W4 Lenses for Astigmatism

What is the correction of myopia?

As a blur circle is imaged on the retina, with the image focused in front of the lens, myopia is corrected with a negative lens of the same power in all meridians- a spherical lens

What is the correction of hyperopia?

Due to the image behind focused behind the retina, a blur circle forms on the retina and to correct this, we use a positive lens of the same power in all meridians- a spherical lens

How can a hyperope bring the image back into focus on the retina?

Via accommodation resulting in a clear image- no glasses needed unless eye strain occurs

How does astigmatism occur?

2 foci are produced, as an elliptical blur circle is imaged on retina and the image appears distorted

What is the circle of least confusion?

The midway point between the 2 foci produces a blur circle of least confusion

How is astigmatism corrected?

A cylindrical lens must be used

What is a plane surface?

Is afocal- has no dioptric power

What is a Plano CONVEX surface?

Has positive dioptric power

What is the cylinder axis?

Meridian of 0 power

What is the power meridian?

The meridian of maximum dioptric power- the power meridian of any cylindrical lens is 90 degrees from the cylinder axis

How do you do negative sphere cyl form?

The sphere will be the most positive, cylinder is the difference between the 2 (cyl is -ve) and do axis (opposite more +ve sphere)

How do you do positive sphere cylinder form?

Specify sphere as most negative power

Cylindrical power is still difference (+ve cyl)

Axis is same side as sphere (more -ve one)

How do you do sphere cylinder transposing?

Add 2 values to get sphere, switch cyl sign and add/minus 90 degrees