Back from Break and Back to Optics
Phys 1020, Day 21:
Questions?
Cameras, Blmfld 15.1
Digital Cameras, Optical systems 15.2
Last lab this week
Coming Up:
Optical communication
What will happen to image if we increase focal length of lens?
a. Image is same size, same place
b. Image is same size and further from lens
c. Image is bigger and further from lens
d. Image is smaller and closer to lens
f
f
(o)
(i)
Lens
equation:
f
f
1 1 1
 
f o i
(o)
(i)
2
What will happen to the brightness of the image?
a. Less bright
b. More bright
c. Same brightness
f
f
(o)
(i)
Lens
equation:
f
f
1 1 1
 
f o i
(o)
(i)
3
How can we compensate for less light?
1. More light sensitive film
2. Larger lens / Open Iris all the way
3. Longer exposure time
4. Use Flash
f
f
(o)
(i)
Lens
equation:
f
f
1 1 1
 
f o i
(o)
(i)
4
Let’s do an example
If you use a 35-mm focal length lens to take a photograph of
flowers 2 m from the lens, how far from that lens does the real
image of the flowers form?
1. Understand the problem and pick a model (physics)
This is obviously an optics problem involving lenses and images –
We will use ray optics to solve; specifically the lens equation
Let’s do an example
If you use a 35-mm focal length lens to take a photograph of flowers 2 m from the
lens, how far from that lens does the real image of the flowers form?
2. Draw a picture
Object
Lens
With focal length f
Object
Distance
Film
Image
Distance
6
Let’s do an example
If you use a 35-mm focal length lens to take a photograph of flowers 2 m from the
lens, how far from that lens does the real image of the flowers form?
3. Draw a ray diagram
Object
Film
Object
Distance
Image
Distance
7
Let’s do an example
If you use a 35-mm focal length lens to take a photograph of
flowers 2 m from the lens, how far from that lens does the real
image of the flowers form?
4. Solve the equation:
1
.035𝑚
1
𝑓
1
𝑜
1
𝑖
1
2𝑚
1
𝑖
= +
=
+
Two objects different distances from lens
1
f
1
f
2
2
Screen at
Focus of 2.
What will happen at screen if close down iris in front of lens?
a. Images brighter and object 1 more in focus
b. Images dimmer and object 1 more in focus
c. Images brighter and object 1 more out of focus
d. Images dimmer and object 1 more out of focus
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F-number
f number  f / d
d
f
fnumber Large:
Focal length large (light spread over larger area)
Small diameter lens (less light captured)
Dimmer image.
fnumber Small:
Focal length small (light spread over smaller area)
Larger diameter lens (more light captured)
Brighter image.
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Depth of Focus:
Sharp focus!
Rays diverge
quickly!
f number  f / d
Large diameter lens
Small f-number
Bright image
Small depth of focus
Small diameter lens
large f-number
Dim image
Large depth of focus
pretty sharp focus for
a large range of object
distances
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(squinting).
Next: Digital Cameras
You have already seen same physics principles
in other applications:
Lens/Optics
Color/Light
Resolution/Pixels
Semiconductors, P-N junctions, capacitors
Digital vs Film Cameras
Mechanical Electrical Shutter
Mechanical Iris
Dark Box
Object
Lens
With focal length f
Film
Electronic
Light Sensor
(semiconductor
device)
Battery Required!
Object
Distance
Image
Distance
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Film vs Electronic Sensor
36 mm
FILM
24 mm
6.6 mm
Electronic Light
Detector (MUCH SMALLER)
4.4 mm
Object
Object
Distance
How should the lens system be set up in the
digital camera so that the view captured is
the same as the 35 mm camera when I am
about the same distance away from the
subject:
a. the lens and lens-detector distance must be
identical to that of the 35mm
b. the lens curved more and the lens-detector
distance shorter
c. the lens curved more and the lens-detector
distance the same
Image
Distance d. the lens should curved more and the lens14
detector distance longer
The Electronic Detector
Semiconductor Device (converts light to electrical charge)
Most digital cameras use: charge-coupled device (CCD
Array)
6.6 mm
(1600 pixels)
4.4 mm
(1200 pixels)
Lots of tiny light detectors (each 1 pixel)
The more light photons that hit, the more electrical charge builds up on the
pixel
Cannot discriminate between colors… all colors treated equally
