Deviprasad Goenka Management college of Media Studies http://www.dgmcms.org.in/ Subject: Photography Lesson 5: Visual Perception Faculty Name: Partha Pratim Samanta

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Lesson # 5
Visual Perception
Subject:
Photography
Faculty Name:
Partha Pratim
Samanta
FY BA
India’s premier Media -school
Deviprasad Goenka Management College of Media Studies (dgmcms.org.in)

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Visual Perception
 This area of study focuses on perception and the general
characteristics of the visual sensory system.

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What are the stages?
 Reception – eye senses a stimulus
 Transduction – changes it so brain can understand it
 Transmission – sends it to the visual cortex
 Selection – aspects selected of stimulus
 Organisation – grouping of elements to form a whole
 Interpretation – given meaning with the aid of psychological
factors

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Reception
 Electromagnetic energy (light) sensed from the environment
 RODS (black and white) and CONES (C for colour) are
photoreceptors that pick up the electromagnetic energy
 This energy must be CHANGED for the brain to be able to
interpret it…

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Transduction
 The electromagnetic energy is converted, changed or
‘transduced’ into electrochemical energy.
 This is the energy all neurons use.
 This means that information sensed by the rods and cones can
be sent as neural impulses to the visual cortex.

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Transmission
 Neural impulses once triggered are sent to visual cortex of the
occipital lobe
 Like any neuron, the “all or none” law applies – there is no such
thing as a big or small impulse. The stimulus must be
ENOUGH for the threshold to be reached to make the neuron
fire.
 They are sent via the optic nerve.

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Selection
 This is where discrimination and identification of the FEATURES of
the stimulus takes place.
 Feature detectors are cells that are in the retina, optic nerve and
visual cortex, that respond to patterns, lines, edges and angles
 Selection is also happening at a basic level in reception with rods
and cones responding to different electromagnetic energy wave
lengths.

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Organisation
 This can only happen once the brain has received the neural
impulses.
 Single elements are grouped to form a whole, using
perceptual principles that work like ‘rules’ of organisation.
 These principles are called Gestalt principles.

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Interpretation
 The ‘whole’ that has been created is given meaning.
 Memory is used to compare incoming information with what is
already known.
 This creates a perceptual hypothesis.
 Interpretation involves the brain using psychological factors
in order to make sense of what it is considering.

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Absolute Threshold
 The minimum amount of energy needed to detect a stimulus
under ideal conditions 50% of the time.
 VISION: A candle flame 50km away on a clear, cloudless night
with NO light pollution (moon, other lights, cities)

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Differential Threshold
 The smallest difference in the intensity of a given stimulus
that a specific sense can detect 50 % of the time.
 This is about DIFFERENCE in the intensity of a stimulus; there
must be comparison.
 If there is enough change, there will be enough stimulus for
receptor cells to fire and send a message to the brain to be
interpreted.
 VISION: a dimmer switch does not turn a light off but changes
the intensity of the light.

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Physical Structure – the Eye

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Let there be light!
 Receptor cells respond to 380 nm (violet) to 760 nm (red) of
the electromagnetic radiation spectrum.
 This is the visible light spectrum to which humans are able to
respond.
 To remember the pattern of light in order, think ROYGBIV (red,
orange, yellow, green, blue, indigo, violet).

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Eye structure and function
 CORNEA – light enters through the cornea and is bent onto the lens.
 PUPIL – light passes through the SPACE called the pupil.
 IRIS – a band of muscle that contracts/expands to manage the
amount of light entering the pupil and hitting the lens.
 LENS – the lens bends the light more and focuses them on the retina
– in particular, on the FOVEA. The lens can bulge or stretch to help
the light reach the fovea.

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Eye structure and function…
 RETINA – registers the electromagnetic energy, processes the
incoming information and transduces the energy to a form the
brain can interpret.
 BLIND SPOT – where the optic nerve attaches to the retina –
no photoreceptors.
 PHOTORECEPTORS – 2 types:
Rods – 125 Million
Cones – 6.5 Million

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Rods VS Cones
RODS
 Black and white vision
 Operate well in low level light – night
vision
 Sensitive to brightness, darkness and
movement
 Mainly located in outer part of retina
 Poor acuity (low sharpness and
focus)
 Excellent for peripheral vision rather
than direct
 19x the number of cones
CONES
 Daytime and colour vision
 Excellent visual acuity – sharp and clean
images
 Concentrated in the fovea
 Not useful at night – can’t discriminate
colours
 Outnumbered by rods 19:1

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GESTALT Principles
 Theory that the whole is greater than the sum of its parts –
Gestalt means ‘form’ or ‘shape’.
 We group individual elements of a visual stimulus into a complete
form.
 This allows us to perceive objects in the most simple way
possible.
 BOTTOM-UP = building an image from the parts into a whole
 TOP-DOWN = working to move from the whole image to the parts
it is assembled from

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What are the 4 Gestalt
Principles?
 Figure Ground
 Closure
 Similarity
 Proximity

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Figure-Ground
 We tend to separate the important aspects
of the figure from the surroundings
(background). We focus or give our
attention to the figure.
 Real or imagined contour lines separate the
figure and ground
 Camouflage depends on difficulties with
establishing contour lines
 Reversible figures change the ownership of
the contour lines

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Love and Death?

