Physics 1230: Light and Color
Ivan I. Smalyukh, Instructor
Office: Gamow Tower, F-521
Email:
ivan.smalyukh@colorado.edu
Phone: 303-492-7277
Lectures:
Tuesdays & Thursdays,
3:30 PM - 4:45 PM
Office hours:
Mondays & Fridays,
3:30 PM – 4:30 PM
TA: Jhih-An Yang
jhihan.yang@colorado.edu
Class # 20
Last time:
The retinal cells have some interesting collective behavior:
1. We detect relative, not total lightness
a)
b)
Lightness constancy (correct for total amount of light)
Weber’s Law (perception of lightness depends on ratio
between two shades)
2. When we stimulate one cell, it turns down response of
neighbors
a)
b)
Lateral inhibition
We detect edges, not the entire field of view
3. We are sensitive to a moving world
a)
b)
c)
Eye jumps around (“saccades”) to move images across retina
Negative afterimages caused by desensitization of retina cells
as they keep being stimulated
Images disappear if they are in your view constantly
Why do we have a blind spot?
A. The fovea isn’t very large
B. We don’t see things where the fovea blocks our
view
C. We don’t see things where the pupil blocks our
view
D. There are no rods or cones where the optic nerve is
E. Really not sure
Why do we have a blind spot?
A. The fovea isn’t very large
B. We don’t see things where the fovea blocks our
view
C. We don’t see things where the pupil blocks our
view
D. There are no rods or cones where the optic nerve is
E. Really not sure
So, the signal from the retina goes to the BRAIN
How do we process images?
(not just gather the light)
B.
A.
C. Both
So, our eyes have certain mechanisms for making sense of what we
see (lateral inhibition, lightness constancy…)
So does our brain. Today we’ll look at some of brain processes.
A Question
You have two focusing lenses, each with a focal length
of F. You put them close together to make them
behave as a single lens. The new ‘doublet lens’ has a
focal length of:
A)
B)
C)
D)
2*F because the diopters add.
F/2 because the diopters add.
Still F for this special case.
Something else happens.
Is there an experiment you can
try?
6
Are the small gray patches below identical?
A) YES B) NO
Craik O’Brien Illusion
Contrast at the edge affects your
perception of center = simultaneous
lightness contrast
See fig 7.7
7
Craik O’Brien Illusion
Simultaneous lightness contrast
These are the patches without the surround.
8
Clicker question
A white sheet of paper continues to look white
as the light level is reduced. We call this effect:
A. Simultaneous lightness contrast
B. Lateral inhibition
C. Weber’s law
D. Lightness constancy
E. Edge enhancement
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Clicker question
The bands of gray look lighter on
their right side because of:
A.Simultaneous lightness
contrast
B. Lightness constancy
C. Weber’s law
D. Lateral inhibition
E. Both A and D
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1
3
2
The red areas show the
receptive field.
Lateral inhibition is greater at 1
than at 2.
The fovea has a smaller
receptive field. So the lateral
inhibition is the same
everywhere in the white area.
Hermann grid illusion: dark areas are from lateral inhibition
13
Now, the brain….
Brain damage on the left side
hurts vision on the right side.
14
See text fig. 7.3
Brain anatomy
Optic chiasma
Where optic nerves cross
Left field of view goes to right brain
Right field of view goes to left brain
from both eyes
Visual cortex is where you “see”
Brain damage at this location hurts vision.
15
Clicker question
If the left side of your brain is injured, you might
lose vision in your
A. left eye
B. right eye
C. left field of view
D. right field of view
E. some loss in left and right field of view
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Example: Which is the darker patch, A or B?
A
B
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Previous experience effect:
Here, the eye is “fooled” into thinking the light is from
above.
A
B
The panel “A” has lots of light, so it must be really
dark.
But “B” must be lighter because it is in the “shade.”
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Parallax / Binocular disparity
• Human (predator) eyes are in front of our
heads: 130 degrees of overlap. Good depth
perception! But field of view just 208 degrees.
