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81 Cards in this Set
- Front
- Back
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what is the relationship between the refractive index and the wavelength
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refractive index varies INVERSELY with the wavelength
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in the normal eye how many wavelengths can be in focus at any single time
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ONE
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how is longitudinal chromatic aberrations quantified
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the difference between the vergences of the retinal conjugates for the wavelength (1) and a reference wavelength (2)
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what wavelength is the eye emmetropic
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587.6nm
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how is transverse chromatic aberration quantified
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difference in magnification as a function of wavelength
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what is the main difference between monochromatic and chromatic aberrations
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chromatic aberrations depend only on the refractive index of the eye
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how do emmetropic eyes respond to red and green light?
myopic eye? hyperopic eye? |
EMMETROPIC
green: slightly myopic red: slightly hyperopic In a DUOCHROME TEST 1. Myopic Patient will see RED blacker and darker 2. Hyperopic Patient will see GREEN blacker and darker |
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what causes spherical aberrations
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1. from the different "zones" of an optical system having different focal lengths
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what is positive spherical aberration?
what is negative spherical aberration? |
1. more refractive power in the peripheral part of the optical system
2. more refractive power in the central part of the optical system |
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what is the cause of barrel distortion?
what is the cause of pincushion distortion? |
barrel: limiting aperture in front of a positive lens
pincushion: limiting aperature behind positive lens |
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what causes diffraction
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deviation of light as it passes by an edge or through an aperture
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what is rayleigh limit of resolution
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airy disc formed from point objects are just resolvable when peak of one function falls on the first zero point of the second function
this value is equal to the airy disc peak to trough width |
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what are the effects of pupil size on diffraction and aberration
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1. diffraction effects increase as pupil size DECREASES
2. aberration become more significant as pupil size INCREASES |
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what causes scatter effect
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due to spatial variations in refractive index usually on a microscopic scale
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what is fourier transforming used for
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means of transforming a signal defined in the spatial domain to the spatial freq domain
any repetitive stimulus can be represented as a sum of sinusoids. of appropriate amplitude and phase |
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what are the optical functions of the pupil size
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1. retinal image quality
2. depth of field 3. retinal light level |
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what diameter of the pupil provides the best balance between diffraction and aberrations
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2-3mm
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what is depth of field?
what is the relationship between pupil size and depth of field? |
1. distance range from the focused point within which an object can be moved without producing a change in perceived clarity (in object space)
2. the larger the pupil, the narrower the depth of field |
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what is depth of focus
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the greatest distance the image plane can be moved without producing a change in perceived clarity
depth of focus is in image space |
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what is the influence of depth of field on VA
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the better the VA, the smaller the depth of field
the better the VA. the smaller the just-detectable blur circle that can be appreciated for a defocused point in the object |
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what is the hyperfocal distance
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point in space conjugate to the retina when the far depth of field is at infinity
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what are the benefits of wavefront guided treatements
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1. they take into account measured lower and higher order aberration to create a more precise ablation profile
2. to minimize spherical and cylindrical errors over all physiological pupil sizes without inducing new higher order aberrations |
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what is a disadvantage of LASIK surgery
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increases higher order wavefront aberrations...most notably spherical aberrations
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for direct ophthalmoscope in an emmetropic patient what is the magnification?
myopic? hyperopic? |
simple magnifier
erect image magnification=F(eye)/4 myopic patients have higher mag. hyperopic patients have less mag. |
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what is the type of image seen through a BIO condensing lens?
