Digital Photography

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Digital Photography

• Part 1
• A crash course in optics

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Light

• Photo graphy (greek) writing with light
• Light is an electromagnetic (EM) wave. EM waves
  are periodic changes in an electromagnetic field.
  
• Characteristics of light
• speed of propagation c (speed of light)
• wavelength ?
• frequency ?
• For any wave, speed of propagation equals the
  wavelength time frequency

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The electromagnetic spectrum
1 nm 10-9 m 1 billionth of a meter 1
millionth of a millimeter
visible range
energy
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Before we go on

• Optics

Geometric optics relatively easy Wave optics complicated
deals with rays of light deals with wave equations
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Ray of light

• Ray of light/light ray ideally, an infinitely
  thin beam of light
• Propagation light ray travels in a straight line
  at speed c. The speed of light in vacuum is
  300,000 km/s 3108 m/s almost the same in
  air.
• Reflection light ray is bounced back from a
  surface
• Refraction light ray enters a different medium,
  wavelength and speed change
• Dispersion on entering a dispersive medium, the
  components of light become spatially separated

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Dispersion

• White light consists of waves of various
  wavelengths (different color). Separated
  components can be reunited with a lens, regaining
  white light.
• Some colors exist as both single-wavelength
  spectral colors and composite colors some only
  exist as composite colors.

• A dispersive prism
• refracts light (changes its direction)
• resolves light into components of different
  color.

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Shadow

• Light sources in real life are never point-like
  and objects also scatter light, so shadows are
  never really black, not even full shadows, much
  less partial shadows. Shadows can be of any
  color, depending on the color of the light, the
  object and the surface!

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Rough surfaces

• Light incident on non-reflective, matte surfaces
  is scattered in every direction thats how we
  see objects from every angle. This phenomenon is
  called diffuse reflection.
• Not only surfaces scatter light. Seemingly
  transparent media, like air, also do thats why
  mountains in the distance seem hazy.

But why is the sky blue and why are the clouds
white? Research for yourself
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Reflection

• Angle of incidence, angle of reflection are
  measured from the normal, not the surface.
• The Law of Reflection the angle of reflection
  equals the angle of incidence ß a.
• This type of reflection is called specular
  reflection.

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Plane mirror

• Image is
• upright
• virtual
• same size as object
• same distance behind mirror as object before it

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Spherical mirrors

• C center of mirror
• V vertex of mirror
• F focal point
• CV line optical axis
• CV distance radius of the sphere
• FV distance focal length half the radius

http//dev.physicslab.org/asp/applets/opticsmirror
s/default.asp
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Principal rays
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Convex mirroraka diverging mirror

• Image is
• upright
• virtual
• reduced
• smaller distance behind mirror than object before
  it

http//dev.physicslab.org/asp/applets/javaphysmath
/java/dmirr/default.asp
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Concave mirroraka converging mirror
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Refraction

• When light enters a new medium, its direction,
  wavelength and speed changes. Wavelength and
  speed are highest in vacuum.
• Def. index of refraction of a medium n c/v (v
  is speed in medium).
• Snells Law
• (also called Descartes Law, Law of Refraction)
• consequences mirage, different apparent size in
  water, etc.

optically less dense medium
optically more dense medium
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Total internal reflection

• Light moving from a dense to a less dense medium
  bends away from the normal but the angle of
  reflection can be maximum 90 degrees (light is
  refracted along the surface). If the angle of
  incidence is increased beyond that critical
  angle, light is totally reflected rather than
  entering the new medium.

light moves from a dense to a less dense medium
http//dev.physicslab.org/asp/applets/javaphysmath
/java/totintrefl/default.asp
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Plane-parallel plate

• Light rays traveling through a plane-parallel
  plate (e.g. window glass) are shifted but their
  direction remains unchanged.

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Convex lensaka converging lens
http//dev.physicslab.org/asp/applets/javaphysmath
/java/clens/default.asp
http//phet.colorado.edu/sims/geometric-optics/geo
metric-optics_en.html
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Concave lensaka diverging lens

• Image is
• upright
• virtual
• reduced
• closer to the lens than the object
• in front of the lens

http//dev.physicslab.org/asp/applets/javaphysmath
/java/dlens/default.asp
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Thin lens equation
d0 distance from object to center of lens di
distance from image to center of lens f focal
length 1/f (f measured in m) power of the lens,
measured in diopters. 1 D 1/m Magnification of
the lens I image size O object size
d0 is always positive di is positive if image is
behind the lens (real image) di is negative if
image is in front of the lens (virtual image) f
is positive for convex lens f is negative for
concave lens
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Optical illusions
The eye and the brain has the tendency not to see
whats actually there but what it thinks is there
this makes judging color, brightness and
perspective especially difficult when taking
photos
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Imaging without lenses

• Possible!
• The camera obscura (Latin dark room) or pinhole
  camera is a box with a little hole on one side.
  It creates a real, reversed image on the opposite
  side of the box. Image is less bright than with a
  lens, but depth of field is almost infinite the
  smaller the hole, the more so.
• Needs long exposure, but free of distortion.
  Large creative potential!