Spectacle lens aberrations, asperics, and atorics

What base curve will a plano lens have for its front surface?

+6.00D

Which aberration are we utilizing when we ask the patient, which side is clearer, the red or the green?

Longitudinal CA

What are the equations for determining longitudinal CA? (dioptric diff. between the blue and red images)

Longitudinal CA = F/v (power of lens/abbe value) [This is the same as =wF, dispersion power * power of lens]

Plano is a +6.00D. What is the trend of base curves has we go minus in power -1, -2, -2?

Moving in the minus direction will cause the lens to become less curved (numbers will go down from +6D (+5, +4, +3) *base curves don't go down as quickly (compared to the positive numbers going up) and this is because the spherical equivalent is divided by 2.

Plano is a +6.00D. What is the trend of base curves has we go plus in power +1, +2, +3?

Moving in the plus direction will cause the lens to become more curved. (numbers will go up from +6).

Why is in impossible to pick a perfect (single) base curve for a high cylinder lens?

Optical performance will decrease as conventional base curves are applied to a high cylinder lens. The base curve you choose will not be optimall for either meridian or if you should use to correct for one, It will be really bad at the other.

What is the equation for the prism power induced by wrap in frames?

Prism = 100*tan(theta)*(thickness of lens in meters/index)*Base curve -thickness must be in meters! -Note, prism induced in lenses by wrap has nothing to do with the power of the lens!

Wrapping the glasses changes the power of the lenses and also induces what?

Prism.... :(

How do we convert relative VA to the VA we are use to measuring?

Relative VA = measured VA/ 20 *direct relationship btwn Abbe value and VA (peripherally). ^abbe value = ^VA in periphery :(

How do aspheric improve optics?

When looking through a +5.50 lens through the center, the image is clear. Then when looking through the periphery, it will produce 0.75D of astigmatism if I use the same radius of curvature throughout the lens. If I change the base curve in the periphery I can achieve sharp vision in the periphery as well! *outside of +7, you will need to use an aspheric if you want to correct for oblique astigmatism.

When should atorics be used?

When there is a high cylinder (greater than 2.00D) -Aspheric = for high spheres -Atoric = for high cyls

The index of refraction we use when speaking of a given material is for yellow light. Do materials with low chromatic aberration have almost the same index of refraction for each wavelength?

Yes!

The two main categories of aberrations are: 1. Chromatic aberrations 2. Monochromatic aberrations What are the two types of chromatic aberrations?

a. Longitudinal (axial) b. Lateral (transverse)

How do aspheric surfaces differ from normal lenses?

"Asphere" means without a sphere. In other words the have multiple radii of curvature. -multiple curves. -based on a surface curvature that comes from a slice through a cone (conic section)

Aspherics lenses can take this distortion effect out. Why do some patient's complain after switching to aspherics?

- Their brain compensated for the distortion already so not he straight lines viewed through the aspheric lens appears bent at first.

What are some characteristics of coma?

-"spherical aberration in disguise" - light from peripheral areas of the lens focused farther from the true image point than it should be. -Light faster in periphery is displaced increasingly father from the point focus; so there is a single point image but it appears comet shaped. -not a big problem because of pupil.

What power should be ordered if you would like the glasses to end up with -5.75D in polycarb with a 25 degree wrap?

-4.47-0.98x180

When we determine the amount of asphericity, it is dependent on the power of the lens, duhh.. But what happens when you have cylinder? What power do you choose to base it on? The cyl power? The sphere power? or a combination of the two?

-Could have two different amounts of asphericities in the two major meridians. (this is called an atoric lens) *Atorics can correct aberrations in both meridians at once.

Why are aspherics recommended for anisometrops?

-Differences in power and differences in base curve result in different magnifications. (occurs when anisometropia exceeds 200D.) *Can use an aspheric on one lens and a spherical on the other.

What are some rules for fitting aspherics in terms of the MRP?

-Find the MRP at the center of the pupil and then drop the MRP using the rule of 1mm drop for every 2 degrees of pants. -but don't drop MRP more than 5 mm below the pupil center. *can use the method of tilting the head so that the glass are perpendicular with the ground and then mark where the pupils sit.

What is the difference between a full aspheric design and a non-full aspheric design?

-Full aspheric: curve starts to vary from the very center of the lens *BP must be exact! Use mono PDs -Non-full aspheric lens: curvature doens't vary until a specific distance from the center. *Have a little more wiggle room to have the pd off a little.

Chromatic aberration is most important for which kinds of materials?

-High-index materials -Fused glass multifocal segments

What are some characteristic of curvature of field or Power error?

