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Some material about central vision:
As we mentioned, the central vision is conducted by yellow spot and its central part - optic fovea - a pit with a diameter of about 0.2 mm, which presents gathering of maximal quantity of cone cells. At the distance of only 10 degrees from optic fovea acuteness is one/fifth part of central vision.
Practically it’s very important for oculists to measure acuteness of vision. Under the acuteness the ability of eye to distinguish two separate bright points, placed close to, or maximally distant to each-other is meant. That’s to say that acuteness of central vision comes to definition of minimal distance between two bright points, when these points are visible (perceptive) as separate two.
If before eye, at some definite distance from it, two bright points a & b are placed, in order to receive image on the retina obligatory is that light beams passed through optical medium of eye, through the focal point of k up to retina. The points a’ and b’ are images of the points a & b on the retina. The angle akb=a’kb’ and is defined as angle of vision.
According to the great amount of research, normal eye of human can perceive two different irritations as separately two ones, when they are disposed at the angle of 1' and this equals as linear measurement of 0.004 mm between images, i.e. two points are perceived as separate ones, when the distance between their images on the retina is 0.004 mm. This value has its ground in anatomical elements of retina within yellow spot: the cell of pigment epithelium with a few neuroelements is independent light-receiving unit. If two images of both bright points are received in the same neuroelement, or neighboring two ones, then they merge into one.
If the images of bright points are received on two neighboring neuroelements, then the sensation of short line is perceived, as both irritations will interflow together and give one sensation. If images of points a & b on the retina are received on the neuroelements, separated by not irritated single one between them, eye will perceive two points separately.
Therefore, in order to perceive two different points as separate two, the distance between images of these points on the retina must be not less than diameter of one single neuroelement (the size of which in humans equals to 0.004mm, or 1' angle of vision).