Vision

• see also:
  • optics
  • light
  • acute visual disturbance
  • visual illusions:
    • powerpoint demonstration of visual illusions and another here
    • pink dots
  • astronomic viewing - seeing, light pollution, techniques

Refraction of the eye:

• the eye is a nearly spherical structure and consists of two main structures which are responsible for refraction of the incident light:
  • the cornea
  • the lens - changes in its curvature as a result of action of the ciliary muscle, assists are ability to focus on objects & thus is responsible for accommodation - the adjustment of the distance of the image to the exact distance of the retina, in accordance with the external distance to the object of regard.
• refractive errors of the eye:
  • myopia - inability to focus on distant objects (near-sightedness)
  • hyperopia - inability to focus on close objects (far-sightedness):
    • there is a normal dwindling of the power of accommodation with age so that by age 45yrs, reading glasses are often required
  • astigmatism - generally due to unequal curvature of the front surface of the cornea, resulting in the person seeing radial lines parallel to the axis of the cylindrical curvature of his eye less sharply than other lines.
• correction of refractive errors:
  • to correct myopia, a diverging lens is required
    • bifocal lenses for myopics have upper area negative, and lower area less strongly negative or even positive for reading
  • to correct hyperopia, a converging lens is required:
    • eg. if minimum distance is 150cm, and a person wishes to read at 25cm, then:
      • p = 25, q = -150, thus from 1/p + 1/q = 1/f, then f = 30cm, where, f = focal length of lens required.
      • and thus it has a power of 1/0.3 diopters = 3.3 diopters
    • bifocal lenses for hyperopics have both areas positive, with the lower one of greater power - usually by about 2 diopters

Sensitivity of the eye to light:

• colour vision sensitivity:
  • you cannot see faint objects such as nebulae in colour unless the telescope aperture is at least 16" diameter
  • The night sky at Full Moon (about magnitude 18 per square arcsecond or about magnitude 9 per square arcminute) must be very close to the color threshold for at least some people to see the night sky as "sky blue". 
  • The canonical "textbook" figure for visual colour threshold is magnitude 15 per square arcsecond, which really isn't all that much different from the Full Moon level.
  • The incident light within about 24 hours of the moment of Full Moon allows one to see colours (on paper, pavement markings, etc.) without ambiguity from a place without other lights. But that light level is much higher than for the sky itself.
  • ability to see threshold objects such as stars during daylight:
    • cannot use averted vision as for night time vision as rods are insensitive in bright light - try looking at this point and use averted vision to determine the 1st letter on this line - not very good!
    • using a well-focussed 8-10" telescope, one can see stars at noon, if one can locate a bright enough star (eg. magnitude 2) and look straight at it.
• scotopic vision - the dark adapted eye:
  • the eye can detect extremely small amounts of light, with a dark adapted eye being able to detect the light from a candle at ~20-30km, in which case the retina is receiving only a few quanta of light. 
  • can read newspaper small headline type from starlight from the Milky Way.
  • although its sensitivity equates to approx. 400,000 ASA photographic film, it fatigues quickly and "exposure duration" equates to approx. 1/30th sec and equates to a 120 megapixel digital camera in resolution
  • the sensitivity varies for different wavelengths, being greatest for ~5600A wavelength (green-yellow)
  • visibility is improved if you use the more peripherally based retinal rods by using "averted vision" - not looking directly at the object
  • achieving dark adaptation:
    • it usually takes ~ 30 to 60min for your eyes to adapt to the dark and this process must restart if there is significant exposure to lights, especially bright lights (minimise this by using faint red lights but if you can see that it's red on the paper your looking at, it's too bright)
    • if you go out on for long on a sunny day, expect to lose about three-quarters of a magnitude in your magnitude threshold the succeeding night---after extended exposure to high-intensity scenes (beach, snow-skiing on sunny days), it takes more than 24 hours to become fully dark-adapted! The usual half-hour or hour won't do. Wear "glacier glasses" when outside during daytime.
    •  
  • ability to perceive very dim small objects such as stars, galaxies, etc:
    • ability to perceive a dim object is dependent upon:
      • brightness of the object
      • contrast in brightness between that of the object & that of the background
      • size of the object
    • the optimum telescope magnification needed to perceive such dim objects is dependent on the telescope aperture as well as the above factors and a balance of:
      • the decreasing object and background brightness as magnification increases
      • the increasing size of the object as magnification increases & thus more visible as long as magnified object size does not exceed the optimal size for the telescope aperture & this is usually less than 6 degrees
      • see http://www.mapug-astronomy.net/AstroDesigns/MAPUG/VisualDet.htm 

Persistence of vision:

• the visual system lags a bit in response to a stimulus & the sensation lasts for approx. 0.1sec after the stimulus ceases.
• the retention of the mental image, referred to as persistence of vision, prevents any "flicker" when a motion film is projected on a screen at the rate of at least 16 frames / sec (most commercial films are shown at 24 fps or 30fps)

Visual acuity:

• the visibility of an object depends on size, contrast, intensity, time & the adaptation of the eye
• the fovea is the most sensitive part of the retinal mosaic of rods & cones, being about 1mm in diameter & its central part, 0.2mm in diameter contains only cones
• the angular field of most distinct vision is about 1degree, which is subtended by a circle of 4.4mm at a distance 25cm from the eye
• within this field, the eye has a resolving power of ~ 1 minute of arc, resulting in the centers of the image lines on the retina being only a few thousandths of a millimeter apart.
• for close objects, this can be improved by using a magnifier glass:
  • as linear magnification = q/p & 1/p + 1/q = 1/f, then q/p = q/f - 1, and as we wish to read at 25cm, q = -25,
  • thus magnification = 25cm/f + 1
  • example: using 5cm focal length converging lens, producing a virtual image 25cm from the eye:
    • p = fq/(q-f) = 4.2cm (thus object should be placed 4.2cm from the lens)
    • magnification = 25/4.2 = 5.9
• for minute objects, a microscope can be used:
  • an optical microscope consists of an objective lens with magnification M_o & an eyepiece with magnification M_e
  • magnification = M_oM_e = q_o/p_o * (25cm/f_e +1)
• for distant objects, a telescope may be used:
  • an optical refractive telescope consists of an objective lens with magnification M_o & an eyepiece with magnification M_e
  • for a distant object, angular magnification = f_o/f_e