OPTICS AND REFRACTION

The Schematic Eye : Auxiliary Lenses and Vertex Distance

We will use auxiliary trial lenses to create refractive errors greater than those obtainable by changing the length of the schematic eye. We call these phantom lenses, and they sit in the rack in front of the eye. These lenses create an ametropia of equal power but opposite sign; that is, plus lenses create a myopicerror and minus lenses a hyperopic error. The power of the phantom is added to the setting of the schematic eye. For example, if you set the scale at –2 D and put a +10 D phantom before the eye, the combination would simulate a myopia of –12 D. The +10 D phantom simulates a 10 D myopia by converging retina rays to the myopic FP of 10 cm. We also use auxiliary lenses to correct refractive errors, as you have already seen. To correct an ametropia, we add lenses of appropriate sign and power, with allowance for our working distance. Problems arise, however, in creating or correcting errors when you stack several lenses together. Thenominal power of a lens (marked on the handle) presumes a short vertex distance (the distance between the back of the lens and the front of the cornea). The effective power changes as the vertex distance (VD) increases. The difference is slight with weak lenses, but increases dramatically as the lenses become stronger. As a rule of thumb, the effective power of a strong lens (±10 D) changes about 1% for each millimeter it is moved. The power of a 10 D lens moved 10 mm changes by about 1 D.† When you increase the VD, pluslenses get stronger (effective power increases) and minus lenses become weaker (effective power decreases). Since effective power relates to VD, make it a habit to keep the strongest lenses closest to the eye. It is especially important to be aware of this change in effective lens power when working with the schematic eye, where the distance between the front and rear cells may be as much as 25 mm. Thus, on some models, the effective power of a strong lens changes by 25% when it is moved from the rear to the front cell. Obviously, you will want to keep the lenses close together. However, errors will still creep in when using strong lenses, and failure to appreciate this leads to a lot of unnecessary weeping. Since the distance between cells is at least 5 mm, a –10 D phantom in the rear cell is slightly overcorrected by a +10 D lens in the next cell. With lens power less than +3 D, the effect between adjoining cells is insignificant. Thankfully, refracting machines have vertex compensation to prevent all these errors. You will only need to consider the VD of the machine itself (compared to the VD of the spectacles) in writing the final prescription. As if this were not enough, each additional lens gives two more surface reflections for you to cope with. Each lens also reduces the brightness of the reflex, since the light has been refracted four times in the two-way path through the lens. Always use the fewest lenses to achieve a desired result; for example, replace a stack consisting of +1.50 D, +0.75 D, and +0.25 D lenses with a +2.50 D lens. You will have problems with your schematic eye in the pages ahead; everyone does, but you will catch on. Be a little forgiving of its calibration and inconsistencies; it is imperfect, but it will teach you fundamentals. By the time you are truly thwarted by the model eye’s limitations, you will be ready to retinoscope patients. Since many of you may already be employed in an ophthalmic office, or have access to an eye refracting lane, I’ve asked Rich Reffner to explain his teaching method. He uses a refractor in retinoscopy of the training eye.