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. (August 2012)
Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying or light-collecting optical instrument, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is still considered "naked".
In astronomy, the naked eye may be used to observe celestial events and objects visible without equipment, such as conjunctions, passing comets, meteor showers, and certain near-Earth asteroids, including 4 Vesta. Sky lore and various tests demonstrate an impressive variety of phenomena visible to the unaided eye.
The basic accuracies of the human eye are:
- Quick autofocus from distances of 25 cm (young people) to 50 cm (most people 50 years and older) to infinity.
- Angular resolution: about 1 arcminute, approximately 0.02° or 0.0003 radians, which corresponds to 0.3 m at a 1 km distance.
- Field of view (FOV): simultaneous visual perception in an area of about 160° × 175°.
- Faint stars up to +8 magnitude under a perfectly dark sky.
- Photometry (brightness) to ±10% or 1% of intensity – in a range between night and day of 1:10,000,000,000.
- Symmetries of 10–20' (3–6 m per 1 km), see the measurements of Tycho Brahe.
- Interval estimations (for example at a plan on paper) to 3–5%.
- Unconscious recognizing of movement (that is "alarm system" and reflexes).
Visual perception allows a person to gain much information about their surroundings:
- the distances and 3-dimensional position of things and persons
- the vertical (plumb line) and the slope of plain objects
- luminosities and colors and their changes by time and direction