Visual Communication in Computer Graphics and Art http:www'eagereyes'orgVisComm

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Visual Communicationin Computer Graphicsand Art

• Photography IV - Aperture, Exposure

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Thin Lens vs. Thick Lens
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Depth of Field (DOF)
http//www.photocourse.com/
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Depth of Field

• Perceptual Effect
• Separates image into foreground (sharp) and
  background (blurry)
• Guides attention to sharp parts
• Eye also has DOF, but ignores blurry parts
• Influenced mainly by aperture
• Range of DOF smaller when focused closely, and
  larger when farther away
• Important for macro photography

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Aperture
D
f

• Numeric aperture k (the denominator in 15.6,
  etc.)
• Effective lens diameter D
• Focal length f

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Aperture

• Part of the lens
• Shape of aperture
• Ideally round
• Really a (more or less) regular polygon
• Important for appearance of blurred regions
• Aperture blades
• Stopping down means closing aperture

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Numeric Aperture

• Number that gives the ratio between effective
  lens diameter and focal length
• Does not describe the ratio of incoming/outgoing
  light!
• Each step means half/double the area, so aperture
  numbers change with v2 1.411, 11.4, 12,
  12.8, 14, 15.6, 18, 111, ...

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Maximum Aperture

• Maximum aperture important property of a lens,
  given with focal length
• E.g., 50/1.4, 85/2, 300/4, 18-70/4.5-5.6
• Maximum/open aperture used for shallow depth of
  field and in low light
• Focusing easier with faster lens (lower maximum
  aperture number)
• Requires larger front lens, more weight

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Circle of Confusion
C
D
v
Solid line sharp imageDotted Line blurred
image
v
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Shallow Depth of Field
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Deep Depth of Field
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What is Sharpness?

• Every point is depicted as small disc
• Image appears sharp if circle of confusion small
  enough to be seen as point
• For 35mm and typical enlargements, a diameter of
  less than 0.03mm gives a sharp image

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Circle of Confusion

• In practice mostly a polygon
• Aperture blades rounded
• Diffraction occurs at corners where aperture
  blades meet
• Occurs at slit that is small compared to
  wavelength
• Narrow corner acts like slit
• Aperture designed so blades meet at obtuse angle

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Diffraction at Aperture
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Bokeh

• The quality of blurred regions of the image
• Japanese Word
• Relatively subtle in most images
• Criterion for lens quality

http//wvs.topleftpixel.com/
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Sieve Aperture
Heinrich Tauscher
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Mirror Lens

• Aperture has doughnutshape

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Mirror Lens Bokeh
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Exposure

• Amount of light to reach medium during exposure
  determined by
• Aperture/light flow
• Exposure time
• Exposure times are close to powers of two 1s,
  1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250,
  1/500, 1/1000, 1/2000,

Aperture/ Light Flow
Exposure
Time
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What is correct exposure?

• Generally 18 overall grey
• Zone System (Ansel Adams)
• Find brightest and darkest point
• Expose for point in the middle
• Control contrast to use whole range of
  contrast/grey levelswith exposure and
  development
• High contrast (usually) needs exposure correction
• Depends on film/chip speed

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Exposure Metering

• Guessing
• Sunny Sixteen Aperture 16 and exposure time
  1/film-speed
• Light meters
• Incident light meters (handheld)
• Reflective light meters (built into cameras
  orhandheld)

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Exposure Metering

• In-camera light meters
• Metering
• Spot metering one small spot, usually in the
  center of the frame
• Center-weighted averages over whole frame, with
  more weight given to center area
• Matrix/evaluative measures light in many areas,
  and determines exposure from database of correct
  exposures

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Matrix Metering

• Measure brightness in many spots
• five to thousands
• Calculate absolute brightness
• Requires information from lens
• Take color and distance into account
• Calculate exposure based on
• Zone system
• Database of correctly exposed examples

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Sensitivity/Speed

• Speed of film/chip described in ISO
• Normal speed 100 ISO
• Films available 25-3200 ISO
• Digicams 50-3200
• High ISOs
• Film grain, more contrast
• Noise
• Film can be pushed and pulled during development