Topic 4: Spectacle Magnification Flashcards
How is SM expressed
As a ratio of retinal image size in corrected eye (with specs) to retinal image size in uncorrected eye (wo specs)
Plus lens magnify, SM >1
Minus lens minify. SM <1
Formula for SM
Shape factor multiply power factor
(1/1-(F1*t/n) * (1/1-dFv) where all measurements in m
Define aniseikonia
A condition where there is relative difference in size and/or shape of the images seen by R & L eyes.
Explain what an anisometropic spec wearer might experience
(1) Diff power for R&L lens: size of image of object seen is different in R&L eyes. Double vision occurs because brain is incapable of fusing two images of a different sizes to one single object
(2) High cyl power difference: image shape in each eye differs, also hindering binocular single vision
State signs & symptoms of aniseikonia
Signs: anisometropia, higher astig in one eye, different cyl orientation in each eye, presence of IOL/scleral buckling, corneal transplant, refractive surgery in one eye
Symptoms: Asthenopia &/or diplopia, better comfort when only one eye used, spatial distortion ie. sloping floors & slanting walls (happens when cyl in one eye significantly different)
Explain which eye conditions can lead to aniseikonia and how
(1) corneal transplant in one eye: corneal from donor could have different shape hence changing initial prescription (add/change cyl orientation)
(2) pseudophakia: prescription changes after lens is removed and replaced with IOL ie. -5.00 OU becomes L plano
(3) scleral buckle: shape of eyeball is altered (sclera pushed inward), causing refractive status to change in that eye
State percentage SM difference range that can hinder binocular single vision
3.5% to 5%
State and explain strategies recommended to mitigate aniseikonia
(1) consider contact lens as it has 0 VD
(2) frame with short VD, if nose pads present further reduce VD
(3) frame with small eye size to thin down plus lens centre thickness & reduce VD slightly
(4) aspheric lens can flatten the front curve to reduce SM of plus lenses
(5) high index lenses reduce centre thickness of plus lenses
(6) iseikonic lenses are specially designed to have same SM value by modifying the shape of each lens (TIP: choosing a thick rimmed plastic frame can disguise appearance)
how to make minus lens minify less
steepen base curve/increase F1 but move bevel position forward to reduce VD (when F1 increases, F2 is more concave and VD increases the more concave)
thicken lens by reducing lens index