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Proximity
Proximity is the grouping of elements that are close to each other to form
an overall figure or pattern. Also known as ‘nearness’.

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Similarity
When the elements of a stimulus or pattern have similar features (size, shape,
colour) we tend to group them together. Similarity is stronger than proximity
if the two are together in a stimulus.

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Closure
What do we see?
We perceive an object as a whole, despite it being actually incomplete. We
group the individual elements to make ‘one’ by filling in the missing contour
lines so that it makes sense.

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Depth Perception
 Depth cues which are internal and involve the eye and the brain
and are known as primary cues.
 Depth cues which are external to the body and part of the
environment or pictures are known as secondary cues.

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Binocular Cues
 Binocular cues are primary cues and involve the eye and its
functioning.
 Binocular cues ALWAYS involve BOTH eyes
 Convergence
 Retinal Disparity

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What do I need to know
about convergence?
 We use this for objects up to 6m away from our eyes.
 When we view objects close to us, our eyes turn inwards or
‘converge’ so that a single image is formed on both retinas.
 Changes in muscle tension are detected and interpreted by the
brain and used to determine the depth and distance of an object.
 Our brain detects that the closer the object, the greater the
convergence.

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How does retinal disparity
work?
 Because our eyes are 6-7cm apart, the retinas receive slightly
different images. An object must be under 10 metres away for us to
use retinal disparity.
 The retinal images are combined together and compared by the
brain.
 Any disparity or difference between the 2 images provides us with
information about the depth of the object and its distance from us.
 You can check this by only using one eye, in turn, to view the same
object on your desk.

29. +Monocular Depth Cues
Accommodation:
This is a primary monocular depth cue that involves the lens of the
eye changing shape. Information about how much the lens bulges
or flattens is used by the brain to determine depth and distance of an
object.
Object close? Lens bulges to direct the image clearly onto retina.
Object distant? Lens flattens or elongates to direct the image clearly
onto retina.

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Secondary Monocular Depth
Cues
 Linear Perspective
 Interposition
 Texture Gradient
 Relative size
 Height in the visual field
Pictorial Cues are secondary
cues, as they occur in the
environment – they are NOT a
function of the eyes!

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Secondary Monocular Depth
Cues
Linear Perspective
 Parallel lines appear to
converge towards the
horizon and an imaginary’
vanishing point, creating a
sense of distance.
Interposition
 The object that is closer
obscures part of another,
more distant object.
 It is useful to indicate which
objects are closer than
others but not actual
distance.

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Secondary Monocular Depth
Cues
Texture gradient
 The amount of detail in a
scene decreases as the
distance increases or moves
away from the viewer.
 The foreground is highly
detailed and the background
is less detailed and less
focussed.
Relative Size
 If separate objects are
predicted to be the same
size, then the one that
appears larger, is closer.
 The retinal images of the
objects are different and we
can only use relative size if
we KNOW the size of an
object to be able to
compare.

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Secondary Monocular Depth
Cues
Height in the
Visual field
 NEVER just call this ‘height’!
 If land objects appear
smaller and closer to the
horizon, they appear to be
further away.
 If air objects appear smaller
and closer to the horizon,
they appear to be further
away.
 HOWEVER – land based
objects will be LOW in the
visual field if close and air
objects will be HIGH in the
visual if close.

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Henri Cartier-Bresson

35. +Perceptual Constancies
 These are learned visual perception principles that let us perceive
or make sense of stimuli.
 This means when objects change in shape, size or brightness, we
still see them as being what they are – a ‘constant’ and familiar
thing.
 Our perception stays the same even though the retinal images
may have altered; we are familiar with the objects.

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Size Constancy
 We maintain an object’s perceived size even though the size of
the retinal image alters due to the object’s distance from us.
 If we look at a truck outside our homes, we KNOW that the
truck isn’t smaller if we stand and look at it as it drives down
the street. We don’t think it ‘shrinks’ as it is driving away from
us.

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Brightness Constancy
 We maintain the perceived brightness of an object, despite actual
changes in the amount of light being received by the retina.
 Because we look at objects in their immediate environment, we
perceive that everything else has altered by the same amount of
brightness too and so know that the brightness has remained
constant.

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Shape Constancy
 We maintain an object’s perceived shape even though the image
cast on the retina changes if the object is observed from a different
angle.
 If we see a closed door, a half open door and a fully open door, we
still understand that the door has a rectangular shape, even if it
appears different when closed, half open and fully open.

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Orientation Constancy
 This is our tendency to maintain an object’s location in ‘space’ as
constant – and perceive the world as being upright and vertical.
 If we hang upside down on monkey bars, we don’t believe that the
world is no longer vertical and that the trees are upside down and
sideways! Again, we compare and use our body’s signals to help us
work out what’s going on.

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OPTICAL ILLUSIONS
OR
HOW EASY IT IS TO TRICK
HUMAN BEINGS….

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How do Optical Illusions work?
 A visual stimulus misleads our perception (or meaning-making) of
that stimulus.
 This happens because we APPLY perceptual constancies to what we
are seeing – they are our RULES.
 We make a false judgement because we misjudge length, position,
speed, direction or curvature.

43. Is this picture REALLY moving?