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the eye captures

As you are reading these words, your eyes are moving all the time. This is because the human retina has only a small region of high acuity, the fovea, allowing you to see sharply and in colour over an area about the size of your thumbnail at arm's distance. To explore a scene, the eye must move over the scene.
The eye moves by rapid saccades, each taking about 0.1 seconds, from one fixation to another, each lasting about 0.4 seconds.
The image above is a simulation of the painter's view of his model at a distance of 60cm, indicating the quality of foveal (smaller circle), parafoveal (larger circle) and peripheral vision (outer area).
The eye-tracker is a biomedical research tool, used in areas as varied as dyslexia studies and jet pilot training. It measures with great accuracy where the eye fixates and for how long. Humphrey wore the eye-tracker while selecting his model and for short intermittent periods while drawing portraits.
Humphrey first chose his model out of four candidates whom he was seeing for the first time. His eye explored each face with a very rapid series of short fixations, at a rate of 140 fixations per minute, each fixation lasting an average of 0.4 seconds.

Fixations during the first 2 to 4 seconds.
S = start, P = pause.

Humphrey: I probably do tend to start with the eyes. I would say that that's no different when I'm drawing to when I am talking to somebody. When you're bicycling, the way to get the attention of a driver in a car you
make eye contact, and then they will watch out for you. It's not an art thing... it's about survival!
Humphrey selected the fourth candidate, Nick, and the next day drew his portrait. The eye-tracker was used for short sampling periods between which Humphrey worked as usual.
How to read an eye-tracker graph.
Nick's portrait: the first 2.5 minutes

Humphrey's saccades were always precisely targeted, and he fixated just one position on the model's face each time; this is quite different from the non-artists tested in similar situations.

During the initial 35 seconds, he was scanning the blank paper to define the location of Nick's eye, suggesting that a visualisation process was operating (expanded graph a).

At 55 seconds, he drew a 4.5cm line (5, 6, 7), with the pencil stopping twice while the eye glanced back at the model, suggesting that in this case the eye captured about 1.5cm of detail per fixation (expanded graph b). The drawing of the eye, seen also at two later stages, proceeded by the accumulation of such small marks.

The eye movement patterns may vary, but the rhythm remains fundamentally unchanged with about 12 fixations on the model each minute. During the five hours it took to make Nick's portrait, one whole hour was thus spent looking at the model, the eye capturing, one by one, each visual detail. Fixations were of the order of 1 second.
Eye movement patterns during the drawing of Nick's portrait. Notice the long fixations on the model at the start, the very rapid sequences as he is drawing the hair and the regular and spaced sequence as he is drawing the lips.

These long fixations are quite different from normal eye movements. When Humphrey is not drawing, his fixations are briefer and more frequent: 0.4 seconds and 140 per minute.

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