
Increases in RI will reduce the thickness of a lens.
Constringence/ Abbe number/ V value refers to?
To the dispersion of a material.
This is the amount the material spreads out the different wavelengths of light passing through it.
V-value is the reciprocal of?
The dispersive power of the material and indicates the degree of TCA.
How is TCA calculate?
TCA = prismatic effect/ V-value = P/V
What is density?
Density is weight per unit volume (in g/cm3).
Aim of anti-reflective coating?
To prevent the loss of light by reflectance at the lens surfaces i.e., to increase light transmission. Another aim is to improve on the cosmetic effect of reflections.
What is CVF and how is it calculated?
*Ratio describing the variation in surface power of an optical medium when compared with crown glass.
*CVF= 0.523 / (n-1)
What are super-lenti
RX 11D+
*Custom- made aspherised lenses for high myopes.
*Aims to eliminate thick edges.
*Available in range of high RIs.
*Requires mono-PDs and heights.
*Little or no decentration.
*Lens thickness does not go above 5mm.
*The higher the power the smaller the aperture.
Who suffers from Jck-in-the-box effect?
High hypermetropia
How is blended lenticular different from conventional lenticular lenses?
Blended lenticular lens is different from a conventional lenticular lens as the carrier portion is blended into the corrective portion to improve the cosmetic appearance.
Describe Jack-in-the Box Effect?
Objects seem to jump in and out of field of view as it moves out if and into the ring of scotoma.
Sudden disappearance and reappearance of the objects in visual space with ring scatoma.
