Jackson’s crosscylinder test is used for ?
Correct Answer: Subjective refining
Description: Ans. is 'b' i.e., Subjective refining Subjective refraction Subjective refraction is meant for finding out the most suitable lenses to be prescribed. The proper subjective refraction includes three steps. 1)Subjective verification of refraction It is done by `trial and-error 'methods. In this one eye is occluded and the appropriate lens combination (as indicated by retinoscopy or refractometry) is placed in front of the other eye. By increasing or decreasing the power of lens, the most suitable spherical lens is chosen. The strongest convex lens and the weakest concave lens providing best vision should be chosen in patients with hypermetropia and myopia respectively. 2)Subjective refining of refraction The most suitable combination of lenses chosen after the subjective verification of refraction is refined before final prescription is made. It is always better to first refine the cylinder and then sphere. A) Refining the cylinder : - Cylinder can be refined by either crosscylinder or by astigmatic fan test. Jackson's crosscylinder test (more commonly used) Crosscylinder is used to verify the strength and axis of the cylinder prescribed. The cross cylinder is a mixed combination of various strengths in which spherical component is one half the power of the cylindrical and of opposite sign to with the axis at right angles. The most convenient form is a combination of a -0.5 D sphere with a +1.0 D cylinder which has an effective strength of + 0.5 D cylinder in one meridian and - 0.5 D cylinder in the meridian perpendicular to the first. The cross-cylinder is identified by the strength of its cylinderical component. So, above described cross cylinder is referred as + 0.5 crosscylinder. Thus, cross cylinder has following feature : - It has one spherical and one cylinderical lens. This combination creates two cylinders of equal strength but with opposite sign placed with their axes at right angle to each other. Astigmatic fan test : - It is used to confirm the cylindrical correction. B) Refining the sphere : - After refining the cylindrical power and axis, the spherical correction is refined by : - The fogging technique, Duochrome test, Pin-hole test. 3)Subjective binocular balancing The final step in the subjective refraction is binocular balancing, i.e., 'equalization of vision ' or 'equalizing the accommodative effo'. This allows both eyes to have the retinal image simultaneous in focus.
Category:
Ophthalmology
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