Optical System and Image Formation
The optical system of the eye is a complex apparatus that ensures the transmission of light rays and the formation of an image on the retina. During visual perception, light rays pass through the optical axis of the eye, the nodal point, the principal anterior and posterior foci, and the principal plane of the lens.
When a person views a relatively distant object, the optical system adapts: the lens transitions to a less refractive state, becoming flatter. As a result of light refraction, an image is formed on the retina with the following strictly defined characteristics:
- Real
- Diminished
- Inverted
To calculate the physical size of this retinal image, physiologists apply the mathematical rule of similar triangles, whose vertices meet at the nodal point. The calculation is performed using the formula X = J · (a / b). In this equation, X denotes the desired size of the object's projection on the retina, J is the true size of the object itself, and a represents the precise distance from the nodal point of the optical system to the retina.
Visual Resolution and Standard Values
Visual acuity is based on the concept of resolving power. The angular resolution limit of the human eye—the minimum possible angle at which two points can still be seen separately—is 0.62 arcminutes. However, in physiology, the standard baseline norm, or unit, is set at exactly one arcminute (1').
Based on this standard, the following normal parameters are distinguished:
- Normal visual acuity: evaluated as 1.0.
- Reduced visual acuity: equals 0.8 and below.
- Increased visual acuity: ranges between 1.5–2.0, exceeding standard baseline values.
Calculation of Visual Acuity
The classical Snellen-Donders formula is used for the quantitative assessment of visual function:
V = d / D
In this formula, the variables have the following meanings:
- V (Visus) — the patient's measured visual acuity.
- d — the actual distance (in meters) from the subject to the testing chart.
- D — the standard distance (in meters) at which a specific line on the chart must be read perfectly by a normal eye (an eye with a value of 1.0).
Retinal Topography and Visual Perception
Visual acuity is not uniform across the fundus; it demonstrates a marked dependence on the location of the projection on the retinal surface. The ability to resolve details is distributed extremely unevenly.
The absolute maximum of visual perception (an acuity equal to 1.0) is recorded exclusively at the very center of the retina (0° angle). As one moves away from the central zone (fovea) toward the peripheral regions, visual acuity drops sharply and precipitously. Graphical analysis shows that with a shift of just 10–20 degrees from the center, the value drops to critical levels of 0.2–0.3 or even lower. This is precisely why the optical system must focus the image of an object strictly onto the fovea centralis for detailed examination.