Conic Sections

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1.
(a) Indications
-The stenopaic slit refraction is useful for confirming the results of other refraction techniques for patients with irregular astigmatism or reduced visual acuity. - It is helpful for patients who have difficulty understanding the complex instructions associated with other subjective techniques.  -It is important to note that, like the pinhole, the stenopaic slit may be used diagnostically to determine a patient's potential visual acuity. -The astigmatism present in the patient’s old spectacles should be considered -The small amount of cylindrical power is of little consequences, in that the subjective end point can quickly be rechecked after the stenopaic slit is removed (b) Techniques

Axis determination
i. remove cylinder power from retinoscopy and
ii. identify BVS or MPTMV, VA
iii. fogged the patient (F=1/2 CYL + 0.50 DS) and watch the acuity chart iv. put the stenopaic slit at any position
v. the slit is rotated until acuity is maximized. The slit now lies along the minus cylinder axis. Spherical power
vi. with the slit in this position, the fog is reduced to best acuity. vii. the lens in place is the sphere power of the patient’s lens formula (pt’s final sphere) viii. the slit is rotated 90 degrees. This will fog the patient again viiii. the fog is again reduced to best acuity. The algebraic difference between the power of the lens in place at the end of this operation and the lens power in place at the end of step one is the minus cylinder power (c) Advantages

- recommended for refracting patients with significant optical anomalies, including keratoconus and lenticular distortion - recommended for patients that have irregular astigmatism
- can be done on patients that have difficulty in understanding the instructions on other subjective techniques (d) Disadvantages
- this technique is crude
- should be only be used for cylinders > 1.00 DC
- advisable to favour objective measures of astigmatism from autorefraction,...
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