If y is the max radial size of the image then θ is the field of view of the lens.
The quality of the image is limited by the aberrations in the lens and the diffraction created by the finite aperture stop.
The entrance pupil is the image of the aperture stop created by the optical elements on the object side of the lens.
The light scattered by an object is collected by the entrance pupil and focused onto the image plane via a series of refractive elements.
The cone of the focused light at the image plane is set by the size of the entrance pupil and the focal length of the lens.
In the case of a monochrome sensor, the light incident on each pixel is integrated and the resulting image is a grayscale like picture.
Physically, diffraction due to the aperture stop will limit the resolvable spatial frequencies as a function of f-number.
In the frequency domain, Modulation Transfer Function (MTF) is a measure of the quality of the imaging system.
In the case of computer vision the light incident on the sensor comprises the image.
In the case of visual perception, the human eye has a color dependent response to light so this must be accounted for.
When the eye focuses on an object farther away than about 3 m, the lens exhibits its lowest refractive power.