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real and inverted
image has depth |
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how is magnification calculated with BIO and SLIT LAMP?
what is the relationship between magnification and field of view |
lateral mag=-F(eye)/F(condensing lens)
higher the dioptric power, wider the field of view |
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what is a critical angle?
what is the critical angle of the cornea |
1.incidence>critical=> reflection
2. incidence=critical=> 90 deg 3. incidence<critical=> refraction arcsin(1/1.376)= 46deg |
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how does a gonioscopy work
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1. because of the existence of the critical angle at the air-cornea, light from the ant. chamber is reflected back into the eye
2. a gonioscopic lens in contact with the eye (with or without fluid) virtually negates the critical angle effect and permits visualization of the angle |
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what are the effect of IR and UV
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1. thermal (IR) effect: too much heat burns tissue
2. photochemical (UV) effect: oxidation induces toxic by-products grotthus-draper law: radiation must be absorbed before a photochemical tissue effect takes place |
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LEARN SAFE TIMES FROM ALL THE OPHTHALMIC INSTRUMENTS
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LEARN SAFE TIMES FROM ALL THE OPHTHALMIC INSTRUMENTS
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radiation effects on the retina
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1. phakic eye can see light 310nm, UVB light reaches the retina!
2. len in phakic eye may absorb UVB, but optical protection required |
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how do LASERs work
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1. ACQUIRED MEDIUM contains atoms whose electrons are excited to a metastable energy level (lasers are often named after their active medium)
2. EXCITATION MECHANISM is a power source that pumps energy into the active medium 3. FEEDBACK MECHANISM allows the energy of the election to build up 4. a photon of the proper wavelength induces the cascade that causes laser radiation to escape through the partially transmitting mirror |
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1. spontaneous emission?
2. stimulated emission? 3. what is used for LASERs? |
1. excited atoms release photons without stimulation
2. excited atoms release photons as a result of stimulation by a photon at the same frequency |
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tissue effects of laser therapy
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1. ABLATION:
precision localized tissue vaporization eximer laser, 193nm, used in LASIK 2. COAGULATION: heat tissue inflammatory response results in scarring, reducing oxygen demands to a tissue (PRP and ALT) causes pigmentation 3. DISRUPTION instantaneously superheats and EXPLODES tissue Nd:YAG (1064nm) capsulotomy blasts a hole in the opaque capsule behind an IOL |
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what are the characteristics of LASERs
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1. monochromatic:
enhance tissue absorption/transmission 2. directionality: high directional: narrow beam with very little spread 3. coherence: highly coherent |
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properties of Excimer laser
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1. rare gases (ArF) in excited state
2. UV (193nm) 3. very high energy pulses 4. converts tissue into vapor (ablation) |
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properties of Argon laser
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used for photocoagulation or retinal blood vessels, iridotomy, trabeculoplasty
1. visible 2. argon gas 3. continuous beams 4. heats tissues (burns) |
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properties of Nd:YAG laser
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used for posterior capsulotomy
1. solid: neodymium ions in yttrium aluminum, garnet 2. near IR 3. high energy pulses (Q-switching) 4. converts tissue into vapor (ablation) or disruption |
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what causes "corneal corona"
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1. seen as halos around bright light sources
2. disruptions in corneal endothelium due to ocular diseases (high IOP) can induce swelling and loss of transparency maybe contact lens problem or high IOP (glaucoma) |
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what causes floaters
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1. opacities in the vitreous gel which cast shadows in the retina
2. due to vitreal liquifaction and collapse associated with age and refractive error 3. increase with age, sudden increase maybe due to retinal detachment 4. opacity is more visible if, its large, near retina, or the pupil size is small |
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blue field entoptic phenomenon or "flying spots"
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1. WBC or RBC traversing within retinal capillaries
2. seen as spots moving in slightly curved path 3. optimally perceived when looking at empty blue field around 400nm |
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what are phosphenes
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visual sensation when the retina is stimulated by energy other than light
direct pressure on the globe with finger |
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which is the brightest purkinje image
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PI
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which purkinje image is the closest to the cornea
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PII...hardest to see
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which purkinge image is largest with accommodation relax
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PIII...furthest away from the cornea
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which purkinje image is the smallest
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PIV
smallest only real image |
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what type of image of the purkinje images
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I-III: virtual and erect
IV: REAL |
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what is a catoptric image...which purkinje image(s)
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formed by reflection alone
PI is the easiest to find |
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what is a catadioptric image...which purkinje image(s)
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formed by refraction and reflection
PII, PIII, PIV |
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relative size of purkinje images in UNACCOMODATED eye?
