A basic camera consists of an aperture to control the amount of light getting in, usually adjustable by means of a diaphragm, a lens or system of lenses to capture and focus the light, and some form of chemical or electronic detector — all wrapped up in a dark box or chamber — from which the term camera comes — to stop light getting to the detector other than through the aperture & lens. All of these elements are also present in the eyes of many animals, including us.
The eye’s aperture is the pupil, adjusted by the sphincter and dilatory muscles of the iris to let in more light in dim conditions, less when it is bright.
The eye has two lens elements. The more rigid, fixed focal length cornea does the bulk of the focusing, while the softer, deformable lens provides adjustability. A ring of ciliary muscle around the lens can contract to increase the lens curvature, decreasing its focal length and allowing nearer objects to be in focus. This reshaping of the lens is known as accommodation.
The distances that an individual can bring into focus range from the far point, when the ciliary muscles are fully relaxed, to the near point when they are maximally contracted. These distances depend on the shape of the eye, the focusing power of the cornea and lens, the deformability of the lens and the strength of the ciliary muscles.
Impairments of focal range are very common. People with myopia or nearsightedness can’t focus on distant objects — their far point is significantly nearer than infinity and light from beyond that point comes to focus in front of the retina. Those with hyperopia or farsightedness can’t focus on nearby objects, the light from which comes to focus behind the retina. Both of these conditions can be corrected with glasses or contact lenses, introducing extra focusing elements in the optical path to change the baseline focusing power of the whole system. As people age, their lenses become increasingly stiff and their ciliary muscles weaken, reducing their capacity to accommodate, shifting the near point further away. This age-related farsightedness is known as presbyopia, literally “old eye”, and it’s why so many people need reading glasses as they get older.
Eyes are directional and there are several other sets of muscles connecting the eyeball to its socket that enable eye movements of various kinds — importantly, these movements are generally coordinated for both eyes. The types of movement include:
- saccade — a fast skip or jump of the gaze from one static location (fixation point) to another, as when glancing between the faces of people you are talking to, or scanning along the words in a line of text
- smooth pursuit — a steady continuous gaze shift that follows some moving object
- vergence — movements that adjust the angle between the two eyes so that they are both pointing at the same target, rotating the angle closer together for nearby objects and further apart for more distant ones
- gaze stabilisation — eye movements that compensate for changes in head position or view content to maintain a consistent field of view. A key example is the vestibulo-ocular reflex, which rotates and shifts eye position to oppose rotations and shifts of the head, driven by positional information detected in the inner ear.
Notably, most of these movements are not under conscious control. You can choose to look at some location, but you can’t choose to point your eyes in arbitrary directions independent of one another, or rotate your eyes about their view direction, or contract your irises. You can make your gaze trace a path in a series of saccades, but for smooth pursuit you pretty much need to watch something that’s actually in motion and let your brain move your eyes automatically. You can — just about — voluntarily shift focus or vergence, but it’s much much easier when you have some actual object to focus on, or at least can clearly imagine one.
Already we can see that the brain is imposing a huge amount of structure on the processes of vision that we just take for granted, and these percolate all the way through our visual perception. Things like saccades and the vestibulo-ocular reflex define the subjective experience of seeing — chopping up and smoothing out the world like a slick Hollywood production team, not some rough Blair Witch shakycam. And this is still just at the stage of the mechanics of light capture. We haven’t even got to the point of transduction yet. To do that, we need to talk about the eye’s detector component: the retina.