Visual Optics 2007/2008. Chapter 4 Emmetropia and Ametropia. For 5% bonus points in Visual Optics II, I would prefer:. Clicker question format as in Visual Optics I Pop quizzes ~ once per week Clicker questions and Pop quizzes
n = 1
n' > 1
Figure 3.52 – Oblique (radial) astigmatism. A point object (B) below the optic axis (A) creates an image spread along the optic axis direction from the tangential to sagittal focus.
Nature of the defocus-sensing mechanism is not known.
May be some combination of the above factors
No proof in humans that the visual system can differentiate hyperopic from myopic defocus
Higher prevalence of myopia in Taiwanese cities vs. villages __
Children with one or two myopic parents 4 more likely to be myopic __
Higher rate of myopia in mono- than di-zygotic twins __
Number of daily “book hours” definitely a factor in myopia __
Myopia genes recently identified __
Recent studies indicate that hereditary factors are likely to drive both susceptibility and resistance to environmentally-induced myopia (only susceptible patients will develop myopia if exposed to environmental “risk factors”) __
Increased prevalence of myopia in Asian communities has occurred more rapidly than can be accounted for by Asian gene pool turnover __
Myopes (ages 8-13) have a deficient accommodative response to a near target relative to emmetropes, on the order of 0.20 D difference for a 4 D stimulus
Best single predictor of myopia is spherical refraction at age 8-9 years (threshold > or < +0.75 D hyperopia)
Sporting activities in youth appear to protect against myopic progression (equal “book” hours to non-participants)
~ +20 D
Feff (CrLens)~ +17 D
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Fig. 4.4 - Reduced eye representation of an astigmatic eye (relaxed) showing focal lines formed by parallel incident light (distant point object).
Why would this type of cataract be associated with significant loss of visual function?
Measuring Axial Length – A-Scan Biometry
Fig. 4.6 - A-Scan (time-amplitude) ultrasonograph. Time intervals between peaks (in sec) are multiplied by sound velocity in each medium to compute axial length).
Immersion A-Scan biometry. Note two corneal peaks (C1 and C2)