Transcript Visual Communication in Computer Graphics and Art

Visual Communication
in Computer Graphics
and Art
Photography IV - Aperture,
Exposure
Robert Kosara
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
Thin Lens vs. Thick Lens
Robert Kosara
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
http://www.photocourse.com/
Depth of Field (DOF)
Robert Kosara
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
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
Robert Kosara
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
Aperture
f
k
D
D
f
• Numeric aperture k (the denominator in 1:5.6, etc.)
• Effective lens diameter D
• Focal length f
Robert Kosara
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
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
Robert Kosara
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
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 √2 ~ 1.4:
1:1, 1:1.4, 1:2, 1:2.8, 1:4, 1:5.6, 1:8, 1:11,
...
Robert Kosara
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
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
Robert Kosara
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
Circle of Confusion
v
CD
v  v'
C
D
v
Solid line – sharp image
Dotted Line – blurred image
Robert Kosara
v‘
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
Shallow Depth of Field
Robert Kosara
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
Deep Depth of Field
Robert Kosara
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
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
Robert Kosara
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
“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
Robert Kosara
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
Diffraction at Aperture
Robert Kosara
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
Bokeh
• The quality of
blurred regions of
the image
• Japanese Word
• Relatively subtle in
most images
• Criterion for lens
quality
http://wvs.topleftpixel.com/
Robert Kosara
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
„Sieve Aperture“
Heinrich Tauscher
Robert Kosara
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
Mirror Lens
• Aperture has doughnut
shape
Robert Kosara
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
Mirror Lens: Bokeh
Robert Kosara
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
Exposure
Aperture/
Light Flow
• 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, …
Robert Kosara
Exposure
Time
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
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 levels
with exposure and development
• High contrast (usually) needs
exposure correction
• Depends on film/chip speed
Robert Kosara
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
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 or
handheld)
Robert Kosara
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
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
Robert Kosara
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
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
Robert Kosara
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/
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
Robert Kosara
Visual Communication in Computer Graphics and Art
http://www.eagereyes.org/VisComm/