• Dog: overlap is 100 degrees.
• Rabbit (prey) field of view = 360 degrees!
Only 24 degrees of overlap.
Dominant eye
A bundle of parallel rays enter the
eye's pupil. No other rays enter. What
does the eye see?
Eye
A) A single pt of light, surrounded by
black.
B) A uniformly illuminated wall of
light, like a white wall.
C) Scattered points of light, like stars in
the sky.
D) None of these.
A bundle of parallel rays enter the
eye's pupil. No other rays enter. What
does the eye see?
Eye
A) A single pt of light, surrounded by
black.
B) A uniformly illuminated wall of
light, like a white wall.
C) Scattered points of light, like stars in
the sky.
D) None of these.
Robert Hooke’s two-lens microscope
A magnifying glass (the eyepiece) magnifies the first image further.
An image of an image!
lens 2 image
lens 1 image
lens 2
eyepiece
object
lens 1
Nosepiece
or Objective
The first lens, the nosepiece, is
used as a projection lens.
22
Lens Power (or diopters)
Lens power: D = 1/F
Units of D are 1/meters, also called diopters
Eyeglass lenses are measured in diopters.
This is handy because the diopters of lenses add:
D (total) = D1 + D2
I. Camera parts
1.
2.
3.
4.
5.
Lens – focuses light
Shutter – times the exposure
Diaphragm – controls amount of light
Focusing screen – image created here
Film (or digital imaging chip) – saves the
image
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24
From last time
If you remove the lens, the image on the screen...
A) remains the same B) gets a bit dimmer
C) becomes fuzzier. D) becomes upright
E) disappears
E. We need the lens to get an image.
This is a simple camera.
This is why we care about real images!
How can a virtual photon develop film? (It can’t)
Digital Image Storage: The CCD
CCD: “Charge Coupled Device”.
Electronic device for
storing images
In Film, light moves electrons from one atom
to another to do CHEMISTRY.
In a CCD, light moves electrons from one
atom to another, traps the charge, and
moves it to an amplifier to MEASURE IT.
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Focal point
Focal point
If you move the screen towards the
lens, the image:
A) remains the same B) gets a bit
dimmer
C) becomes fuzzier. D) becomes
upright
E) disappears
Focal point
Focal point
Imagine the image is unfocussed. What do we need to do?
Move the screen forward or back.
This is how a camera focuses
It adjusts the distance between lens and sensor
Photography principles
1. Lens focuses by moving closer/further from
film
What will happen if the focal point is moved further
away from the lens?
If you’re mathy, use this:
1
1
1


XO X I F
A: Image moves further away. Image is always outside focal point
Lens focal length in cameras
You have a camera and can use two different focal length
lenses to produce an image on the film. (The film has to be
closer to the short focal length lens to make an image.)
Which lens makes a bigger image?
50 mm normal lens
(A) 50mm
85 mm portrait lens
(B) 85mm
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Focal length of lens determines magnification
and field of view
• Short focal length = wide angle (more things in the image)
• Long focal length = “telephoto” (larger, but fewer things)
50 mm normal lens
85 mm portrait lens
Which is best for taking a picture of a bird
in a tree?
A. focal length 200 mm
B. focal length 50 mm
Longer focal length, larger image of a smaller field of view. Telephoto
lens.
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Photography principles
1. Lens focuses by moving closer/further from
film
2. Long focal length = larger image (telephoto)
What is depth of field?
Depth of field is how far objects can be apart
and still be nearly in focus.
F
Are not in
focus at the
same place,
but film is in
a fixed spot!
36
Large lenses have smaller depth of field than small lenses
F
in focus
fuzzy
big blur
small blur
More like a pinhole camera
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Large lenses have smaller depth of field than small lenses
That goes for smaller apperature too
Narrow depth of field
Large depth of field
Photography principles
1. Lens focuses by moving closer/further from
film
2. Long focal length = larger image (telephoto)
3. Bigger lenses / aperature = narrow depth of
field /focus
Camera parts
We are
here
1.
2.
3.
