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Association of British Dispensing Opticians Heads You Win, Tails

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Association of British Dispensing Opticians Heads You Win, Tails Agenda Heads You Win, Tails You Lose • The correction of ametropia • Magnification, retinal image size, visual Association of British The Optical Advantages and acuity Disadvantages of Spectacle Dispensing Opticians • Field of view Lenses and Contact lenses • Accommodation and convergence 2014 Conference Andrew Keirl • Binocular vision and anisometropia Kenilworth Optometrist and Dispensing Optician • Presbyopia. 1 2 3 Spectacle lenses Contact lenses Introduction • Refractive errors that can be corrected • Refractive errors that can be corrected using • Patients often change from a spectacle to a using spectacle lenses: contact lenses: contact lens correction and vice versa – myopia – myopia • Both modes of correction are usually effective – hypermetropia in producing in-focus retinal images – hypermetropia • apparent size of the eyes and surround in both cases • There are of course some differences – astigmatism – astigmatism between modes, most of which are • not so good with irregular corneas • better for irregular corneas associated with the position of the correction. – presbyopia – presbyopia • Some binocular vision problems are • Binocular vision problems are difficult to manage using contact lenses. easily managed using spectacle lenses. 4 5 6 The correction of ametropia using Effectivity contact lenses • A distance correction will form an image • Hydrogel contact lenses at the far point of the eye – when a hydrogel contact lens is fitted to an eye, The Correction of Ametropia the lens “drapes” to fit the cornea • Due to the vertex distance this far point – this implies that the tear lens formed between the will lie at slightly different distances from contact lens and the cornea should have zero the two types of correcting lens power and the ametropia is corrected by the BVP of the contact lens – the powers of the spectacle lens and the – not always the case but usually assumed in contact lens required to correct a particular practice eye will therefore be different. –FCL = K 7 8 9 The correction of ametropia using Correction of ametropia - RGP The tear lens and the BOZR contact lenses • RGP contact lenses • The contact lens-tear lens system • Important to determine the likely magnitude of – the back surface of a rigid lens maintains its shape formed when a RGP contact lens is the tear lens and how it varies as the BOZR is – a tear lens of predictable form and power is placed on an eye means that three changed formed between the rigid contact lens and the – helps determine BVP required and aids fit cornea elements are involved in the formation evaluation – ametropia is corrected with a contact lens/tear of the final retinal image: • Rules of thumb: lens system – contact lens – tear lens power increases by 0.25 D for each 0.05 –F ≠ K CL – tear lens (liquid lens, lacrimal lens) mm that the BOZR is steeper than the cornea – eye – tear lens power decreases by 0.25 D for each 0.05 mm that the BOZR is flatter than the cornea. 10 11 12 Spectacle magnification Spectacle magnification The ratio of the retinal image size in the corrected When considering hydrogel contact lenses, the tear lens is ametropic eye compared to the retinal image size considered to be plano and the lens thin. in the same eye when uncorrected Magnification, Retinal Image We also assume that the power of the hydrogel contact lens is equal to the eyes ocular refraction. / Size and Visual Acuity hc K SM = SM = h / F u sp K SMcl = = Unity Fcl SM = K/Fsp only applies to thin lens systems and model eyes where the entrance pupil coincides with the cornea or the reduced surface In contact lens systems the entrance pupil is normally considered to be 3 mm behind the corneal surface and both the contact lens and the tear lens have a thickness. 13 14 15 Spectacle magnification Spectacle magnification Spectacles Vs CLs For a contact lens-tear system Assumptions regarding visual acuity The monocular static visual field for the right eye SM = power factor x shape factor / 1 Fv SM = x _ / 1 aF Feq Ametropia Spectacles CL v 60 degrees UP / / 75 degrees down Fv is the BVP of the lens system (L4 ) Myope SM < 1 SM ≈ 1 100 degrees Temporal Feq is the equivalent power of the lens system 60 degrees NASAL a is the distance from the back vertex of the lens system to the entrance pupil of the eye Hypermetrope SM > 1 SM > 1 < Specs For spectacles a = vertex distance + 3 mm As enjoyed by an emmetrope or a contact lens wearer For CLs a = 3 mm as d = 0 / / / Fsp +10.50 D SM ≈1.25 L2 L3 L4 (Assuming that the contact lens or its optic zone is not very small in diameter) Feq =L1 x x x Same patient wearing CLs SM ≈1.05 L2 L3 L4 16 17 18 Factors affecting the field of view Field of view - Spectacles Real and apparent fields of view of a spectacle lens • The static field of view of a spectacle lens is • Aperture size • Apparent the total area visible through the lens • Lens power – the angle subtended by the empty frame aperture at the eye’s centre of rotation – usually expressed in angular measure • Vertex distance – defined as the maximum angular extent of vision – to obtain the maximum field, whatever the size of • Real obtainable through the lens. the aperture might be, the spectacle lens should – the field of view obtained when a spectacle lens is be fitted as close to the eyes as the lashes permit. glazed into the frame. 