Understanding the Distance between the Lens and the Retina in Human Eyes

The distance between the lens and the retina in the human eye is a critical component of visual mechanics. Typically, this distance is around 22 to 24 millimeters, a range that can vary slightly among individuals. This measure is pivotal for the focusing ability and overall vision of the eye.

Focus Length and the Eye's Mechanics

When an image is in focus, the measurement of the focal length of the eye lens when an object is in focus is essential. Discussing blurred images doesn't make much sense in this context. Therefore, I will focus on sharp images only. The lens surfaces in the eye are thicker in the center compared to the edges, making them converging lenses. They form an inverted image on the photosensitive layer at the back of the eye - the retina. The retinal image is shared by two lenses: 1) the cornea, which has a fixed focal length, and 2) the eye-lens, which has a variable focal length through shared changes called accommodation mediated by the ciliary muscles.

The Role of Ciliary Muscles

When the ciliary muscles are relaxed, the focal length of the eye-lens is at its maximum, and distant objects are in focus naturally. When the ciliary muscles contract, they shorten the focal length of the eye-lens to bring nearer objects into focus. The two limits of this range are called the far-point when the muscles are relaxed, and the near-point when the maximum accommodation is achieved. Source: University of Colorado Boulder.

The Typical Focal Length and Strength of Eye Lenses

The distance between the eye-lens and the retina is about 20 millimeters. When an object is far away from the eye, the image is located essentially at the focal point. Therefore, the focal length of the cornea and the eye-lens should be about 20 millimeters when the muscles of the eye are relaxed. The lens strength is the reciprocal of its focal length in meters. Hence, the strength of the cornea and the eye-lens at the far-point is about 1/0.020 50 diopters.

Near-Point Accommodation and Diopter Strength

When an object is located at the near-point, the closest point at which an object can be brought into clear focus on the retina, the focal length of the cornea and the eye-lens must be changed so that the image is formed on the retina which is still 20 millimeters away. The near-point for an adult is 25 cm, corresponding to a focal length of the cornea and the eye-lens of 18.52 millimeters using the standard ray-tracing rules of lenses. Hence, the strength of the cornea and the eye-lens must now be about 1/0.01852 54 diopters. In other words, the muscles of the eye can provide an accommodation range of 4 diopters.

Convergence of Vision: A Special Case

Knids, however, can focus to points as close as 6.5 cm away, which is 15 diopters of optical power, equivalent to a minimal focal length of 15.39 millimeters. This dramatic difference in the range of accommodation highlights the adaptive capabilities of different eye types, with significant implications for lens design and ophthalmic care.