1 Physics 1230: Light and Color Chuck Rogers, [email protected] Ryan Henley, Valyria McFarland, Peter Siegfried physicscourses.colorado.edu/phys1230 Lecture 22: Start cameras No Online or Written HW this week. Project update due after the break.
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1
Physics 1230: Light and Color
Chuck Rogers, [email protected] Henley, Valyria McFarland, Peter Siegfried
physicscourses.colorado.edu/phys1230
Lecture 22:
Start cameras
No Online or Written HW this week.
Project update due after the break.
2
Project Progress Reports
Will be due Friday after break
15 points towards the 65 point project total
• Turn in an outline of your project paper
• Try to include one picture or figure for your
project.
• Let us know how your observations are
going. Are there questions you have? Are
there some confusing things that you’d like
feedback on? This point in the project is a
great time to ask us if you need special
feedback.
3
Physics 1230: Light and Color
Chuck Rogers, [email protected] Henley, Valyria McFarland, Peter Siegfried
physicscourses.colorado.edu/phys1230
Lecture 22:
Start cameras
Last Time: Compound Lenses
• A modern lens can have 16 elements and can “zoom”.
• Can have reduced aberrations.
4
“stop”Reduces aberration
Image
plane
For closely spaced lenses, just add the lens diopters
(powers) to find the total power and focal length.
Aberrations
What are aberrations?
Examples:
Mis-shaped or damaged lens or mirror, Spherical aberrations: For spherical mirror - go far enough out on the circle – no single focus pointChromatic aberrations: different focal length for diff colors.
And others…
Anything that makes the image less than perfect is an aberration.
Called Spherical
Aberration
Actually, the light rays from a spherical mirror that come from the edge focus closer to the mirror
Spherical aberration
Fix by using a parabola
shape rather than sphere
OR
Don’t use the edges…
Chromatic aberration
OK, So Much Information!
Now you know loads about lenses,
how they produce images, and at least
a few non-ideal properties that cause
images to be not perfect.
All these things have impact on the important
artistic and scientific practice of photography,
through the main tool: Cameras!
Pinhole Camera
Light bulb
Image of
light bulb
blocked rays
• What is an image?
• A real image is formed on a screen when rays from each point on the object reach the corresponding points on the screen and no other rays from other points on the object reach those points
Camera with a pinhole
Camera with a lens
An image forms at the back of the camera. The distance between the lens and the back of the camera, xi, is determined by the lens equation, in other words… by xo and f.
f
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
Focus Focus
Image,
on screen
Object Focal point Focal point
Lens focusing in a camera
Cameras existed for hundreds of
years (pinhole and lens versions)
The revolution (early 19th century) was FILM:
The method to store and reproduce the
images.
• Early cameras had a glass screen (you drew the picture!)
Detecting and Saving the Image
• Film
• Digital Chip (CCD, CMOS)
• Cells in your Eye: Rods and Cones
Cameras existed for hundreds of
years (pinhole and lens versions)
The revolution (early 19th century) was FILM:
The method to store and reproduce the
images.
First, let’s understand cameras.
1515
Lenses can do amazing things: Optics in the hands of an artist
1942 Ansel Adams
1. Lens
2. Shutter
3. Diaphragm
4. Focusing screen
5. Film (or digital
imaging chip)
Camera parts
Lens
Sheet filmholder
Bellows
Shutter &DiaphragmFocus
knob
This is an old-fashioned
view camera.
Camera parts
Sheet filmholder
Bellows
Shutter &Diaphragm
Focus knob
Lens
Produces the
image that you
want to record.
Camera parts
• Point and shoot
One lens, viewfinder or
liquid crystal display (LCD)
Point and shoot digital
• Single lens reflex (SLR)Interchangeable lenses
Single-lens reflex
LCD is on the back.
Two common kinds of cameras
Different focal length lenses are
a major part of photography.
Let’s see why!
What happens when we change lens focal length?
22
50 mm normal lens
85 mm portrait lens
Back o
f cam
era
You have a camera back and can use two different focal
length lenses to produce an image on the camera back.
(A) 50mm
(B) 85mm
Which lens produces the larger image size?
Question
23
50 mm normal lens
85 mm portrait lens
Back o
f cam
era
You have a camera back and can use two different focal
length lenses to produce an image on the camera back.
(A) 50mm
(B) 85mm
Which lens produces the larger image size?
Question
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• 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
Back o
f cam
era
Focal length determines
Long lens focal lengths - vary
Point and shoot digital
Here is a camera
Here is a camera
Here is a camera
Here is a camera
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By the way: As a practical issue, when you are
taking a picture with a camera, the situation is:
1942 Ansel Adams
A) The object is farthest away from the lens
B) The focal length is farthest from the lens
C) The image is farthest away from the lens
D) Any of these could be correct depending on focus.
