The Human Eye – i

Sitting alone in a hotel room on new year’s day towards the rear end of a long shoot can be a good thing – and bad, it’s just in the way one perceives it. Surrounded by a deluge of white, the sheets, covers, pillows, the walls, towels, and whatnots, my brain only sees the lack of colour when a clumsy bit of festive dessert and coffee finds its way to break the… icey white bed sheet. 

Isn’t the act of seeing a constant choice? Aware or not, we are constantly scanning through data removing the peripherals, to arrive at the object of our focus. In a crowd, our brain will spot a loved one from afar just by their gait, on a film set one will notice the nuances of our own department (is the right pan 1mm more than it should be?), on lunch break we’ll gravitate towards the sweets perhaps (or proteins in some cases).

Which brings us to question on how we perceive.

To all art, there is a technical and a creative side. And the human senses in their abilities are the finest, most perfect little instruments to the magic artists have created over the ages. While vision is a larger subject, a cumulation of all the life we see, hear, taste, touch, and smell, see-ing itself, at its very basic is a close dance between the brain and our beautiful ‘windows to the soul’ – the eyes. Let’s look at the technical aspects of what makes the eyes have it.

Anatomy of the Human Eye

The cornea is the outermost, transparent layer of the eyes. Its purpose is to refract the light entering the eyes onto the lens. Among its lesser acknowledged purposes is to deal with a spot of dirt brought on by an unruly bit of wind and the eye rubbing which ensues, or to allow for a clouded vision brought on by tears of joy and grief. Along with the sclera – the white part of the eye which makes for a tough cover, the cornea forms the external protective coat of the eye.

How many of these refracted light rays shall enter the lens of the eye is controlled by the iris. So whether it is your brown eyed girl, or the bonny blue eyed baby, the iris unaware of the songs written on it just controls the light levels falling on the lens inside by constricting or dilating the pupil, the black centre of the perceivable human eye. If you’re a cameraperson, pupil is the aperture and iris is well, the iris of the lens, and there’s a forever diligent AC compensating for exposure on them with iris control.

At the next step is the lens of the eye, it’s right behind the pupil, in a transparent casing, focusing the incoming light rays onto the retina – the parallel to the sensor of a camera. And like all advanced lens and camera systems, the choroid absorbs excess light and reflections post the lens before it reaches the retina.

How does the lens focus? It is held on to by ciliary muscles which allow the lens to change its thickness and curvature, and focus correctly on different distances. In cinematography, it may feel a bit counterintuitive, but a telephoto lens has lesser power than a wide-angle lens, because a telephoto lens is receiving rays from far, in a longer, narrower body, and does not need to bend the incoming light rays as much to be focussed on the sensor. But a wide-angle lens needs to focus a much wider number of rays coming in from all directions and refract them with a rather stern hand to focus them quickly onto the sensor nearby. 

It’s like filling a bucket from a robust tap. If the mouth of the tap is at a height (distance) to the bucket, a slender bucket will be enough to gather the water without any spillage because the distance lends the water to organise in a single file, just like rays of light arriving from infinity are already collimated. But if we collect the water near the source, where it is more spread out,  a much wider tub will be needed to collect the water and avoid wastage. 

Now imagine all the camera lenses we ever have, and place them in one little transparent layer behind the pupil, controlling in milliseconds the rays we want to focus on to the retina just by shifting our gaze. Like when we’re at the auditorium with some bad seats at the back looking onto the concert, our ciliary muscles are relaxed, the lens of the eye is almost flat and at its minimum focusing power. However, when we switch our look to focus on the amazing date by our side, the ciliary muscles get to speed, contract, and the lens becomes thicker with a stronger curvature, to allow for close focus.

This sheer genius of the work done by the ciliary muscles, changing the shape and curvature of our lens in milliseconds to make us see, goes by the unassuming term ‘Accommodation’. 

As the camera sensor is made of photosites which receive light energy to form an image, so are our retinas made of rods and cones. Rods provide for low light vision – scotopic vision, and cones are responsible for colour vision in brighter light levels – photopic vision. While it remains a topic of research if rods evolved before cones, we have these two to thank for all we see. 

Cones are concentrated in the fovea centralis, or the fovea, the part of the retina with the highest visual acuity. Cones form the central vision of our eyes, with a rapid fall off towards the edges, and are responsible for creating the clearest colour images we see. There are three types of cones corresponding to the three primary colours of light – red, green, and blue.It basically means these cells are sensitive to these wavelengths of light over others. 

There are no rods in the fovea but their numbers grow rapidly towards the edges. Peripheral and night vision is achieved all with rods, which require merely a single photon to get stimulated into action. There are about 6 million cones and about 120 million rods on our retina.

Each cone is connected to a single neuron, while multiple rods share a neuron. It’s like a one-on-one versus a conference video call. On a one to one call, there’s lesser data – that is fewer people speaking, but which also leads to more clarity in the conversation. On a group video call – and we’ve all suffered through a few of those – the amount of data is a lot, but the clarity of conversation is lesser. 

Similarly, a cone offers a high clarity of vision due to having a single assigned neurotransmitter to it, while a bunch of rods, though they are collecting more light information, only give us a rough idea of what a thing looks like – but having more data are responsible for motion detection.

While looking straight just take note of what the periphery of your eyes is seeing. It’s an unfocussed dull blur isn’t it? It’s the rods at work, always on standby and ready to alert us, in case a tiger suddenly lurches at us from the dark, or a rickshaw driver with neon lit interiors and a himesh reshammiya song on full blast decides to swerve in from the wrong side in front of our vehicle.

And when our brain senses an urgent or important information on the peripheries, we turn our heads and look to the side, effectively shifting from peripheral to foveal vision to get a clear picture of the situation at hand.

  • to be continued    – 

blog and memes: Monica Tiwari

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