Chapter 4: Cameras and Photography
• Depth of Field
– Circle of Confusion
– Effect of aperture
• Exposure
– Shutter speed
– Balancing f-stop and
shutter speed
• Apertures
– F-stop
– Area
– Depth of field
• Lens types
– Telephoto
– Wide-angle
Depth of Field
size of acceptable blur
depth of field
It is a measure of how far apart two objects can be and
still both be in reasonable focus on the film
Depth of Field
The “circle of confusion” is because instead of focusing to a single
point, single points on the object appear on the sensor as circles or
disks of light, which overlap, resulting in blur.
Depth of Field
Here we can see how the circle of confusion is reduced for a
smaller aperture lens. This translates to a longer depth of field.
Focus: Circle of Confusion
increasing depth, increasing blur (circle of confusion)
Controlling Light: Aperture
• We saw how changing the diameter of the lens can
affect the depth of field of a lens.
• It’s not practical to change the diameter of the lens
itself, so we change a mask in front of the lens.
• This is called the iris, diaphragm, or aperture of the
camera lens assembly
Iris and Pupil: The Human Eye
Aperture: Camera Iris
• In a camera lens, a set of overlapping “leaves” rotate
to change the diameter of an inner open space.
• This allows the camera to smoothly adjust the open
space while keeping it roughly circular.
Aperture: F-stop
• You will see this aperture listed as an f-stop or f-number
• This is defined as the focal length of the lens (f) divided
by the diameter of the lens (d)
f-number = f/d
Clicker Questions
What is the f-number (f-stop) of a
40 mm focal length lens at a full
aperture, diameter d=10 mm?
A.
B.
C.
D.
E.
f/2
f/4
f/6
f/8
f/10
f-number = f/d
What if we stop down the
aperture to 5 mm? (same focal
length)
10
40 mm focal
length lens
at full aperture
of diameter
d = 10 mm
5
40 mm focal
length lens
with aperture
of diameter
d = 5 mm
f-number = f/d
f-number = f/d
=40mm/10mm
=40mm/5mm
f-number = f/4
f-number = f/8
Proportionality
Proportionality
Practice Problems
1. The volume, V, of a
spherical balloon is
proportional to its
diameter, d, cubed:
– V d3
•
If the diameter is
doubled by how much
does the volume change?
a)
b)
c)
d)
a factor of 2
a factor of 4
a factor of 6
a factor of 8
•
Answer:
– (2d)3 = 23·d3 = 8·d3, so the
answer is a factor of 8
2. The diameter of a circle is
proportional to the square
root of its area
– d √A
•
If the area is made 4 times
larger, by what factor is the
diameter increased?
a) 2 b) √2 c) 4
• Ans: √(4A) = √4·√A = 2·√A
F-stop and Area
F-stop and Area
f/22
f/16
• This is why f-stops have such
weird looking numbers
f/11
f/8
f/5.6
Smaller
f-stops
f/4
f/2.8
f/2
f/1.4
• Each f-stop down the list (the
circles are not to scale)
– has a diameter, d, larger by factor
√2 than the previous stop
– √2 is approximately 1.4
– has an aperture area (d2) larger by
factor 2
– Lets in twice the light
F-Number and Depth of Field
• Why might we want to adjust the aperture of our
lens?
• There are cases when it is just too bright, and you
have to reduce the light coming into the camera
• There are also artistic reasons for adjusting the
f-number.
Depth of Field Comparison
large f-stop,
small aperture,
large depth of field
small f-stop,
large aperture,
short depth of field
f/32
f/5.6
Depth of Field Comparison
Depth of Field Comparison
• http://graphics.stanford.edu/courses/cs178/appl
ets/dof.html
Controlling Light: Shutter Speed
• There is a second way that we can control the
amount of light that hits the film or CCD sensor
• All cameras have a shutter that allows light to hit
the film or sensor for a controlled amount of time
• This amount of time is called the “shutter speed” or
“exposure time”
• Just like f-stop, there are both practical and
aesthetic reasons for adjusting the shutter speed.
