In taking pictures, in order to get a clear picture, the photographer has to adjust the camera according to the distance of the person from the camera -- everyone knows that. The eye, which is physically a miniature camera, also has to make adjustments for different distances -- no one seems to know that.
Lenses, which are curved glass, will bend or refract rays of light and heat towards the thicker part of the lenses. If it is a convex lens, which is thicker in the center, light rays will bend towards the center and come to a point or focus some distance from the lens. If it is a strong lens, it will come to a point near the lens. If it is a weak lens, it will come to a focus farther away.
The lenses in your spectacles or glasses affect and change the course of light rays entering your eyes. They affect and change the focus in your eyes. If it is a convex lens, the focus will be closer -- if a concave lens, the focus will be farther. A neutral lens will not affect the focus, and will not make you see better or worse. A neutral lens is one that is neither convex or concave. It is one that has the same thickness over the whole surface of the lens. It is called plano lens.
Spectacles or glasses that affect our vision in any way are focus-glasses. They have optical power to change the focus in our eyes. They have curvature, either convex or concave.
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