LENSES

A convex lens converges light and is also called a plus lens or converging lens and appear as black in refraction box

It is a lens which has power in one axis and zero power in other axis. It is used to correct astigmatism. It focuses light on a line instead of a point focus. It converges light at right angle to its axis. So the axis must be mentioned in prescription.

Spherical aberration

Rays passing close to centre of lens are converged less than the rays passing more peripherally through the lens. Thus no point image is formed. In ocular lens pupil control the peripheral rays to avoid spherical aberration In dilated pupil the patient becomes slightly more myopic than smaller pupil. 

treatment: aspheric design of lenses

Chromatic aberration

short wavelenth rays are converged more than long wavelength rays. Thus blue light converges early then green light which converges more than red light. Thus light is not focused on a point called chromatic aberration. principle is used in duochrome test where The patient is asked to compare the clarity of the letters on the green and the red side. If the letters of the green side are clearer +0.25 D sphere is added and if the letters on the red side are clearer -0.25 D sphere is added. With optimal spherical correction, the letters on the red and green halves of the chart appear equally clear. 

a spherocylindrical lens like +2 sphere and -1 cylinder at 90 degree can also be written in transposed form in which

both values are added to get new sphere

the value of cylinder remains same but different sign

the angle is changed by 90 degree. if more than 90 then reduce by 90 and if less than 90 than add 90 degree.

The above transposed form will be

+1 sphere with +1 cylinder and axis 180

pronounced as fre enel prism. To avoid the weight and some of aberrations of conventional prism, clinicians use fresnel prisms. plastic sheets of side-by-side long, narrow, thin prisms. One form of Fresnel prism, the Press-On membrane prism, is applied with water to the back surface of a spectacle lens. 

if sphere is added to half of cylinder, we get spherical equalent of that spherocylindrical lens