Accommodation (eye)

Light from a single point of a distant object and light from a single point of a near object can be seen clearly when the curvature of the lens changes.

Accommodation is the way the eye increases optical power (the degree to which the lens converges or diverges light). This is necessary to produce a clear image (focus) on an object when it draws near the eye. A lens that is more convex( fatter in the middle) would refract more light rays than a less convex lens (thinner lens). The lens can change shape because the cells of the lens contain an elastic crystalline protein. The young human eye can change focus from distance to seven centimeters from the eye in 350 milliseconds.

The eye focuses on a given object by changing the shape of the eye lens through accommodation. This is controlled by ciliary muscle,which surrounds the lens.

Accommodation (eye) Media

Minimum (top) and maximum accommodation (bottom).
Two horse lenses suspended on water by cling wrap with four approximately parallel lasers directed through them. The 1 cm spaced grid indicates a near focus focal length of around 6 cm
The path of light through the eye calculated using four refractive indexes, cornea and lens curvatures approximating components of real eyes. Note objects in some size ranges and distances do not require the light path to bend noticeably to achieve focus.
Virtual eye showing the contribution to focus of different components.
3D reconstruction based on measurements taken from a 20 year old human male focusing from 26mm to infinity (4.85 dioptre change). Side & back views shown. Most image distortions near the center are due to model being limited to 512 faces to make up the lens. Peripheral distortions are also present in animal lenses though are typically covered by the iris (anatomy)
The previous video of the eye lens changing shape with focus is placed into context as the lens in this video is placed into the context of a virtual eye.
Schachar model of lens focus
Tracing of Scheimpflug photographs of 20 year old human lens being thicker focusing near and thinner when focusing far. Internal layering of the lens is also significant
Wrinkled lens fibers in picture below compared to straight fibers above
Duane's classical curves showing the amplitude or width of accommodation as changing with age. Mean (B) and approximate lower (A) and upper (C) standard deviations are shown.

Other websites

pupilEyes - Learn about accommodation of the eye Archived 2011-07-15 at the Wayback Machine