The eye has a visual acuity threshold below which an object will go undetected. As light enters the eye through the pupil, it passes through the lens and is projected on the retina at the back of the eye. Muscles called extraocular muscles, move the eyeball in the orbits and allow the image to be focussed on the central retinal or fovea. The retina is a mosaic of two basic types of photoreceptors, rods and cones. Rods are sensitive to blue-green light with peak sensitivity at a wavelength of 498 nm, and are used for vision under dark or dim conditions.
Cones are highly concentrated in a region near the center of the retina called the fovea region.
The maximum concentration of cones is roughly 180,000 per square mm in the fovea region and this density decreases rapidly outside of the fovea to a value of less than 5,000 per square mm. Note the blind spot caused by the optic nerve which is void of any photoreceptors. In this 288 micrometers dimension, there are 120 color sensing cone cells packed. With a little trigonometry it is possible to calculate the resolution of the eye at a specific distance away from the lens of the eye. When visually inspecting an object for a defect such as a crack the distance d might be around 12 inches.