Chapter 4: Cameras and Photography

• Depth of Field– Circle of Confusion– Effect of aperture

• Apertures– F-stop – Area– Depth of field

• Exposure – Shutter speed– Balancing f-stop and

shutter speed

• Lens types– Telephoto– Wide-angle

Depth of Field

depth of field

size of acceptable blur

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

40 mm focallength lensat full apertureof diameter d = 10 mm

40 mm focallength lenswith apertureof diameterd = 5 mm

10 5What is the f-number (f-stop) of a 40 mm focal length lens at a full aperture, diameter d=10 mm?

A.f/2B.f/4C.f/6D.f/8E.f/10

What if we stop down the aperture to 5 mm? (same focal length)

f-number = f/d

=40mm/10mm

f-number = f/4

f-number = f/d

=40mm/5mm

f-number = f/8

f-number = f/d

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) a factor of 2b) a factor of 4c) a factor of 6d) 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

f/11

f/8

f/5.6

f/4

f/2.8

f/2

f/1.4

Smallerf-stops

• This is why f-stops have such weird looking numbers

• 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

f/32 f/5.6

large f-stop, small aperture, large depth of field

small f-stop, large aperture, short depth of field

Depth of Field Comparison

Depth of Field Comparison

• http://graphics.stanford.edu/courses/cs178/applets/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 motionbut gives darker image

Shutter Speed Comparison

1/13 sec1 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

1/8 sec

1/15 sec

1/30 sec

1/60 sec

1/125 sec

1/250 sec

1/500 sec

1/1000 sec

1/2000 sec

f/22

f/16

f/11

f/8

f/5.6

f/4

f/2.8

f/2

f/1.4

Smallerf-stopsEach step is twice the area (twice the light) of the previous one

Faster speedsEach step is twice as fast (half the light) as the previous one

Large depth of field

Fast actionShallow depth of field

Slow action

Too Dark? Shift the pairs

1/8 sec

1/15 sec

1/30 sec

1/60 sec

1/125 sec

1/250 sec

1/500 sec

1/1000 sec

1/2000 sec

f/22

f/16

f/11

f/8

f/5.6

f/4

f/2.8

f/2

f/1.4

Smallerf-stopsEach step is twice the area (twice the light) of the previous one

Faster speedsEach step is twice as fast (half the light) as the previous one

Large depth of field

Fast actionShallow depth of field

Slow 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 focusB. Your camera has an upper limit on how fast it can

open and close the shutterC. 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

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

focal length of lens

focal length of lens

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