The Human Eye – ii

Film people are caravanners. Somewhere between the incessant call sheets, grueling schedules, and turnovers, we find enough time to unknowingly absorb the minute habits of our fellow crewmates. Details such as who likes sugar in their tea, who’s kid attends the same school as someone else’s, who are the worshippers of the circadian rhythm and who are the night owls, who will appreciate the latest poor joke birthed by a sleep deprived high, all become so relevant as we spend the better parts of each day attempting to make something good together.

And then one day, almost abruptly, these particulars become superfluous with the three golden words “It’s a wrap!”.

We return home, try to settle in, and suddenly it’s time to get onboard a new journey with a fresh set of people we’ll call home.

We relive these encounters in nooks of our hearts, in a little smile on some half-remembered on-set joke. Some friendships persist, few others stagger for a bit aided by the frail bonds of social media, some are forgotten – names and all, some suddenly convert into work a few years later over an impromptu phone call.

Films demand a suspension of disbelief, equally from the audience and its makers. And we oblige, by riding the wave of rigorous work and emotions demanded by this consuming illusion. Project to project, what remains is the knowledge gained, and a lingering thirst for newer experiences.

Less dramatically, but equally fascinating is the way things linger in the physical human eye a moment longer, precisely 1/16th of a second longer, after the object we are viewing is removed. Persistence of vision, or, retinal persistence is a key phenomenon which makes motion pictures possible. A lowest number of still frames per second is deployed – it should be above the 10-12 frames we can actually spot as separate stills – to create the illusion of motion, a phenomenon known as stroboscopic vision. The magic of seeing the world move on a 2D screen is all thanks to the series of quick moving static frames which smoothen out the intermittent staccato.

I remember drawing on the lower edge of used notebooks in school and flipping through the pages real fast to deliver quite a show to my little sister, and to an unimpressed elder brother who chose to persist with a “been there done that” attitude on the matter of my DIY flipbook.

However, motion perception is not entirely due to this single principle. There are multiple factors which come into play for the brain to inform us that something is moving. To understand more on motion perception let’s look more deeply into depth perception (pun 1/1).

We perceive depth via a combination of these two visions::

Monocular – depth as perceived by inputs from a single eye

Binocular – depth perception aided by inputs from both eyes

For seeing things at long distances, the monocular vision takes over since our eyes are receiving collimated rays which are not going to vary in information with the small gap in between our eyes. 

Monocular depth relies on:

  1. Relative size: Objects appear smaller if they are further away from us. Seems so common-sensical and mundane, but if we really notice, so perfectly used by the brain to inform us on motion. When we are crossing the road and notice a truck which should actually be the size of a regular Mumbai 1bhk, but is currently the dimensions of a lego toy, we instinctively feel free to loaf on the road while crossing it. In addition to this, bigger objects appear to move slower than smaller objects. Common occurrences such as a bigger aeroplane seemingly moving slower than a private jet viewed at the same distance is an example of this.
  1. Texture: Closer objects will reveal more textures than farther ones. More texture is the brain’s cue for an object being closer to us. In harsh sunlight a mountain might appear closer to us than on a hazy morning. And how many times do we fill a background with lobaan for some ‘atmosphere and depth’. So, the grass may not be greener on the other side, but it sure is always smoother. And so are the pores and pocks of a loved one’s face held close immeasurably dearer than the porcelain of an unknown’s skin.
  1. Interposition/Occlusion: If something in the depth is blocking (occluding) something else from our vision, we automatically understand that the item blocking is nearer to us than the one being blocked. Look at the artwork titled ‘The Killing’ by the genius artist Sudhir Patwardhan. The work doesn’t rely on linear perspective, but we get all the depth cues needed to know who’s nearer and who’s away, simply from occlusion. 
  1. Linear Perspective: “Parallel lines converge at infinity” – while this is just a polite way to say they never converge, in effect, it is a huge cue for the brain’s depth perception. Things which are really far from us “seem” to be reaching us from a common vanishing point, and in addition to the other cues above, inform the brain on the proximity or distance of the object. 
  1. Planar height: Far away objects will tend to appear higher on our visual field, this comes from the simple fact that to look really far, at the horizon, the stars, more distance is needed. Closer objects will mostly be lower in the visual plane unless vehemently blocking our further view – which is so uncomfortable as a viewing experience.
  2. Light and shadow: Objects reveal their 3D shapes through shadows, the occluded light forms the shape of the object which informs us of its depth. Also, we are conventionally at home with shadows forming from a light coming from above, rather than below. Light along with her shadows informs all vision.

