Physiological optics 13 th lecture Dr. Mohammad Shehadeh

Spectacle Magnification The optical correction of ametropia is associated with a change in the retinal image size. The ratio between the corrected and uncorrected image size is known as the spectacle magnification.

Clinically, it is more useful to compare the corrected ametropic image size with the emmetropic image size. This ratio is known as the relative spectacle magnification (RSM).

In axial ametropia, if the correcting lens is placed at the anterior focal point of the eye, the image size is the same as in emmetropia. The RSM is therefore unity. In axial myopia, if the correcting lens is worn nearer to the eye than the anterior focal point, the image size is increased. The relative spectacle magnification is therefore greater than unity. Contact lenses in axial myopia thus have a magnifying effect.

Relative spectacle magnification. Axial ametropia with correcting lens at anterior focal point of the eye. Ametropic state (solid lines) compared with emmetropia (dotted lines).

Relative spectacle magnification. Axial myopia with correcting lens nearer the eye than the anterior focal point (contact lens). Ametropic state (solid lines) compared with emmetropia (dotted lines). l em is the image size when correction is at the anterior focal point, which equals the emmetropic image size; and l CL is the image size when correction is closer to the eye than the anterior focal point.

In contrast to axial ametropia, the image size in refractive ametropia differs from the emmetropic image size even when the correcting lens is at the anterior focal point of the eye. The image size in refractive hypermetropia is increased, thus the relative spectacle magnification is greater than unity. In refractive myopia the image size is diminished, and thus the relative spectacle magnification is less than unity

Furthermore, in refractive ametropia, if the correcting lens is worn nearer to the eye than the anterior focal point, the image size approaches the emmetropic image size. The relative spectacle magnification thus aproaches unity.

Relative spectacle magnification. Refractive ametropia with correcting lens at anterior focal point of the eye. Ametropic state (solid lines) compared with emmetropia (dotted lines).

Spectacle correction in aphakia (refractive hypermetropia) produces a relative spectacle magnification of 1.36 when placed at the anterior focal point of the aphakic eye (23.2 mm in front of the principal plane). However, when a contact lens is used, the relative spectacle magnification is reduced to 1.1 Spectacles are usually worn 12–15 mm in front of the cornea and the aphakic relative spectacle magnification at this position is approximately 1.33.

Optical Problems in Correcting Aphakia with Spectacles (approx D or more). relative spectacle magnification produced by aphakic spectacle correction is approximately This means that the image produced in the corrected aphakic eye is one third larger than the image formed in an emmetropic eye. This magnification causes the patient to misjudge distances. Objects appear to be closer to the eye than they really are because of the increased visual angle subtended at the eye

The image magnification also results in an enhanced performance of standard tests of visual acuity, e.g. Snellen test type. For example, a level of 6/9 for an aphakic spectacle wearer is equivalent to 6/12 for an emmetropic eye. The use of a contact lens, or an intra-ocular implant, which reduces the RSM to 1.1 or 1.0 respectively, overcomes these problems.

The lenses used in aphakic spectacles are subject to the aberrations ;pin cushion effect. The prismatic effect of aphakic spectacle lenses produces a ring scotoma all around the edge of the lens.This scotoma may well cause patients to trip over unseen obstructions in their path.

the direction of the ring scotoma changes as the patient moves his eyes, and objects may appear out of the scotoma or disappear into it – the 'jack-in-the-box' phenomenon.

shows the eye in the primary position and the location of the ring scotoma. An object O is visible to the patient through the periphery of the spectacle lens. Let us now consider what happens when the patient tries to look directly at object O.

shows that as the eye rotates, moving the nodal point from a to b, the ring scotoma moves in the opposite direction, from A to B. Thus when the patient tries to look at object O it disappears in the ring scotoma B only to reappear in his peripheral vision when he looks away. This disappearance and re-emergence of an object is known as the 'jack-in-the-box' phenomenon.

high powered glass lenses are heavy and may consequently cause the spectacles to slip down the patient's nose thus altering the effective power of the lenses. (Heavy spectacles are also uncomfortable to wear.) Weight may be reduced by the use of plastic lenses, but these tend to scratch especially if laid 'face' downwards when not in use. Another means of reducing lens weight and thickness is by the use of lenticular form lenses. A lenticular lens has only a central portion or aperture worked to prescription, the surrounding margin of the lens acting only as a carrier. However, the field of vision is necessarily reduced.

The foregoing aberrations and the prismatic effect and its consequences can all be eliminated by the use of contact lenses or intra-ocular implants. The advantages of these two forms of correction stem from the fact that in each case the correcting lens becomes an integral part of the optical system of the eye.

Spectacle correction of a unilateral aphakic eye can achieve a clear retinal image, but with an RSM of 1.33 the image in the aphakic eye is one third larger than the image in the normal fellow eye. The patient is unable to fuse images of such unequal size (aniseikonia) and complains of seeing double. The use of a contact lens or intra-ocular implant reduces the RSM to 1.1 or 1.0 respectively. The image in the aphakic eye will thus be the same size (intra-ocular implant) or only one tenth larger (contact lens). In either event the images can be fused and binocularity restored. Iseikonic lenses??