Shutter Speed
• 1/15 sec
• 1/30 sec
• 1/60 sec
• 1/125 sec
• 1/250 sec
• 1/500 sec
• 1/1000 sec
• 1/2000 sec
Faster speeds, better able
to stop (freeze) fast motion
but gives darker image
Shutter Speed Comparison
1 sec
1/13 sec
Fast Shutter Speed
1/800 sec
Intermediate Shutter Speed
Long Exposure
Shutter Speed and F-stop
• You can adjust the light hitting the film in two
ways, the shutter speed and the f-stop.
• Choosing values for each is a matter of both
practical concerns and aesthetics.
• There will be combinations that are just not
possible in certain lighting conditions.
For example, you can’t take a large depth of field (requires
small aperture) of a fast moving object (requires fast shutter
speed) in low light conditions.
Speed and F-stop Combinations
Large depth of field
Smaller
f-stops
Each step is twice
the area (twice
the light) of the
previous one
Shallow depth of field
f/22
1/8 sec
f/16
1/15 sec
f/11
1/30 sec
f/8
1/60 sec
f/5.6
1/125 sec
f/4
1/250 sec
f/2.8
1/500 sec
f/2
1/1000 sec
f/1.4
1/2000 sec
Slow action
Faster
speeds
Each step is twice
as fast (half the
light) as the
previous one
Fast action
Too Dark? Shift the pairs
Large depth of field
Smaller
f-stops
Each step is twice
the area (twice
the light) of the
previous one
Shallow depth of field
Slow action
f/22
f/16
1/8 sec
f/11
1/15 sec
f/8
1/30 sec
f/5.6
1/60 sec
f/4
1/125 sec
f/2.8
1/250 sec
f/2
1/500 sec
f/1.4
1/1000 sec
1/2000 sec
Faster
speeds
Each step is twice
as fast (half the
light) as the
previous one
Fast action
Clicker Question
• What might limit your ability to take a short
depth of field (small f-stop, larger aperture)
photograph on a bright sunny day?
A. Lens can’t focus
B. Your camera has an upper limit on how fast it
can open and close the shutter
C. The object is moving too fast
Lenses
focus ring
aperture
adjustment
Indicates
approximate depth
of field for range of
f-stops
Zoom Lens: 80-210 mm Focal Length
These sets of lenses move with respect to one another to both change the focal
length and to focus the object onto the film or sensor
Telephoto Lenses
• Telephoto lenses are used to take pictures of
distant objects, and make a big image of a small
portion of the scene.
• We know that distant objects are imaged
approximately at the focal point of the lens.
• What does this mean about the focal length of
telephoto lenses?
Telephoto Lenses
focal length
of lens
focal length
of lens
Because the image will focus at the focal point of the lens, the
longer the focal length, the larger the image. In general, telephoto
lenses have a long focal length
Telephoto Lenses
• One way to remember that telephoto lenses have a
long focal length is to think of the enormous lenses
used by the paparazzi!
• Because a telephoto lens enlarges a small portion of
a large scene, it has a very small “angle of view”
Canon EF500mm f/4.5L
This is a fixed focal length (500mm) telephoto lens. Notice how much simpler
it is without an adjustable zoom! Focusing is easier because with a telephoto
you can assume that the objects are at infinity.
Wide-Angle Lens
• The opposite effect is achieved by a lens with a very
short focal length, called a “wide-angle lens”
• These are often called “fish-eye” lenses
Nikon 14-24mm Wide-Angle Lens
This lens has optics both to focus and to change the focal length.
Canon EF14mm f/2.8L
This is a fixed focal length (14mm) ultra wide-angle lens. This is more complex
than the telephoto because it has to have focusing optics, because you might
want to take a wide-angle photo of things at different distances.
Lens Comparison
Lens Comparison