E) None of the above is correct
The lens equation:FXX IO
111
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For an object that is rather far away, the
image will be found:
A) A bit farther than the
B) Exactly at the
C) A bit closer than the
focal length of the lens
Point and shoot digital
D)
A) B) C)
The long focal length lens is:
Common lens focal lengths
31
(o) (i)
f
f
What will happen to brightness of image for longer f ?a. Less brightb. More brightc. Same brightness
(o)
(i)
f
f
HINT: Same amount of light captured by lens, but now spread over larger area…
Longer f
ANSWER, a): less light per area… = dimmer. How can we compensate for less light per area of film?
32
(o) (i)
f
f
(o)
(i)
f
f
How can we compensate for less light?
One way:
Make the long focal
length lenses Larger!
Common lens focal lengths
Different focal length lenses are
a major part of photography.
Great place for questions!
• The aperture is the opening of a lens’s diaphragm through which light passes.
What are apertures, f-numbers, and stops?
36
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.
f-number =focal length
diameter of opening=
f
d
f-number controls amount of light
37
1 Big opening
1.4
2
2.8
4
5.6 Factors of √2 apart
8
11
16 Small opening
more light
less light
Diaphragm has variable opening.
f-number scales
Why these
numbers?
38
1 Big opening
1.4
2
2.8
4
5.6 Factors of √2 apart
8
11
16 Small opening
more light
less light
Diaphragm has variable opening.
f-number scales
Why these
numbers?
39
1 Big opening
1.4
2
2.8
4
5.6 Factors of √2 apart
8
11
16 Small opening
more light
less light
Diaphragm has variable opening.
f-number scales – factors of 2 in light
Why these
numbers?
40
f number Amount of light
1 100%
1.4 50%
2 25% or 1/4
2.8 1/8
4 1/16
5.6 1/32
8 1/64
Amount of light
Quick summary of camera choices:
The apertureLens focal length.
Sets image size and field of view size.
Sets lens opening size, light and ‘depth of field’… More soon!
Great place for a break.
Enjoy your day!
43
F
fuzzyin focus
Rays of light diverge on either
side of the imaged point.
screen
Not perfectly
sharp, but
acceptable.
Why can images be out of focus?
44
F
Are not in focus at the same place
screen
What is depth of field?
Depth of field refers to how far apart
objects can be as still be ‘in focus’.
Narrow depth of field
Large depth of field
Large versus small aperture
You can operate some lens
either with a large aperture
or small aperture.
Which has the larger depth
of field?
A) Large aperture.
B) Small aperture.
C) They have same DoF.
Large aperture has a larger cone of light
Narrow depth of field
Large depth of field
Large versus small aperture
You can operate some lens
either with a large aperture
or small aperture.
Which has the larger depth
of field?
A) Large aperture.
B) Small aperture.
C) They have same DoF.
Large aperture has smaller depth of field
Narrow depth of field
Large depth of field
Large aperture has smaller depth of field
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 back of the camera?
50
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.
Shutters
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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
Shutter speed
Faster speeds, better able to stop (freeze)
fast motion but give less light to work with
Demo: Look at a digital camera
Demo: Look at a digital camera
• Focal length: Changed by Zoom lens – 18 to 55 mm• Object we’re photographing much farther away than
focal length
• f5.6 to f36• Exposure determined by shutter
• Can set various automatic or manual modes
• To focus: move lens slightly (move lens away from sensor to focus on closer object)
Questions
For which focal length will the metronomes appear smaller (take up a smaller portion of the image screen)?A)18 mmB) 55 mmC) the same in both cases
Which setting will give us a larger depth of field?A)F5.6B) F36C) the same in both cases
Note depth of field will also change with focal length and object distance
Questions
For which focal length will the metronomes appear smaller (take up a smaller portion of the image screen)?A)18 mmB) 55 mmC) the same in both cases
Which setting will give us a larger depth of field?A)F5.6B) F36C) the same in both cases
Note depth of field will also change with focal length and object distance
56
Different shutter speed
¼ sec. lets subjects move 1/250 sec. stops motion
Background out of focus
Equivalent exposure settings – fixed focal length
Snapshot of motionGreater depth of field
The photo was taken most likely usingA) Large f-number & fast
shutter speed.
B) Large f-number & slow
shutter speed.
C) Small f-number & fast
shutter speed.
D) Small f-number & slow
shutter speed.
E) None of above.
Question
Break for questions
Pinhole versus lens
Pinhole = selecting one set of rays. Dim but always in focus!
Many pinholes = many images
Lens gathers light from multiple rays to focus to one image
6262
Lenses can do amazing things: Optics in the hands of an artist
Ansel Adams
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