Binocular Depth:

Our pair of eyes allow us to have stereoscopic vision. This is a key aspect in depth and motion perception. The retina, like a movie theatre screen, is flat (actually it is slightly curved like the screen in a dome theater). What lends us to perceive the world as 3D, is the slight displacement and change in angle of the things being viewed owing to the actual physical distance in the placement of our eyes. 

The closer our viewing object is to us, the greater the displacement between objects in the images formed by our eyes individually, and this is the very information used by the brain to inform us of proximity. If our eyes are converging a lot to focus on an object, our brain concludes that the object must be near, and if our eyes are not converging, staring at a distance, our brain understands that we are focused at a far off view. One can, for instance, rest assured that these beautiful leading ladies are testing the limits of binocular depth perception in the following montages.

Stroboscopic vision, persistence of vision, beta movement, phi phenomenon are some dedicated motion informers, along with all faculties involved in depth perception.

I don’t follow football, but one thing which fascinates me is the ‘Wave formed by the cheering crowds’. To make the wave, the spectators in the stadium get up, throw up their arms in cheer, and sit down again, a few columns at a time, and the effect is quite literally a human wave in motion. This is an ace example of Beta Movement, β “Described as an apparent movement of a figure (object), whose physical properties (shape, size, color) are identical to the properties of the objectively stationary targets.”

Another such illusion is Phi Movement, ϕ. The examples run similarly, except that the phi phenomenon is noticeable at much higher frequencies. 

Notice the animation below, the slower one is an example of Beta movement. Our brains are able to process each white circle switching on and off, giving the illusion of a round motion around the entire circle against the blue background.


Now notice this faster animation. The frequency of switching on and off of the circles is so quick, that they actually appear stationary, but what we do perceive is a motion going through them, around in a circle. In technical terms, “Phi phenomenon It is a pure (objectless) movement in the sense that there is nothing in the stimulus that specifies the shape of the occluding object. The ϕ-movement, itself, ‘accounts for’ the flicker of the lights. The contour-less, pure movement, called ϕ, always has the color of the background.

 Simply put, with “phi phenomenon, our brains are filling a gap where there isn’t really an image, and in beta movement, the brain sees a series of images as one image changing location.” 

Always remember, that our brain is sitting in a dark box, with no actual idea of the world outside, and only drawing meaningful conclusions from the information fed to it. It’s like an editor who never came on set, and now has to make sense of the footage and relay it to the audience as a meaningful film. The editor wouldn’t (and shouldn’t) care how challenging it was to get the few shots she’s now blamelessly discarding… the shot in which the focus puller gave a glorious single take at full open aperture on a 300mm lens (more on the the heartlessness of us DoPs later), that other one in which the secondary artist inconsolably cried in the 20th take till his eyes ran dry, for a scene which didn’t make it to the final cut, how the spot dada awkwardly juggled four hot cups of tea till ‘cut’ was called for, for a shot lost at the proxy stage.

Much like a great editor, unmoved by the challenges of receiving footage – or neurosignals – the brain constantly skims through the vastness of information relentlessly received by it to arrive at a comprehendible story for us, a framework to understand the world by. Aren’t we then, each of us, the greatest storytellers of our own lives!

Sergei Eisenstein inspired editor brain, doodle by: Vinita Katdare

I will conclude this post with a mention of the ‘Blind Spot’, a pretty interesting experience which occurs purely due to the anatomical structure of the human eye, but rather a dry one to write on. Technically, ‘Blind Spot is the region where the optic nerve passes through the optic disk and out of the eyes. Also, it is at this very region that the blood vessels enter the eyes. It lacks photoreceptor cells (rods and cones) in the retina so the light falling at this spot does not form any image.’ But today, going through a favourite book of poems by Gulzar sahab, the master percipient of most everything, i figured it’s much better to let him take over on the matter, especially with the last two lines: 

From ‘Selected Poems’, Gulzar

by: Monica Tiwari

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