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To capture information about color: put colored filters
in front of each pixel
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To capture information about color: put colored filters
in front of each pixel
Question:
If the camera is taking a picture of an
orange, light will be detected by:
a. Blue pixels
b. Red pixels
c. Green pixels
d. Both b and c
e. Both b and a
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Interpreting Color
Computer determines true color at center
pixel by looking at amount of light collected
at surrounding pixels … that way you have
info on red, green and blue.
Resolution
36 mm (~3000 pixels)
24 mm
~2000 pixels
FILM
6.6 mm
(1600 pixels)
4.4 mm
(1200 pixels)
Traditional film (~200 iso) has effective
resolution of ~3000 x 2000 pixels
….Captures more detail than most CCD
arrays.
Question: To improve the amount of detail
captured by the digital camera, you need to:
a. Increase number of pixels on the CCD array
b. Increase the physical size of the CCD array,
keeping the total number of pixels the same
c. Both a and b.
2.1 million pixels
2.1 Megapixels
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Optical Zoom vs Ditigal Zoom
Optical Zoom: Real
Image is larger on
array
Digital Zoom: Real
Image is same size on
array, uses subset of pixels and then
guess at detail…. NO EXTRA
INFORMATION.
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Paul Revere's Ride
Henry Wadsworth Longfellow
Listen my children and you shall hear
Of the midnight ride of Paul Revere,
On the eighteenth of April, in Seventy-five;
Hardly a man is now alive
Who remembers that famous day and year.
He said to his friend, "If the British march
By land or sea from the town to-night,
Hang a lantern aloft in the belfry arch
Of the North Church tower as a signal light,-One if by land, and two if by sea;
And I on the opposite shore will be,
Ready to ride and spread the alarm
Through every Middlesex village and farm,
For the country folk to be up and to arm."
next verses talk about moon, night wind, … i. e. noise that could obscure signals
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alternatives? (shows benefits of light used today)1) “yell really loud at me”- Does rhyme with “sea”. but sound not carry as far as light.
2) “Row over and tell me”
- still rhymes, but slow. Light much faster.
Uses another trick of modern optical communication- digital information.
Flash a light. Just seeing it tells Paul his friend is there to send signal.
All information contained in second pulse of light.
If on = by sea, if off, no light= by land. “binary digital signal”two choices (on/off) . One bit of information. Information “digital”.
Big Advantage-Very distinct and unambiguous.
on or off -- can distinguish even if fog, moonlight, etc.
Now days- information coded as series of on and offs that give numbers.
Signal is insulated from presence of noise.
(same as hiss on tape recorder, etc.)
We Shall return to this idea and how it happens . . .
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Why use light ?
1. light- fast. Transmit signals rapidly.
2. Can go long way without being absorbed.
(under special conditions!)
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Principles behind design of optical communication system
Want to send telephone signals from Boulder to Ft. Collins.
Put sound information into light pulses.
(lots of lantern flashes, a al Paul).
What are some problems with sending this way?
(Write down 3 per group.)
Then be prepared to offer ways to get around the problems.
1. need lots of light flashes in a hurry. Need good way to turn light on and off very fast and
detect pulses.
2. Light spreads out all over the place, not much gets to Ft. Collins. Also, clouds and dust
block it.
3. Want to send many conversations, no way to separate one light from another .
4. not be able to see light from location.
5. ….
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Solutions:
1. confine light to pipe, send it right where want, keeps
clouds, dust etc. out, keeps light from spreading out (laser).
Send light pulses down thin glass optical fibers.
(“light pipes”) Light does not spread out, can be directed
to exactly right place, can have multiple fibers to separate
conversations, no clouds, etc.
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