Table 1.
How different elements of the visual system limit visual acuity.
| Physiological limits (acuity type) | Nature of limit | Anatomical spatial limit | Frequency limit (c/deg) | |
|---|---|---|---|---|
| Optics (detection limiting) | ||||
| Monochromatic aberrations | Astigmatic defocus | Magnitudea of residual astigmatic defocus: <0.10 D. | <0.15 µm (Villegas, Alcón, & Artal, 2008) | >900 |
| Spherical defocus | Magnitude of residual spherical defocus: ∼0.25 D. | ∼0.29 µm (Thibos, Hong, Bradley, & Cheng, 2002) | >500 | |
| Higher-order aberrations (HOAs) | Magnitude of the HOAs (coma, trefoil, spherical aberrations): <0.35 µm and approaches 0.045 µm at 3-mm pupil diameter. | <0.35 µm (Salmon & van de Pol, 2006; Villegas, Alcón, & Artal, 2008) | >400 | |
| Chromatic aberrations (CAs) | Transverse CA | Magnitude of TCA: ∼0.61 arcmin. | 3 µm (Thibos et al., 1990) | ∼82 |
| Longitudinal CA | Magnitude of LCA: ∼1.56 D (0.36 arcmin) across the visible spectrum. | 1.8 µm (Thibos et al., 1990) | ∼50 | |
| Pupil | Diffraction | The spatial diameter of the diffraction-limitedb Airy's disc at a 2.3-mm pupil sizec is about 0.5–1 arcmin. | ∼4.94 µm (Campbell & Gubisch, 1966; O'Brien, 1951) | ∼30–60 |
| Neural limits: Retina/geniculate (resolution limiting) and visual cortex (recognition limiting) | ||||
| Cone cells (fovea) | Density, spacing,d aperturee | Peak cone density of ∼164 K–199 K cones/mm2, center-to-center spacing of ∼2.7 µm, and an aperture of ∼1.6–2.2 µm. | ∼2.7 µm (Curcio, Sloan, Kalina, & Hendrickson, 1987; Wells-Gray, Choi, Bries, & Doble, 2016) | ∼55 |
| Retinal ganglion cells and LGNf | Receptive field size | The smallest RF size of midget-RGCs is ∼1 arcmin. | ∼5 µm (Dacey, 1993) | ∼30 |
| Central V1 cortical neurons | Receptive field sizeg | Cells have RF sizes of ∼4–5 cones wide but have a preferred stimulus width of ∼2–3 cones wide (0.7–1 arcmins). | ∼3.4–5.1 µm (Dow, Snyder, Vautin, & Bauer, 1981) | ∼30–43 |
For an optical system fully corrected for astigmatism and defocus excluding the effects of other factors such as diffraction and at a 6-mm pupil diameter.
In a perfect diffraction-limited optical system, the cutoff frequency given as 1.22*λ/aperture size is about 60 c/deg for an optical system of focal length 17 mm with light of a 550-nm wavelength. The point spread function for this optical system will have a diameter of about 1 arcmin at half height.
At pupil diameters of between 2 and 3 mm, the effects of aberrations and diffraction are approximately balanced.
The Nyquist limit of the foveal cone mosaic is about 60–85 c/deg.
A smaller cone aperture dictates finer sampling resolution of the cone mosaic.
Lateral geniculate nucleus (LGN) ganglion cells number about 1.2 M cells/mm2 in humans. Receptive field (RF) properties in the LGN are similar to retinal ganglion cells (RGCs). P (midget) ganglion cells project to the four dorsal layers of the LGN and are responsible for high-acuity performance.
Receptive field size of central cortical neurons is smaller but more numerous while those representing the peripheral field are larger but less numerous, contributing to a higher foveal cortical magnification, a limiting factor for vernier and other forms of hyperacuity such as recognition acuity.