Lenses as optical System.

Lenses optical behavior and types

The key stone part of any prescription eyeglasses is lenses. It's solution to not normal curvature of (cornea) astigmatism or focus for presbyopia (near vision).

How it works.

Positioning

The lenses positioned in front of the eyes by frame that sits on the bridge of the nose and is held in place with arms that rest on or are hooked behind the ears.

Optical surfaces

A lens consists of two optical surfaces of revolution enclosing a transparent optical medium, usually glass or on of the plastic resins. Any such lens or combination of lenses Surrounded by the same medium (such as air) on both sides is referred to as a symmetric optical system. although the two suffices need not be identical, as would usually be inferred from the use the word symmetric. A thin lens is a hypothetical construct that assume zero thickness of the lens at it center. In ophthalmic practice, only negative lenses approach this ideal. The thickness of most lenses used in low vision care departs from the hypothetical zero. However, treating these lenses as “thin” will simplify understanding of their general properties. Although the clinician will encounter many varieties of surfaces of the revolution, an understanding of spherical surfaces is sufficient. Near their center, a spherical and depart from a true sphere only as their diameters increase. Thus far, we have defined the optical device that is going to act on the light to make it behave in certain way.

Optical axis

So far, object points located on the optical axis of the thin lens system considered. The optical axis of a lens system is defined as straight line that passes normally through every surface in the system. And also passes through the center curvature of every surface on this system. The size of the object and image can be measured to scale, which gives an indication the linear magnification properties of the optical system. Liner magnification is defined as the ratio of image size to object size.

Lenses types

Today mostly commertialy available four type of lenses: single vision, bifocals, trifocals, or multifocals rogressive.
Today mostly commertialy available four type of lenses: single vision, bifocals, trifocals, or multifocals rogressive.
Single vision lenses have equal focal power throughout and used to correct astigmatism , nearsightedness, farsightedness, or a combination of these disorders. Most people before the age of 40 have single vision disorders.
Bifocal lenses have two parts: the upper part normally used for distance vision and the lower part used for near-vision tasks such as reading.

Trifocal lenses have three different focus areas: the top for distance vision, the center for intermediate vision, and the bottom for near vision.

Progressive addition (no-line bifocal) lens that increases in power from top to bottom. Progressive lenses have no clear dividing lines as the focus changes from bottom to top. They are popular in recent years, they look like single vision.