19 20 21 Field of view: Positive lens Field of view: Minus lens Example • The static real field of view provided by a • The static real field of view provided by a minus • Round lens, 48 mm in diameter fitted at a positive spectacle lens is less than the spectacle lens is greater than the apparent field of distance of 25 mm from the centres of apparent field of view implied by the empty view implied by the empty spectacle frame rotation of the eyes spectacle frame • This means that myopes benefit from an increase in field of view • 10.00 myope and 10.00 D hypermetrope • This means that hypermetropes suffer from a • This will result in an annular area around the lens – apparent static FOV 87.7° decrease in field of view periphery where objects will be seen in diplopia. – myope real static FOV 102.7° • There will be an area around the edge of a – hypermetrope real static FOV 71.5° lens from which no light can enter the eye –a ring scotoma. CL wearer has the widest field of view! 22 23 24 Ocular accommodation Example • The ocular accommodation is the •F = +5.00 D at 15 mm accommodation required to neutralise sp Accommodation and • An object is placed 1/3 m from the spectacle negative vergence from a near object plane convergence measured in the plane of the eye • Calculate the required ocular accommodation •Aoc is the symbol for ocular accommodation • Compare this to a contact lens wearer •A = K - L where K is the ocular refraction oc 2 viewing the same near object. and L2 is the vergence arriving at the eye from the near object. 25 26 27 Accommodation Example Spectacles Vs contact lenses Convergence •Fsp = -5.00 D at 15 mm • As contact lens wear results in an ametropic • An object is placed 1/3 m from the spectacle patient being considered “artificially “The movement (rotation) required emmetropic” plane from the primary position, for the • Calculate the required ocular accommodation – the myopic patient will need to accommodate more when wearing contact lenses than when eyes to fixate an object point on • Compare this to a contact lens wearer wearing spectacles the mid-line.” viewing the same near object. • could be an issue for the emerging presbyope! – the hypermetropic patient will need to accommodate more when wearing spectacles than when wearing contact lenses. 28 29 30 Convergence Convergence Example •Fsp = -5.00 D at 15 mm centred for distance • Convergence and accommodation should be • The “base in” effect of minus lenses means vision, the distance PD is 66 mm and the equal for normal binocular vision that myopes corrected with spectacle lenses centres of rotation of the eye lie 27 mm • Convergence can be expressed in degrees, converge less than emmetropes or contact behind the spectacle plane prism dioptres or metre angles. lens wearers • An object is placed on the mid-line and • The “base out” effect of plus lenses means 1/3 m from the spectacle plane that hypermetropes corrected with spectacle • Calculate the convergence required for a lenses converge more than emmetropes or subject corrected using (i) spectacle lenses contact lens wearers. and (ii) contact lenses. 31 32 33 Convergence Accommodation and Example Spectacles Vs contact lenses convergence - A Summary •Fsp = +5.00 D at 15 mm centred for distance vision, the distance PD is 66 mm and the • As contact lens wear results in an ametropic • Myopia centres of rotation of the eye lie 27 mm patient being considered “artificially – accommodation and convergence behind the spectacle plane emmetropic” • more with contact lenses than spectacles • An object is placed on the mid-line and – the myopic patient will need to converge more • Hypermetropia 1/3 m from the spectacle plane when wearing contact lenses than when wearing – accommodation and convergence spectacles • Calculate the convergence required for a • less with contact lenses than spectacles – the hypermetropic patient will need to converge • So, when changing from CLs to spectacles subject corrected using (i) spectacle lenses more when wearing spectacles than when wearing and (ii) contact lenses. contact lenses. (and vice versa) the accommodation convergence ratio is only minimally disturbed.
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