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91 Cards in this Set
- Front
- Back
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spectacle magnification
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retinal image size relative to the optical lens, either spectacle or CL
-form of angular magnification |
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relative spectacle magnification
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retinal image size in corrected ametropia relative to emmetrope retinal image size
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magnification
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comparing image size with optical system to image size without optical system
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What are the 4 types of ratios in magnification?
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1. comparing image size to object size
2. comparing image angle to object angle 3. comparing image distance to object distance 4. comparing object vergence to image vergence |
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relative distance magnification
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comparison between one distance to another (no optical system)
-object size remains constant |
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relative size magnification
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comparison between sizes of an object at a given distance
-distance remains constant (enlargement) |
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angular magnification
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comparison between angular subtense of image formed by optical system to angular subtense of object
-reduces visual field -maintains working distance |
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nominal magnification
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-commonly seen among any simple HH magnifiers
0based on object reference distance of 25cm -angular subtense of image to angular subtense of object at 25cm M=P/4 |
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trade magnification (manufacturer's, conventional)
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M=P/4 + 1
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what kind of magnification is from an afocal telescope?
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angular magnification
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What are the object and image vergences from an afocal telescope?
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object vergence: 0
image vergence: 0 secondary focal point of objective coincident with primary of ocular |
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What are the lenses in a galilean telescope?
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plus-objective
minus-ocular |
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What is the image from a galilean telescope?
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erect and virtual image
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What are the lenses in a keplerian telescope?
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both plus lenses
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what is the image formed in the keplerian telescope?
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inverted and real image
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Is Galilean or Keplerian w/ or against motion?
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Kelperian: against motion
Galilean: with motion |
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what is an exit pupil?
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image of objective lens
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where is the exit pupil of the galilean?
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located behind ocular lens (inside T/S)--virtual
-further away from eye's center of rotation -smaller FOV |
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where is the exit pupil of the keplerian
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"floating" in front of ocular lens--real
-closer to eye's center of rotation -typically larger field of view |
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what happens when you put the Rx at the OCULAR
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no change in telescopic magnification
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what happens when you put the Rx at the OBJECTIVE
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changes magnification; tube length stays the same; effectively the same as putting a cap over obj lens
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Turning the tube length CW vs CCW does what to the t/s?
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CW (shortens)= myope (adds minus)
CCW (lengthens)=hyperope (adds plus) |
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when you lengthen or shorten the t/s how do you change the mag in a GALILEAN?
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shortening it decreases the mag
lengthening it increases the mag |
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when you lengthen or shorten the t/s how do you change the mag in a KEPLERIAN?
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shortening it increases the mag
lengthening it decreases the mag |
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some points about what telescopes do
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increase image (angular) size
increase movement decrease field decrease object brightness perception of closer distance small depth of focus increase peripheral aberrations |
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What is the Equivalent Viewing Distance (EVD)?
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the distance at which the object would subtend an angle that is equal to the angle that the image subtends at the eye
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In terms of the SHAPE component in spectacle magnification, what increases or decreases the mag?
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increase in thickness and power increases the M
higher index of refraction decreases M |
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aberration
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failure of refractive surface to bring all rays from an object point to a desired image point: image blur
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What is the simplified theory?
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point object--point image
monochromatic light 1st order theory sin theta~theta appropriate for on-axis |
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What is the complex theory?
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on axis vs. off axis
3rd order theory, sin theta=theta-theta^3/3! |
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What is chromatic aberration controlled by?
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lens material
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what is monochromatic aberration controlled by?
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lens form
affected by: 1. lens aperture size, 2. vertex distance 3. panto tilt 4. OC position 5. lens thickness 6. oblique rays |
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What is the axial chromatic aberration (ACA)?
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dioptric separation between red and blue focus
blue in front, red in back P(blue)-P (red)=P(yellow)/nu |
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Lateral Chromatic Aberration (LCA) is what?
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linked with ACA
size difference--difference in linear magnification between chromatic images LCA=Z/nu |
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Can chromatic aberration be eliminated in an optical element made of a single material?