ACCOMMODATED eye? |
1. III>I>II>IV
2. I>II>III>IV |
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optical axis
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line that connects the centers of the curvature of the refracting surface (perpendicular to the cornea and lens surface)
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visual axis
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line that connects fixation point and the fovea and passes though the nodal points
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angle alpha
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formed by optical and visual axis @ nodal point
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line of sight
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from the fixation point to the center of the entrance pupil and the from the center of the exit pupil to the fovea.
clinical counterpart of the visual axis |
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pupillary axis
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center of the entrance pupil that intersects the corneal center in the a perpendicular manner
clinical counterpart of the optical axis |
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angle lambda
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between the pupillary axis and the line of sight at the center of the entrance pupil
clinical counterpart of angle alpha |
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angle kappa
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between the pupillary axis and the visual axis
angle kappa and angle lambda are used interchangeably |
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fixation axis
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fixation point to the center of rotation
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angle gamma
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between the fixation axis and the optical axis at the eye's center of rotation
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prolate cornea
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center: steeper
periphery: flatter |
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oblate cornea
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center: flatter
periphery: steeper |
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what do you have to do to the correcting lens if you decrease the vertex distance?
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decrease vertex distance=INCREASE POSITIVE POWER
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what do you have to do to the correcting lens if you increase the vertex distance?
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increase vertex distance=INCREASE NEGATIVE POWER
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what are the three types of schematic eyes
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1. gullstrand exact eye (2C, 4L)
2. gullstrand simplified eye (1C, 2L) 3. emsley reduced eye (1) |
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what is an A-scan...what is it used for
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1. amplitude scan
2. used to determine axial length of the eye and intraocular lens power for cataract surgery 3. each surface will yield a sharp spike |
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how must the A-scan be applied
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must be perpendicular to the apex of the cornea with a methylcellulose coupling media
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what is an B-scan...what is it used for
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1. brightness scan
2. produce 2D image of the anterior segment of the eye |
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what is optical pachymetry used for...what method does it utilize
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1. assess thickness of cornea with doublet prisms...like kerotometry
2. use amplitude method |
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what FOUR things happen to the lens during accommodation
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1. Decrease radius of curvature, anterior surface decrease more than the posterior surface
2. Increase anterior-posterior thickness -- effective forward displacement of the whole lens; Decrease in equatorial diameter 3. May sink in direction of gravity 4. Increase in lens power causes a decrease in the eye’s focal lengths |
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how do you calculate accommodative amplitude
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accommodative amplitude= (far point vergence) - (near point vergence)
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what is Amplitude of Accommodation
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the amount of accommodation exerted in diopter power to move the focus from the far point to the near point
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what is Range of Accommodation
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It is the linear distance between the far point and the near point of accommodation.
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what is Ocular Accommodation
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Accommodative amplitude or demand is measured with respect to the eye’s principal plane
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what is Spectacle Accommodation
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Accommodative amplitude or demand is measured with respect to the spectacle plane (phoropter)
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what happens anatomically in the ACCOMMODATED eye
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a contraction of the ciliary muscle causes a release of resting zonular tension and a decrease in the lens equatorial diameter allowing the young lens to take a more convex form
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what happens anatomically in the UNACCOMMODATED eye
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the ciliary muscle is relaxed, the suspensory zonules of Zinn is at its greatest tension, and the lens takes its flattest curves
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what is Range of Accommodation
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It is the linear distance between the far point and the near point of accommodation.
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what is Ocular Accommodation
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Accommodative amplitude or demand is measured with respect to the eye’s principal plane
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what is Spectacle Accommodation
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Accommodative amplitude or demand is measured with respect to the spectacle plane (phoropter)
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what happens anatomically in the ACCOMMODATED eye
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a contraction of the ciliary muscle causes a release of resting zonular tension and a decrease in the lens equatorial diameter allowing the young lens to take a more convex form
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what happens anatomically in the UNACCOMMODATED eye
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the ciliary muscle is relaxed, the suspensory zonules of Zinn is at its greatest tension, and the lens takes its flattest curves
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