4.
5.
Lens, focal length
Shutter (on the lens)
Diaphragm (on the lens)
Focusing screen or LCD
Film (or CCD chip)
Controlling the light that enters.
40
The Shutter: Opens the camera lens for a
controlled amount of time
You can open the shutter on your camera for 1
sec. or for ½ sec. Which one lets more light
energy hit the film?
A) 1 sec. because the light power is on longer
B) ½ sec because the light power is on less
C) The same, because the power hitting the lens is the
same in both cases.
D) None of these.
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Shutter speed
The shutter controls the duration of the light.
Common shutter speeds
1/15 sec
1/30 sec
1/60 sec
1/125 sec
1/250 sec
1/500 sec
Each change lets in half the light.
FACTORS OF 2 in light
energy entering the
camera
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Photography principles
1. Lens focuses by moving closer/further from
film
2. Long focal length = larger image (telephoto)
3. Bigger lenses / aperature = narrow depth of
field /focus
4. Faster shutter speed = less light
The lens itself also controls the amount of light:
F-Number
focal length
F
f

diameter of opening d
Small F-number = Bright image
Example: 200 mm focal length / 25 mm opening = f-number 8
If you have a long focal length and want a small f-number, for bright
image, do you need large or small diameter lens?
Big. That’s why telephoto lenses are so big.
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The lens itself also controls the amount of light:
F-Number
focal length
F
f

diameter of opening d
Small F-number = Bright image
Example: 200 mm focal length / 25 mm opening = f-number 8
Big diameter lens = low f-number, like f-number 1.4
Heavy. Cost more! Works in low light and indoors.
Small diameter lens = big f-number, like f-number 3.5.
For outdoors, weigh less.
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Change the diaphragm size = change “d” so
change F-number
1
Big opening
1.4
f
focal length
F

diameter of opening d
2
2.8
Diaphragm has variable opening.
4
5.6
Factors of √2 apart
8
11
16
Small opening
Amount of light scales with area
Area scales with (diameter)2
F-numbers go up in factors of 2
46
Which gives more light?
A. f-number 4
B. f-number 22
f= F/D so SMALLER f-number is a larger D and therefore, more light.
47
Photography principles
1. Lens focuses by moving closer/further from
film
2. Long focal length = larger image (telephoto)
3. Bigger lenses / aperature = narrow depth of
field /focus
4. Faster shutter speed = less light
5. F-number = focal length / diameter. Small Fnumber = bright image. Large F-number =
dim image. Change F-number with lens
and/or aperature.
Photography principles
1.
2.
3.
4.
5.
Lens focuses by moving closer/further from film
Long focal length = larger image (telephoto)
Bigger lenses / aperature = narrow depth of field /focus
Faster shutter speed = less light
F-number = focal length / diameter. Small F-number = bright
image. Large F-number = dim image. Change F-number with lens
and/or aperature.
If you have a big F-number (dim image) but want a bright
image, what do you need to do to the aperature?
A. Open it more
B. Close it a bit
What does that do to the depth of field?
A. Increases it
B. Decreases it
You can get a lot of light in two ways:
•Low F-number (large aperature)
•Long shutter speed
This determines film’s EXPOSURE
But why not just use a large aperature and long
shutter all the time?
Equivalent exposure settings (same light)
¼ sec. lets subjects move
1/
Background in focus
(good depth of field)
Background out of focus
250
sec. stops motion
51
Which gives more light?
A. shutter speed 1/4 sec
B. shutter speed 1/16 sec
Longer time open is more light.
52
Which f-number gives half the light of f-4?
A.
B.
C.
D.
E.
f-2
f-2.8
f-4
f-5.6
f-8
f = F/D, so you need a LARGER f-number for less light. 5.6 is sqrt(2)
smaller than 4, so twice the light.
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Which pair gives the same amount of light?
1/30 sec. and f number 4 and
a)
b)
c)
1/120 sec. and f-1
1/120 sec. and f-2
1/120 sec. and f-4
4x more light for ¼ the time.
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