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no
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What is the achromatic system
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two elements (doublet) of DIFFERENT materials that produce EQUAL BUT OPPOSITE dispersion
-correct: blue and red wavelength -for CA but secondary CA remains |
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What is the apochromatic system?
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combines THREE elements to eliminate secondary CA.
-attempt to correct: blue, red, and green wavelengths into a single point |
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What's the formula for reducing CA with achromatic lenses?
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(P1/v1)+(P2/v2)=0
Ptotal=P1+P2 |
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What is the formula for reducing CA with achromatic prisms?
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Ztotal=Z1-Z2
(Z1/v1)-(Z2/v2)=0 |
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What are the two types of monochromatic aberrations?
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1. image degradation
2. image deformation |
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What are the 3 types of image degradation?
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spherical aberration
coma marginal astigmatism (oblique, radial) |
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What are the two types of image deformation?
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curvature of field, distortion
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what are the rays in spherical aberrations?
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axial rays
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what are the rays in coma aberration?
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peripheral off axis rays
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what are the rays in marginal astigmatism
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oblique paraxial rays
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What do the longitudinal and transverse spherical aberration depend on?
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LSA: SQUARE of aperture
TSA: CUBE of aperture |
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What are some characteristics of spherical aberration?
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dioptric separation
dependent on aperture size controlled by lens form independent of stop location |
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What are four ways to correct spherical aberrations?
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aspheric surfaces
non-aspheric surfaces crossed lens single spherical surface |
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How do aspheric surfaces correct SA?
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also reduce oblique astigmatism and distortion
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how do non-aspheric surfaces correct SA?
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SA may be reduced but not eliminated
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How do single spherical surfaces eliminate SA?
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they eliminate SA and COMA if:
1. obj and image are at the surface 2. obj and image are at the center of curvature 3. the sys is aplanatic (sys designed to reduce coma and SA) |
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What is COMA?
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variation in magnification between peripheral and paraxial rays
aperture stop location INDEPENDENT if SA corrected. aperture stop location DEPENDENT if SA not corrected |
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What are the two types of marginal astigmatism?
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oblique
radial |
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What is oblique astigmatism
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a type of MA.
inability of a lens (spherical) to form a point image of an oblique point object -depends on SQUARE of object height and pupil diameter |
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What is the Petzval surface?
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flat object
image formed by lens, curved located at SECONDARY focal point of lens -a theoretical perfect surface where MA ABSENT |
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How do you change marginal astigmatism?
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control lens form.
variable factors: viewing distance, aperture stop location, lens material |
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What is the Tscherning ellipse
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relationship between surface power and back vertex power of a thin lens for which oblique astigmatism is eliminated
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What is each Tscherning ellipse based on:
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fixation distance
angle of view distance of lens from center of rotation of eye refractive index lens thickness |
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What are the two branches of the Tscherning ellipse?
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Wollaston--steeper, upper
Oswalt--flatter, lower |
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Far point sphere is a function of what
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the eye
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petzval surface is a function of what
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the lens
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What is curvature of image?
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inability of a lens to form a plane image of a plane object
-if there is no marginal astig, the curved image will lie on the petzval surace |
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What is the definition of far point sphere?
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locus of points conjugate to the fovea as the eye rotates, where r=0.027-f'
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What is image shell (power) error
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dioptric difference between Petzval surface and far point sphere
-curvature of image |
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What is the mean oblique power (MOP)?
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where MA=0
(Pt +Ps)/2 |
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What is the mean oblique error (MOE)?
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difference between MOP and lens back vertex power
MOE=(Pt+Ps)/2-Pv MOE=MOP-Pv |
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What are the three ways to minimize MA and CI?
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Point focal lens
Percival lens form compromise between the two |
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What is the point focal lens?
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correct oblique astigmatism completely, but leave power error uncorrected
MOP~Pv MOE does not equal 0 |
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What is the percival lens form?
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eliminates (mean) power error
-leaves some residual astigmatism MOE=0 |
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What is Vogel's formula?
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best choice for base curve
(+): SE +6.00 (-): SE/2 +6.00 |
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What is distortion?
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inability of a lens to form an image of the same shape as the object
-depends on cube of object size -depends on placement of stops in the optical system |
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What kind of lens gives pincushion distortion?
