The human eye is an optical device that has a high sensitivity to differences in lighting. An important characteristic of a person’s optical device is the resolution of the eye. Dots are perceived differently when exposed to sensitive receptors.
What is the resolution of the eye?
The human eye is an organ with a complex structure. The eyeball has the shape of a ball with a length of 24–25 mm and contains a light-reflecting and light-receiving apparatus.
The resolution of the human eye is the distance between two objects or lines that are visible separately. You can evaluate the resolution in minutes or millimeters, most often reveal the number of lines visible separately in the interval of 1 mm. The reason for the change in resolution of the eye is the anatomical size of the receptors and their connection.
The resolution of the human eye depends on factors:
- Nerves process the signal received by the retina.
- Optical - corneal irregularities, impaired focus, diffraction by the iris, light scattering, and eye disturbances.
Contrast of objects affects resolution. The difference can be seen in daylight and night lighting. During the day, the influence of diffraction increases due to the narrowing of the pupil, and the deviation of the cornea from the correct shape does not affect the image. At night, the pupil expands and becomes part of the peripheral zone of the cornea. The quality of vision decreases with a violation of the cornea, which is due to light scattering on the photosensitive areas of the eye.
Resolution Definition
To identify the formula for the resolution of the eye, it should be understood that the resolution is the inverse of the smallest angle between the directions by 2 points, at which different images are obtained.
The diffraction of light on the entrance pupil looks like a bright circle in the center. The first diffraction minimum is at a certain angle from the center. To determine the resolution of the eye, it is necessary to know the diameter of the pupil and the length of the light wave. The pupil diameter is many times the wavelength.
More than 84% of the line of light passing through the pupil enters the Airy circle. The maximum indicator will be 1.74%, the remaining maximums show a fraction of the first. Thus, the diffraction pattern is considered to be composed of a central bright spot with an angular radius. This spot projects the image onto the retina. This is how diffraction is formed.
Vision angle
It is established that the influence of the angle of view on the resolution of the eye is great. In space there are 2 points that pass through the refractive medium of the eye and are connected on the retina. The rays after refraction form an angle called the angle of view.
The magnitude of the angle of view will depend on the size of the object and its distance to the eye. The same subject, but at different distances, will be displayed at different angles. The closer the subject, the greater the angle of refraction. This explains that the closer the subject, the more detailed the person can examine it. It is also known that the human eye distinguishes between 2 points if they are displayed at an angle of not less than 1 minute. The light beam should thus fall on the 2 nearest nerve receptors so that at least one nerve element remains between them. Therefore, normal vision depends on the resolution of the eye. After refraction, the angle of view remains 1 min.
Refraction
One of the characteristics of the organ of vision is considered to be refraction of the eye, on which the sharpness and clarity of the resulting image depends. The axis of the eye, the sides of the lens and cornea affect refraction. These parameters will depend on whether the rays on the retina converge or not. In medical practice, refraction is measured physically and clinically.
The physical method calculates from the lens to the cornea, not taking into account the features of the eye. In this case, it does not take into account what characterizes the resolution of the eye, and refraction is measured in diopters. Diopter corresponds to the distance through which the refracted rays converge at one point.
For the average value of refraction of the eye, an indicator of 60 diopters is taken. But the calculation is not effective for determining visual acuity. Despite the sufficient refractive power, a person may not see a clear image due to the peculiarities of the structure of the eye.
If it is broken, then the rays may not hit the retina at the optimal focal length. In medicine, the calculation of the relationship of eye refraction and the location of the retina is used.
Varieties of refraction
Depending on where the main focus is located, in front or behind the retina of the eye, the following types of refraction are distinguished: emmetropia and ametropia.
Emmetropia is normal eye refraction. Refracted rays converge in the retina. Without tension, a person sees objects removed at a distance of several meters. Only 40% of people do not have visual pathologies. Changes occur after 40 years. With normal eye refraction, a person can read without fatigue, which is due to focus on the retina.
With disproportionate refraction - ametropia, the main focus does not coincide with the retina, but is located in front or behind. So distinguish farsightedness or myopia. In a nearsighted person, the farthest point is located nearby, the cause of incorrect refraction is hidden in the enlargement of the eyeball. Therefore, such people do not see objects far away.
Hyperopia occurs with weak refraction. Parallel rays converge behind the retina, and the image of a person is seen blurry. The eyeball has a flattened shape and clearly displays distant objects. The disease most often develops after 40 years, the lens loses elasticity and cannot change curvature.
Eye color sensitivity
The human eye is sensitive to different parts of the spectrum. The relative luminous efficiency in the spectral circle is equal to the ratio of the sensitivity of the eye to light with a wavelength of 555 nm.
The eye sees only 40% of solar radiation. The human eye has a high adaptation. The brighter the light, the smaller the pupil becomes. A pupil with a diameter of 2-3 mm becomes optimal for high sensitivity.
In the daytime, the eye has great sensitivity to the yellow part of the spectrum, and at night to blue-green. For this reason, night vision becomes worse and the sensitivity of colors decreases.
The disadvantage of the optical system of the eye
The eye, as an optical device, is not without flaws. The smallest linear distance between two points at which images merge is called the linear period of the resolution of the eye. Violation of the structure of the lens and cornea leads to the development of astigmatism.
Optical power in the vertical plane does not equal the power in the horizontal. Typically, one is slightly larger than the second. In this case, the eye vertically can be shortsighted, and horizontally - farsighted. If the difference in these lines is 0.5 diopters or less, then it is not corrected by points and referred to physiological. With a larger deviation, treatment is prescribed.
Uncentered optical system of the eye
The resolution of the eye depends on the structure of the optical system of the organ of vision. For the optical axis take a straight line passing through the center. The visual axis is a straight line that passes between the nodal point of the eye and the foveola.
At the same time, the central fossa is not located on a straight line, but is located below, closer to the temporal part. The optical axis crosses the retina without touching the central fossa and the optic nerve head. The normal eye creates an angle between the optical and visual axes from 4 to 8 about . The angle becomes larger with farsightedness, with myopia less or negative.
The center of the cornea rarely coincides with the optical center, respectively, the eye system is considered off-center. Any deviation prevents the rays from converging on the retina and reduces the resolution of the eye. The dispersion of eye disorders is great and may vary for each person.