The average effective focal length of the relaxed human eye is approximately 17 mm. This figure is derived from the eye's total refractive power, which is typically around 60 diopters (1 meter / 60 diopters ≈ 0.0167 meters or 16.7 mm).
While 17 mm represents the effective focal length from the eye's principal plane, the physical distance from the front of the eye (cornea) to the retina where the image is formed is longer, averaging about 24 mm. This axial length is also often referenced in discussions about the eye's dimensions.
The Eye's Focal Length Compared to Photographic "Normal" Lenses
It's interesting to compare the eye's effective focal length to the concept of a "normal lens" in photography, although they represent different things.
In photography, a "normal lens" is generally defined as a lens whose focal length is roughly equal to the diagonal measurement of the film or sensor format it is used with. This provides an angle of view and perspective that is often considered similar to natural human vision.
- Reference Information: For a 35 mm camera format, which has a diagonal of 43 mm, the most commonly used normal lens is 50 mm. However, focal lengths between about 40 and 58 mm are also considered normal for this format, as stated in typical photographic guides.
Key Difference:
- The eye's actual effective focal length is around 17 mm.
- A photographic "normal lens" for a 35 mm format is typically in the 40-58 mm range.
These photographic normal lens focal lengths are not the eye's focal length. Instead, they are chosen for a specific camera format (35mm) to produce images with a perspective that approximates what we see, despite the eye's much shorter physical focal length. This difference arises because the eye's "sensor" (the retina) is effectively much smaller than a 35mm film frame.
In summary:
- Average Effective Focal Length of the Eye: Approximately 17 mm.
- Axial Length of the Eye (Cornea to Retina): Approximately 24 mm.
- Photographic "Normal" Lens (for 35mm format): Typically 40-58 mm.