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plus lens
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What kind of lens gives barrel distortion?
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minus lens
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How do you minimize distortion?
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back curve of -20D
-aspheric lens -orthoscopic lens |
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What is the difference between spectacle accommodation and ocular accommodation?
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spectacle accommodation: at plane of spectacle
ocular accommodation: at principal plane of eye (15mm behind pole of spectacle lens). difference between vergence at distance and near reference to principal plane of eye |
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What are the 7 methods for vertical imbalance compensation?
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DOC positions
2 pairs of SV lenses Dissimilar segments Ribbon segments contact lenses Fresnel prism Bi-centric grinding (slab off) |
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What are the prism options for a Frensnel Prism?
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1-10, 12, 15, 20, 25, 30, 35, 40.
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What is the method of conventional slab off:
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base down removed from most minus lens
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what is the method of reverse slab off:
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base down added to least minus (most plus) lens
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What is the limit for vertical imbalance compensation in slab off?
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1.5 to 6 prism diopters
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What are the two methods of verification?
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1. lensometry--direct
2. lens gauge--indirect |
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What is the method of lensometry verification in slab off
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straddle slab off line over lens stop, ID separation of mire target, yield prismatic separation or approximate mag of slab off
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What is the method of lens gauge?
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assumes 1D=1 prism diopter
place lens clock above slab off line, read base curve of lens. -lens clock with center pin on slab off line. -difference between two readings. |
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What does the Tscherning ellipse tell us with reference to lens design and lens aberrations?
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there is a relationship between the physical form of the lens and the prescription of a lens for which MARGINAL ASTIGMATISM can be minimized. Since the physical form of the lens is related to the design, optimizing these features would directly impact the expected aberration from the lens.
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What is the significance of the family of Tscherning ellipses?
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there are many variables in the lens design that will impact the ellipse.
so for each variable the ellipse will take on a slightly different range of values for which MA may be minimized. variables include: lens thickness, lens index, viewing angle, viewing distance. the family of Tscherning ellipse represent all of these ellipses and the lens designer must choose which one is good for the patient. |
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Define spectacle magnification
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a form of angular magnification where by the magnification of the corrected eye retinal image is compare to the retinal image size of the uncorrected eye
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Define relative spectacle magnification
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magnification is the comparison of the corrected ametropic retinal image size and the emmetrope retinal image size.
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Define what is far point sphere and what is its significance with reference to a specific monochromatic aberration
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the far point sphere is the sphere representing the locus of points conjugate to the fovea and generated by the eye as it rotates about its center of rotation. When the FPS is compared with the petzval surface, any difference that is presented between these two conic surfaces is a representation of the presence of curvature of field or power error
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What does the petzval surface represent?
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a representation of the situation where no marginal astigmatism exists and theorteically lies at the same position where the secondary focal point of the lens is.
-the surface is possible because when a flat object is imaged through a curved lens, the image is not flat but rather curved. |
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In the Tscherning ellipse, ID the name of the upper and lower part of the ellipse. what is the difference between these components of the ellipse as it pertains to the design of the lens?
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Upper--Wollaston, lower--ostwalkt
The graph represents the relationship between the front curve (or back curve) of the lens with the lens vertex power in order to reduce MA. in the calculation for the best form lens, the QUADRATIC EQUATION gives to possible roots. One is steeper and the other is flatter. The steeper ones have a higher value and are on the upper part of the ellipse--Wollaston. The flatter is on the lower part of the ellipse in the Ostwalt curve. |
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with reference to reducing the amount of vertical imbalance, why and how does the use of dissimilar segments represent an appropriate management tool for minimizing the prism?
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it apples the CONCEPT OF TOTAL DIFFERENTIAL DISPLACEMENT AT THE READING LEVEL. So with each lens the goal is to select a segment that helps to reduce the amount of vertical prism from the major lens. the results is that the segment OC needs to belower or higher in the segment to induce the amount of prism in the opposite direction from the major lens. as each lens will have different amount of prism due to the difference in their power, the segment will help to reduce the imbalance between the two lenses will also be different; hence the concept of dissimilar segments.
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