Practical Applications for Spectacle Lens Formulas

Transcription

1 Practical Applications for Spectacle Lens Formulas ED DE GENNARO RICHMOND, VIRGINIA Ed De Gennaro Director, Professional Content First Vision Media Group 1

2 Boring? Formulas don t have to be boring Too often, learned and used but not analyzed You can learn a great deal by analyzing a range of data from a formula What Is A Strong Lens? No clear answer to this Seems thick in the frame Is heavy for the wearer An Rx whose power creates significant aberrations and distortions 4.00D or more (= to 1x mag) Most doctors consider ±3.00D and higher as strong lenses Some go as low as ±2.50D 2

3 What Is A Strong Lens? ANSI Z80.1 Vertical & horizontal prism - used to say, except in higher powers Now it says For vertical: greater than ±3.375 D For horizontal: greater than ±2.75 D Thick Lens Formula 2 P = D 1 + D 2 + T x D 1 N 2 P = D 1 + D 2 + T x D 1 N Nominal Lens Formula Shape & Thickness Factor 3

4 Look What Shape & Thickness Do to a Lens Front Curve: Back Curve: Thickness = 6.8mm Index = = (-4.37) X = Where did the come from? Note: = (not ) The nominal lens formula is wrong! Now, Turn the Lens Around P = (-4.37) P = (0.08) X P = Note: = ( 4.37) still = But now the shape & thickness factor only adds

5 Strong vs. Weak Lenses D B = D 1 t 1 ( ) D n 1 + D 2 D 1 = t = 8.0mm D 2 = -2.00n = D B = ( ) * (-2.00) D B = (0.0627) + (-2.00) D B = (-2.00) D B = D B = D A gain of 0.80D Strong vs. Weak Lenses D B = D 1 t 1 ( ) D n 1 + D 2 D1 = t = 2.0mm D2 = n = D B = ( ) * (-5.25) D B = (0.0082) + (-5.25) D B = (-5.25) D B = D B = +1.05D A gain of 0.05D 5

6 The Optics of Add Focusing Considerations for Strong & Weak Lenses The thickness, index and front curve of a lens add to its plus power Even minus lenses add plus Surfacing computers adjust the backside curve to accommodate the power addition The power addition is not proportional as power increases Minor power increases for low powered lenses are now being corrected with free-form processing Why, when the increase is usually small? Using a lens clock on a strong lens will result in significant power estimating errors 6

7 Magnification Lenses not only refract, they alter the size and shape of the world Magnification (minification) Alters the size, shape and speed of the world Spatial orientation With lenses With changes in power, base curve, thickness, index Patients must adapt to new spatial orientation Magnification Through Spherical Lenses Grid Object & Plano Lens Plus Lens M inus Lens 7

8 Magnification Through a Cylinder Lens Grid Object & Plano Lens Plano +cyl x 90 Plano +cyl x 180 Oblique Cylinder Magnification Grid Object & Plano Lens Plano +cyl x 135 8

9 Symptoms From Equal Magnification In Both Eyes Change in the size of the world Change in spatial orientation Change in the speed of objects Distortion Disorientation, nausea, dizziness Distortion Plus Lens Minus Lens Grid Object Pinchusion Distortion Barrel Distortion 9

10 Unequal Magnification OD: sphere OS: sphere Symptoms From Unequal Magnification in Both Eyes Double vision (diplopia) Visual confusion Independent vision Suppression Headaches, eyestrain, asthenopia (especially in small differences, 1 to 2 %) 10

11 Four Factors of Magnification Power Base curve Center thickness Vertex distance Changing A Patient To Flattened Aspherics What to Do Understand the lens' design Understand how changes made may effect perception Tell patients of potential adaptation DURING delivery, NOT AFTER 11

12 Magnification Formula SM = 1 t 1 ( ) D n 1 [Shape] x D 1 1 hd [Power] SM = Spectacle lens magnification D = Power of the lens D 1 = Front curve of the lens h = Vertex distance plus 3mm *for dist. to the center of the pupil Sm = Shape Mag * Power Mag Changes in Magnification D D 1 t n h Old mm mm New Mag 0.25% Approx. Formulas D D 1 t n h %SM = D 1 (t/15) Old mm mm %SM = D 1 ( t/15) New 8.0mm Mag 0.42% %SM = h(d/10) D D 1 t n h Old mm mm New 11mm Mag 1.00% 12

13 Power s Contribution D D 1 t n h Old mm mm Mag 18.52% New Mag 19.18% Mag 0.66% For Weak Powers D D 1 t n h Old mm mm New Approx. Formulas Mag 0.08% %SM = D 1 (t/15) %SM = D 1 ( t/15) %SM = h(d/10) D D 1 t n h Old mm mm New 3.0mm Mag 0.22% D D 1 t n h Old mm mm New 11mm Mag 0.10% 13

14 D D 1 t n h Old mm mm Mag 2.11% New Mag 2.62% Mag 0.51% Magnification Effects Can you really control mag at the dispensing table? Yes but are you sure you want to? Ruins the corrected curve advantages of the lenses Use lenses that have been designed to reduce mag Corrected curve Aspherics Progressives 14

15 Vertex Distance Positional effect alters a lens power A huge concern with aphakic spectacles The as worn position of eyeglasses is usually different from the refracted position How much impact does it have on the average Rx? Compensated Powers Table 1.00D 2.00D 3.00D 4.00D 5.00D 6.00D 7.00D 8.00D 9.00D 10.00D 11.00D 12.00D 1mm mm mm mm mm mm mm mm mm mm mm mm

16 Compensated Powers Table 1.00D 2.00D 3.00D 4.00D 5.00D 6.00D 7.00D 8.00D 9.00D 10.00D 11.00D 12.00D 1mm mm mm mm mm mm mm mm mm mm mm mm *0.25D Spherical Errors Conclusions on Vertex Distance Minimal effect in low powers D in powers up to 3.00D Adds 0.25D at 5.00D but with 10mm of shift Important around 7.00D or so for 0.25D change Remember: Each meridian gains or loses at a different rate Must compensate each meridian separately 16

17 Vertex Distance Positional effect affects a lens power A huge concern with aphakic spectacles The as worn position of eyeglasses is usually different from the refracted position How much impact does it have on the average Rx? Compensated Powers Table 1.00D 2.00D 3.00D 4.00D 5.00D 6.00D 7.00D 8.00D 9.00D 10.00D 11.00D 12.00D 1mm mm mm mm mm mm mm mm mm mm mm mm

18 Compensated Powers Table 1.00D 2.00D 3.00D 4.00D 5.00D 6.00D 7.00D 8.00D 9.00D 10.00D 11.00D 12.00D 1mm mm mm mm mm mm mm mm mm mm mm mm *0.25D Spherical Errors Conclusions on Vertex Distance Minimal effect in low powers D in powers up to 3.00D Adds 0.25D at 5.00D but with 10mm of shift Important around 7.00D or so for 0.25D change Remember: Each meridian gains or loses at a different rate Must compensate each meridian separately 18

19 Ed De Gennaro