Schematic/Reduced Model Eyes Flashcards
Describe paraxial vs wide-angle eye models
Most eye models are paraxial only considering rays close to theoretical optical axis: study (near) foveal vision as foveola close to optical axis (2 degs V and 5 degs H) ~ assumes all refracting surfaces are spherical
Wide-angle: more complex for off-axis aberrations and correctly assume non-spherical refractive surfaces
Explain the laws for cardinal points/planes in (non)/equifocal systems
Equifocal: nodal and principal points/planes must always coincide
Non-equifocal: points don’t coincide
Explain the law of principle points
for a single, curved refracting surface, principal points coincide with each other and the surface’s vertex, nodal points coincide with each other and Rc (radius of curvature)
for a thin lens all principal/nodal points coincide with the optical centre
Explain Gullstrand’s Exact Eye Model vs his Schematic Eye Model
Exact: 6 refracting surfaces (lens cortex 1.386, nucleus 1.406), principal points close together in AC, nodal points close together straddling post. lens surface ~ used for modelling accommodation
Schematic: 3 refracting surfaces (single corneal surface, homogenous n lens (1.409), identical cardinal points to ‘exact’ eye ~ simple accomm. modelling
Describe Emsley’s ‘Reduced’ Eye Model
single refracting surface, principal points coincide at corneal vertex with entrance/exit pupils, nodal points coincide at Rc
Explain the concept of a far point (k)
always measured as distance away from corneal apex (i.e. location of principal points) applied to ametropic eyes only
Explain the purpose of distance vision correction
ametropic eyes only full corrected for DV by a lens with 2nd focal point (F’) coinciding with far point (distance k) and the 2nd focal length (f’)
Explain 6 key differences between specs/CL corrections
different lens powers (effectivity)
corrected retinal image size varies (spec magnification)
Lenses alter accom,/convergence demands and can modify FoV
Cosmetic effects due to magnification/minification of px eyes through specs
Define spectacle magnification and which factors affect this
Corrected Retinal image height for a dist, object/Uncorrected Retinal image height for a dist. object
the change in RIH size depends on the Fsl and BVD (a)
corrective lenses change the angle (w)
Explain the clinical implications of SM for Specs vs CL in hyperopes vs myopes
Hyperopes: +specs increase uncorrected retinal image (SM>1.0), CL offer a relative reduction in SM which ‘minify’ distant objects
Myopes: -specs reduce uncorrected retinal image (SM<1.0), CL offer relative increase in SM which magnifies distant objects
px over 4.00D may experience spatial distortions when switching from specs to CLs (esp. if anismetropic)
Define Aniseikonia
px perception of inequality in retinal image size between eyes can cause image fusion problems
Explain the design and use of afocal ‘size’/iseikonic lenses
for aniseikonia px, they’re afocal (Fv’ = 0 D so power factor always = x1), specially designed to increase/decrease SM without inducing/correcting any refractive error
Explain the trend of accommodative demand between myopes/hyperopes
myope spec wearers accommodate less than myope CL wearers
hyperope spec wearers accommodate more than hyperope CL wearers
hyperope spec wearers more likely to become presbyopic earlier than age-matched spec wearing myopes
Describe the pros/cons of the accommodative pinhole effect
increases depth of field however much less light received at retina so less useful in dim conditions
Explain the trends in effectivity and cylinder powers for astigmats wearing specs vs CLs
myopic astigmats require less negative cyls in CLs compared to specs
hyperopic astigmats require more negative cyls in CLs compared to specs
Describe 3 common relationships for convergence demand differences between specs/CL wearers
all CL-corrected myopes converge more than spec-corrected myopes
expect esophoric hyperopes to prefer specs over CLs for near vision
expect exophoric hyperopes to prefer CLs over specs for near vision
Explain aphakia
eye which has developed without a natural crystalline lens or it’s been removed without a IOL replanted
generally very hyperopic, form tiny retinal imahes when uncorrected which become significantly enlarged with specs
tend to adapt better with CLs due to less xSM
Explain 4 problems experienced by spec-corrected aphakes
high xSM
jack-in-the-box (‘roving ring’ scotoma shift)
pin-cushion disortion: increased mag moving away from lens OC to edges of high +lenses creating problems for square objects
large apparent size (longitudinal mag)
What is the optimal correction for aphakia?
use CLs as cornea solely responsible for aphakic eye’s total power
refer for iris-fixated IOL (wider FoV, less SM)