-If a designer makes a lens that is completely free of oblique astigmatism, there will be power error that occurs when looking through the periphery of the lens. The dioptric power difference btwn the Far Point Sphere (where the lens should focus light) and the Petzval Surface (where the light is focused when oblique astigmatism is correct) . -"Image shell error"

We know how to find a base curve for a lens of a particular power. What happens when the powers between lenses are different?

-If powers are greater than 2.00D apart = keep the BC as calculated *There will be too much diff. btwn the two. -If powers are less than 2.00D apart = use the BC for the stronger lens for both lenses.

Comparing a plus lens with cylinder with a minus lens with cylinder. How does the sweet spot compare between the types of lenses we have to choose from? (best form poly compared to aspheric poly compared to ViZio atoric pol)

-If you choose the right base curve for a spherical lens, you can get good optics with either the best form of poly and the aspheric poly (the white circle) But if you have an atoric poly, you can get better optics further in the periphery.

What are contour optic lenses?

-Lenses with a 16 base curve used with a wrap-around frame -40% wider field of clear vision claimed -reduced chromatic aberrations b/c the vertex distance stays the same.

How do steep lenses reduce chromatic effects?

-Light travels through he lens at less of an angle than for flat lenses.

What are some characteristics of oblique astigmatism?

-Look through center of lens and see 20/20. Look through periphery and get a reduction in acuity. -Same drop in VA when the lens is tilted. *Most important aberration for spectacle lens!

What are some ways to identify aspheric?

-Noticing the flatness of the front (and back) curve compared to other lenses of equal power -Moving the lens clock sideways. -Viewing a grid attern thorugh a higher plus aspheric lens and not seeing distortion will tell you it is aspheric. -May have identifying markings in the lens

What are some characteristic of distortion?

-Occurs bc there is a different magnification at different areas of the periphery of the lens in proportion to the distance of those areas from the OC of the lens.

What is the difference between Ostwalt's recommendation and Wollaston's for base curve to correct -5.00D lens?

-Ostwalt: found that if you have a -5.00D lens, a +6.00 base curve will eliminate the oblique astigmatism. -Wollaston: found that if you have a -5.00D lens, a +18 base curve will correct the the aberration. *That's a big diff! May be due to a difference in working distances that they used.

What are some characteristics of spherical aberration?

-Peripheral rays focus at diff. points on optic axis than do paraxial rays. *B/c the pupil limits the # of rays entering the eye, spherical aberration is not a problem for us :)

What are the two different types of distortions and what are they caused by?

-Pincushion distortion (formed by Plus lenses) -Barrel distortion (formed by Minus lenses)

What is an achromatic lens?

-Plus lens of one material and a minus lens lens of another material. *different refractive indices chosen to counteract the effects of chromatic aberration. -Lens powers must add up to needed power. -F (blue) and C (red) spectral lines must focus at focal point of lens -Not used for ordinary spectacle lenses.

What aberration can be ignored because of pupil size?

-Spherical aberration

What is a petzval surface?

-when the oblique astigmatism is corrected, the tangential and sagittal foci (the teacup and the saucer) collapse down into a curved surface called the Petzval surface. -until they are corrected the teacup and saucer are always on one side or the other (together) of where the Petzval surface will end up being located.

Example: Rx with 25 degree wap. +8.00D base curve. Polycarb. 2mm center thickness. How much prism and in what base direction?

0.47 prism diopters BO Light enters the "nasal" side of the lens. The lens bends light outward with makes it seem like the image is coming from the nasal portion (BO prismatic effect). *To neutralize, the lens requires 0.60^ BI per eye.

when should Aspherics be recommended? (what powers?)

1. Above +3.00/-3.00 2. Anisometropes *if you have a smaller lens, you don't have to go to an aspheric as quickly. *if you have a larger frame, you will have to go to aspherics more quickly.

What are the (2) benefits of using plus-powered aspherics?

1. Allow the base curve to be flattened 2. Cause a reduction in lens center thickness

Which 3 aberrations are important for ophthalmic lenses?

1. Astigmatism 2. Power error 3. Distortion (least important of the 3)

Other possibilities for using aspherics:

1. Children who are sensitive about how their glasses look (use a poly aspheric) 2. For contact lens wearers so they won't over-wear to avoid thick, ugly glasses. 3. For older individuals in order to decrease lens weight.

What are the two main categories of aberrations that occur in spectacle lenses?

1. Chromatic aberrations 2. Monochromatic aberrations

What are 3 possibilities for Corrected curve lenses?

