MAGNIFICATION PART 1.pptx
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Spectacle Magnification and relative spectacle magnification are measures of how much a correcting lens increases or decreases the size of the retinal image compared to an uncorrected or standard emmetropic eye. Spectacle magnification depends on the shape factor related to the front surface power and thickness of the lens, and the power factor related to the back vertex power. It increases the retinal image size for a plus lens and decreases it for a minus lens. Relative spectacle magnification compares the retinal image size of a corrected ametropic eye to that of a standard emmetropic eye, and depends on whether the ametropia is axial or refractive.
MAGNIFICATION PART 1.pptx
- 2. Spectacle and relative spectacle magnification • SM= change in retinal image size brought by correcting lens. • Ratio of retinal image size with the correcting lens to that without the correcting lens. • SM= retinal image size in corrected eye/basic height of retinal image in uncorrected eye • %SM= (SM-1)*100
- 3. • Depends on 2 factors • Shape factor and power factor • SM= • 1 1 • 1-(t/n)*F1 1-dF`V
- 4. • F1= power of the front surface • F`V= back vertex power • t = thickness • n= refractive index • d = distance from the back vertex and the entrance pupil of the eye
- 5. Shape factor • 1 1-tF1/n SM increases with increase in front surface power and increases with increase in thickness. SM decreases with decrease in index of refraction of the lens material.
- 6. Power Factor • 1/1-dFv’ Increases with increase in back vertex power for Plus lens. Decreases with increase in back vertex power for minus lens. If + lens is moved closer to the eye, d decreases and SM also decreases.
- 7. • If –lens is moved closer to the eye d decreases and SM increases • SM>1 for + lens • SM<1 for -lens
- 8. SM and vertex distance • Retinal image size Sec. Focal length. • 1/power of the correcting lens • As the vertex distance of a +lens increases ,the SM increases and vice-versa. • As the vertex distance of a – lens increases, the SM decreases and vice-versa
- 9. Factors affecting Retinal Image • Change in prescription • Change in vertex distance • Change from spectacle to contact lens • Change in thickness • Change in bend or form of lens
- 10. SM in Astigmatism • Differs in two principal meridians. • The retinal image is larger in direction of the axis of the –cyl that corrects the astigmatism. • Difference between the two meridians= 1.5%per dioptre of astigmatism.(spectacle) • Difference between the two meridians=0.3%per dioptre of astigmatism(contact lens)
- 11. Relative Spectacle Magnification RSM • Ratio of the retinal image size of the corrected ammetropic eye to that of the schematic emmetropic eye or standard emmetropic eye. • Depends on whether the ametropia is axial or refractive • The image size is essentially the same as that for the emmetropic eye in axial ametropia, but is magnified in hyperopia and minified in myopia compared with that for the emmetropic eye in refractive ametropia
- 12. • RSM= fe` /f`ST= FST/Fe • FST= Equivalent power of the standard emmetropic eye • Fe= Equivalent power of the system • Fe= FSP+ FA • Where FSP= power correcting of spectacle lens • FA=refracting power of the ametropic eye
- 13. RSM in axial/refractive ametropia • FA= FST • RSM= FST/ FSP+ FST-dFSP*FST • If spectacles are in the anterior focal point of the eye, then RSM=1 • FAnot equal to FST • Axial length of the ametropic eye is equal to the standard emmetropic eye. • RSM= 1 /1- dFSP • Significance is contact lens in ametropia
- 14. RSM in astigmatism • If corrected by spectacle lens the retinal image size is greatest in the axis meridian of the correcting minus cyl and least in the power meridian.
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