The Camera and The Human Eye

 A.          Components of A Camera
Container - light-proof container
Lenses - in  series
Film - light-sensitive (changes chemically when exposed to light)
Shutter - with variable speed to open and let light in
Diaphragm - controls the quantity of light going through the lens
 B.          Components of the Eye
     a thin transparent layer on the external part of the eye that does most of the focussing for the eye onto a thin, curved layer of light-sensitive cells
     has almost the same refractory index as water
     a transparent, flexible, convergent-shaped structure that also focuses the image on a thin, curved layer of light-sensitive cells
     light-sensitive cells that respond to the various intensities and colours of the light that falls upon them, sending electric signals to the optic nerve
     made of  two types of photoreceptors:
            i)            rods:            approx. 125 million
                                    concerned with colourless vision and dim light
            ii)            cones:            approx 7 million
                                    color vision in ample light
                                    the density of cones directly corresponds to visual acuity
                                    (i.e. humans-150000 cones/mm2; birds-1million cones/mm2)
Optic Nerve
     transfer electric signals from the retina to the brain
Ciliary Muscles          
     ring-shaped muscles that alter the shape of the lens to accommodate vision (i.e. focus on an object)
     a doughnut-shaped ring that controls the amount of light that enters the eye
     the hole (aperture) in the center of the eye
     the stiff, external sheath of the eye that functions in preserving the shape of the eyeball
     resists internal and external pressure (keeps the eyeball in its spherical shape)
Aqueous Humour
     water-like substance between the cornea and the lens that is highly compressible
Vitreous Humour
     jelly-like substance between the lens and the retina that is very resistant
  C.      Vision and Accommodation
       Light from an object is refracted through the cornea and lens of the eye and is focused onto the retina
     The retina sends electrical signals to the optic nerve, which in turn, send these signals to the brain
     The image on the retina is inverted and reversed, but the brain straightens this out and you "see" the image the right way up
(i)   Close Vision
     to focus a close image onto the retina, the ciliary muscles contract, thus changing the shape and curvature of the lens (i.e. compressing it), and the cornea is pushed outward by the force exerted onto the aqueous humour
 (ii)  Distant Vision
     when the ciliary muscles are relaxed, the cornea and lens are automatically focused for distant vision
 D.      Defects in Vision and Their Correction
 (i)        Farsightedness (hypermetropia):      the inability to see nearby objects clearly
1.   distance between the lens and the retina is too small
2.   weakness in the ciliary muscles - those that change the shape of the lens
3.   loss in the elasticity of the lens (old age) - loss in accommodation
      converging glasses or contact lenses that converge the rays of light so that the lens can focus the image clearly on the retina
 (ii)       Nearsightedness (myopia):      the inability to see distant objects clearly
1.   the distance between the lens and the retina is too great
2.   the ciliary muscles are too strong - those that change the shape of the lens
     diverging glasses or contact lenses that diverge the light so that the eye lens can focus the image clearly on the retina