1. Correct the oblique astigmatism completely and leave the power error uncorrected. *Point Focal Lens* 2. Eliminate power error and leave residual astigmatism uncorrected. *Percival form lens* 3. Design a compromise between the above 2 possibilities. *Minimum tangential error form lens*

What is the simplified sequence of lens development?

1. Flat lenses 2. Periscopic lenses (one surface with 1.25 power on it) 3. Meniscus lenses (one curve has a +6) 4. Corrected Curve lenses (lens that tries to take the whole lens into consideration including peripheral vision.) 5. Aspheric lenses (radius of curvature changes thought the periphery) 6. Atoric lenses 7. "position of wear" design philosophy (customization, takes into account all aspects including working demand)

Comparing spherically based lenses, aspherics and atorics. How are their surfaces constructed for a sphere? for spherocylinder?

1. For simple shares (no cylinder), the front surface is spherical 2. For spherocylinders, the front surface is aspheric. The back is toxic.

Comparing spherically based lenses, aspherics and atorics. Spherically-based lenses are the most commonly used of all lenses. How are their surfaces constructed for a sphere? for spherocylinder?

1. For simple spheres (no cylinder), the front surface is spherical and the back surface is also spherical. 2. For spherocylinders, the front surface is spherical and the back surface is toxic.

What are the purposes for using an aspheric design?

1. Improving optics 2. Flattening lens 3. Thinning lens 4. Good fit in frame 5. Producing progressive power changes.

Review: Lateral CA is the difference in image magnification or diff. in prismatic effect between red and blue images. What are some equations for Lateral CA?

1. Prism power formed from blue - Prism power formed by red 2. distance we are from the center of the lens (FF - FC) 3. Prism power/v (abbe value) *The bigger the lens and the farther out you look = ^ CA

The two main categories of aberrations are: 1. Chromatic aberrations 2. Monochromatic aberrations What are the 5 types of Monochromatic aberrations? (Tidal aberrations)

1. Spherical aberration 2. Coma 3. Oblique astigmatism 4. Curvature of Field (Power error) 5. Distortion

We just got done talking about Chromatic aberrations (lateral and longitudinal), now we will talk about monochromatic aberrations. What are the monochromatic aberrations? (5)

1. Spherical aberrations 2. Coma 3. Oblique astigmatism 4. Curvature of field (power error) 5. Distortion *we still get aberrations even though there is only one wavelength of light.

What are the 4 key variables when designing lenses?

1. Vertex distance 2. Lens thickness 3. Refractive index 4. Front and back curve powers

In what situations is it best NOT to duplicate an old base curve?

1. When prescription changes 2. When the prescription stays the same, but the index of the material changes. 3. Don't request a flatter BC to make the lens look better and be lighter in weight (instead go to an aspheric lens) 4. Don't automatically steepen the base curve for people with long lashes (1st try to find a frame that will avoid the lash touch problem) 5. Don't automatically steepen the BC for a wrap frame w/out requesting an aspheric you know is designed for a +8.00D BC lens.

When should you specify a base curve that you want the lab to use for a pair of glasses?

1. When replacing one lens in a pair (want the BC to match the other lens) 2. When ordering an identically-power second pair of the same index material.

Comparing spherically based lenses, aspherics and atorics. There will be both spheres and spherocylinders in a given lens series that uses an atonic design. How are their surfaces conttructed for a sphere? for a spherocylinder?

1. for simple spheres (no cylinder), the lens is really an aspheric. It can't be an atonic, because it has no cylinder. 2. For spherocylinders, the front surface may or may not be aspheric. However, on the topic back surface both of the meridians change their radius of curvature at different rates. Thus both topic surface powers can be corrected for lens aberrations.

What are some important dispensing factors for lenses with low abbe values (polycarb and some high-index materials)?

1. use monocular PDs 2. Measure major reference point heights, considering pantsscopic angle 3. Use shorter vertex distances 4. Have sufficient panto, but not more than 10 degrees for high lens powers 5. Give attention to comparative edge thickness

Dispersive power is a difficult number to work with, it is easier to work with an F value. What do we work with instead?

Abbé value! (v) *Abbe value (v) = 1/dispersive power(w) -Abbe value is sometimes referred to as constringence or Nu value. *Most commonly used number for identifying amount of CA for a given lens material.

Both spheric and aspheric lens designs can only go so far with cylinder lenses. They can only correct aberration in 1 of the major meridians. What lens can correct aberrations in both meridian at once?

Atorics! *lens aberration may be overcome in both meridians with an atonic lens design.

How can we approximate the base curve that a lab will use for a minus lens?

