Optical system and image formation
The optical system of the eye is a complex apparatus that ensures the passage of light rays and the formation of an image on the retina. During visual perception, rays pass through the optical axis of the eye, the nodal point, as well as the main anterior and posterior foci and the principal plane of the lens.
When a person perceives a relatively distant object, the optical system adapts: the crystalline lens transitions into a less refractive state, becoming flatter. As a result of light refraction, an image with the following strictly defined characteristics is formed on the retina:
- Real
- Diminished
- Inverted
To calculate the physical size of this image on the retina, physiologists apply the mathematical rule of similar triangles, whose vertices lie at the so-called 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 exact distance from the nodal point of the optical system to the retina.
Resolving power of the eye and standards
Visual acuity is based on the concept of resolving power. The angular limit of resolving power of the human eye, i.e., the minimum possible angle at which two points are still seen separately, is 0.62' (arcminutes). However, in physiology, the standard norm, or unit, is considered to be exactly one arcminute (1').
Based on this standard, the following baseline values are distinguished:
- Normal visual acuity: evaluated as 1.0.
- Reduced visual acuity: is 0.8 and below.
- Increased visual acuity: ranges within 1.5–2.0, which exceeds standard values.
Calculation of visual acuity
To quantitatively assess visual function, the classical Snellen-Donders formula is applied:
V = d / D
In this formula, the variables have the following meanings:
- V (Visus) — the patient's target visual acuity value.
- d — the actual distance (in meters) from the test subject to the test chart.
- D — the standard distance (in meters) from which a specific line of the chart must be read completely correctly by a normal eye (an eye with a value of 1.0).
Retinal topography and visual perception
Visual acuity is not uniform across the entire fundus; it demonstrates a pronounced dependence on the localization of the projection on the retinal area. The ability to resolve details is distributed extremely unevenly.
The absolute maximum of visual perception (an acuity equal to 1.0) is recorded exclusively in the very center of the retina (0° angle). As one moves away from the central zone (fovea) toward the peripheral areas, visual acuity drops sharply and rapidly. Graphic data analysis shows that with a displacement of just 10–20 degrees from the center, the value drops to critical levels of 0.2–0.3 and even lower. This is precisely why, for detailed study of an object, the optical system is forced to focus its image strictly on the fovea centralis.
Briefly: answers to topical questions
Which types of photoreceptors provide maximum visual acuity in the fovea centralis of the retina?
Maximum visual acuity in the fovea centralis of the retina is provided by cones.
In the retinal macula, photoreceptors are represented solely by cones, and the fovea centralis is the zone of high/maximum visual acuity. A direct impulse pathway is indicated for the fovea centralis: 1 cone → 1 bipolar cell → 1 ganglion cell.
For the Snellen-Donders formula (V = d/D), use the rule "Small in the numerator, Large in the denominator." The lowercase letter "d" is the distance to the patient (they are physically here, the distance is smaller), and the uppercase letter "D" is the standard distance of ideal vision (it is always larger).
Class and exam questions
What kind of image is formed on the retina of the eye?
The optical system of the eye always forms a real, inverted, and diminished image of an object on the retina.
How does the state of the crystalline lens change when looking at distant objects?
Upon perceiving a relatively distant object, the crystalline lens becomes flatter, transitioning into a less refractive (relaxed) state.
Where on the retina is maximum visual acuity recorded?
Maximum visual acuity (1.0) is recorded in the very center of the retina (0°). When shifted to the periphery by 10–20 degrees, it drops sharply to 0.2–0.3.
What is the calculation of retinal image size based on?
The calculation is based on the geometric rule of similar triangles whose vertices converge at the nodal point of the eye's optical system.
What else to study on this topic
- The principle of similar triangles in image calculation (X = J · a/b)
- Diagram of light ray propagation: nodal point, optical axis, and foci
- Resolving power of the eye: physiological limit of 0.62'
- Dependence of visual acuity on retinal area (analysis of the visus decline graph)
- Changes in the refractive state of the crystalline lens upon distance object viewing
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