Base curve = (spherical equivalent)/2 +6.00D

How can we approximate the base curve that a lab will use for a plus lens?

Base curve = spherical equivalent + 6.00D

Why is base curve not really the refractive power of the surface?

Because it is the power of the surface if the lens were made from material having a 1.53 index of refraction.

Since oblique astigmatism is the most important aberration for spectacle lenses, it is a good thing to reduce. How can oblique astigmatism be reduced?

By finding optimum base curve for given lens power.

What is distortion caused by?

Changing magnification in the periphery of the lens. *Aspheric design can often reduce distortion. *by viewing the periphery of a high power lens, you can determine if it is aspheric or not.

True or false. Oblique astigmatism results from rays from an on-axis point.

False! -Rays from off-axis point strike spherical surface at an angle, oblique astigmatism cases light to focus as two line images, instead of a single point, at the tangential focus and sagittal focus. -It is as if light is passing through astigmatic lens instead of spherical lens. *Most important aberration for spectacle lens!

True or false. Higher index material must have steeper curve in order to retain the same "base curve effect" as the old lenses.

False. Higher index must have flatter curve.

True or false. with a lower abbe value, you will have a slower reduction in relative VA in periphery.

False. Low abbe value = ^CA = FASTER reduction in relative VA peripherally.

What should you do if any of these 3 situations come up? -If the frames are large metal or rimless frames -If the frame doesn't hold the lenses securely -If the lens has Rx prism greater than 4 prism diopters

Flatten both base curves by 2.00D (or go into an aspheric lens with a flatter base).

If oblique astigmatism is not corrected, it will form two line images instead of a single point. What do those look like?

Forms a Teacup and a saucer-like image from the oblique astigmatism.

Example: a person orders a pair of wrap -5.75D sunglasses in polycarb. The angle of wrap is 25 degrees. If no compensation is made, what will we have as the effective power?

Fs = -6.07 Ft = -7.39 Axis is of induced cylinder is the same as the axis of the tilt. Face-form wraps around the 90 degree meridian. -6.07-1.33x090

What is the Tscherning Ellipse?

It graphically shows which power goes with which base curve (front surface power). -Which combination will give us the best design.

What is Dispersive power? (omega or W)

It's another way of expressing lateral chromatic aberration. *A measure of how much the lens separates colors of light by refraction.

What is chromatic power?

Lateral CA when referring to prismatic effects *as prismatic effect increases, effects of CA become more powerful and disturb vision more.

Some labs make lenses specifically for wrap frames. What is one example of a lens made for this?

Spazio (made by Zeiss) -They incorporate three different atoric designs to minimize peripheral blur in all powers, to offer wide, clear vision all the way to the periphery.

Lens wrap changes the power of the lens. (creates oblique astigmatism). What are the two equations used to calculate how much the power will change with a wrap?

The F in the equation is the prescription that was ordered. The Fs and the F(T) are the effective powers produced by the wrap in the sagittal and tangential meridians. *So if you want to be certain that a specific power is produced, put the values you want for the Fs and F(T) and calculate what power you should tell the lab to make the lenses.

What is longitudinal chromatic aberration?

The dioptric difference between blue light and red light. -Caused bc the refractive index is diff. for each wavelength. The power of the lens is different for blue and red light. *longitudinal - the horizontal difference

What is lateral chromatic aberration?

The prismatic difference between the blue and red. -Leads to a difference in size between the images formed by red and blue light. *Lateral - think Lat pull downs, vertical difference.

Why are atrocious usually limited to single vision lenses?

They were hard to make!

The aspheric portion was traditionally on the front so it wasn't possible (before) to have a segment on the aspheric lens. What lens produced by three rivers optical can create an aspheric lens with both segment and sphericity on the back?

Tri O Seg! *Type of free-form round segment lens. -Can be produced in any lens material, including poly Transitions.

True or false. Higher abbe value corresponds to lower CA.

True!

True or false. Moving the aspheric curves to the back surface (of a high minus lens), it reduces the lens thickness.

True!

True or false. The base curve is dependent on the working distance.

True!

True or false. Higher base curve in terms of glasses corresponds to a steeper slope.

True! -with spRx the curvature is specified in diopters. *with CL, a higher base curve is measured in mm and corresponds to a flatter lens.

True or false. The Petzval surface is the ideal surface where light will focus.

True.

True or false. When fitted incorrectly, aspheric lenses do not perform any better than spherical lenses.

True. In fact, they may even do worse.

How does the contour optic lenses reduce distortion?

Vertex distance doesn't vary as much.