Cameras

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Cameras
Digital Visual Effects
Yung-Yu Chuang
with slides by Fredo Durand, Brian Curless, Steve Seitz and Alexei Efros
Camera trial #1
Pinhole camera
pinhole camera
scene
film
Put a piece of film in front of an object.
scene
barrier
film
Add a barrier to block off most of the rays.
• It reduces blurring
• The pinhole is known as the aperture
• The image is inverted
Shrinking the aperture
Shrinking the aperture
Why not making the aperture as small as possible?
• Less light gets through
• Diffraction effect
High-end commercial pinhole cameras
Adding a lens
scene
$200~$700
lens
film
Lenses
Thin lens formula
Similar triangles everywhere!
D’
y’/y = D’/D
D
f
y
y’
Thin lens equation:
Frédo Durand’s slide
Thin lens formula
Similar triangles everywhere!
y’/y = D’/D
y’/y = (D’-f)/f
D’
Thin lens formula
The focal length f determines the
1 +1 =1
lens’s ability to bend (refract)
D’ D f
light. It is a function of the shape
D’
D
f
and index of refraction of the lens.
D
f
y
y’
Frédo Durand’s slide
Frédo Durand’s slide
Zoom lens
Adding a lens
200mm
28mm
“circle of
confusion”
lens
scene
film
A lens focuses light onto the film
• There is a specific distance at which objects are “in focus”
• other points project to a “circle of confusion” in the image
• Thin lens applet:
Nikon 28-200mm zoom lens.
http://www.phy.ntnu.edu.tw/java/Lens/lens_e.html
Field of view vs focal length
Focal length in practice
24mm
o
i
simplified zoom lens
in operationFrom wikipedia
Scene
w
α
Sensor
50mm
f
Gaussian Lens Formula:
Field of View:
1 1 1
 
i o f
α = 2arctan(w/(2i))
≈ 2arctan(w/(2f))
Example: w = 30mm, f = 50mm => α ≈ 33.4º
Slides from Li Zhang
135mm
Correcting radial distortion
Distortion
No distortion
Pin cushion
Barrel
• Radial distortion of the image
– Caused by imperfect lenses
– Deviations are most noticeable for rays that pass
through the edge of the lens
from Helmut Dersch
Vignetting
Vignetting
Vignetting
Vignetting
L3 L2 L1
B
L3 L2 L1
A
B
A
more light from A than B !
more light from A than B !
original
corrected
Goldman & Chen ICCV 2005
Slides from Li Zhang
Slides from Li Zhang
Chromatic Aberration
Exposure = aperture + shutter speed
F
Lens has different refractive indices
for different wavelengths.
http://www.dpreview.com/learn/?/Glossary/Optical/chromatic_aberration_0
1.htm
Special lens systems using two or more pieces of glass with different refractive indexes can reduce or eliminate this problem.
Exposure
• Two main parameters:
• Aperture of diameter D restricts the range of rays
(aperture may be on either side of the lens)
• Shutter speed is the amount of time that light is
allowed to pass through the aperture
Slides from Li Zhang
Effects of shutter speeds
• Slower shutter speed => more light, but more motion blur
– Aperture (in f stop)
– Shutter speed (in fraction of a second)
• Faster shutter speed freezes motion
Walking people
1/125
Running people
1/250
From Photography, London et al.
Car
1/500
Fast train
1/1000
Aperture
Depth of field
• Aperture is the diameter of the lens opening,
usually specified by f-stop, f/D, a fraction of the
focal length.
Changing the aperture size affects depth of field.
A smaller aperture increases the range in which
the object is approximately in focus
– f/2.0 on a 50mm means that the aperture is 25mm
– f/2.0 on a 100mm means that the aperture is 50mm
• When a change in f-stop
occurs, the light is either
doubled or cut in half.
• Lower f-stop, more light
(larger lens opening)
• Higher f-stop, less light
(smaller lens opening)
Diaphragm
Point in focus
sensor
Depth of field
lens
Object with texture
Depth of field
Changing the aperture size affects depth of field.
A smaller aperture increases the range in which
the object is approximately in focus
Diaphragm
Point in focus
sensor
lens
Object with texture
From Photography, London et al.
Exposure
Reciprocity
• Two main parameters:
• Assume we know how much light we need
• We have the choice of an infinity of shutter
speed/aperture pairs
– Aperture (in f stop)
– Shutter speed (in fraction of a second)
• Reciprocity
The same exposure is obtained with
an exposure twice as long and an
aperture area half as big
– Hence square root of two progression of
f stops vs. power of two progression of
shutter speed
– Reciprocity can fail for very long
exposures
• What will guide our choice of a shutter speed?
– Freeze motion vs. motion blur, camera shake
• What will guide our choice of an aperture?
– Depth of field, diffraction limit
• Often we must compromise
– Open more to enable faster speed (but shallow DoF)
From Photography, London et al.
Exposure & metering
• The camera metering system measures how
bright the scene is
• In Aperture priority mode, the photographer sets
the aperture, the camera sets the shutter speed
• In Shutter-speed priority mode, photographers
sets the shutter speed and the camera deduces
the aperture
• In Program mode, the camera decides both
exposure and shutter speed (middle value more
or less)
• In Manual mode, the user decides everything
(but can get feedback)
Pros and cons of various modes
• Aperture priority
– Direct depth of field control
– Cons: can require impossible shutter speed (e.g. with
f/1.4 for a bright scene)
• Shutter speed priority
– Direct motion blur control
– Cons: can require impossible aperture (e.g. when
requesting a 1/1000 speed for a dark scene)
• Note that aperture is somewhat more restricted
• Program
– Almost no control, but no need for neurons
• Manual
– Full control, but takes more time and thinking
Sensitivity (ISO)
Summary in a picture
• Third variable for exposure
• Linear effect (200 ISO needs half the light as 100 ISO)
• Film photography: trade sensitivity for grain
From dpreview.com
• Digital photography: trade sensitivity for noise
source hamburgerfotospots.de
Demo
Film camera
aperture
& shutter
See http://www.photonhead.com/simcam/
scene
lens &
motor
film
Digital camera
CCD v.s. CMOS
aperture
& shutter
scene
lens &
motor
• CCD is less susceptible to noise (special process, higher
fill factor)
• CMOS is more flexible, less expensive (standard
process), less power consumption
sensor
array
• A digital camera replaces film with a sensor array
• Each cell in the array is a light-sensitive diode that
converts photons to electrons
CCD
CMOS
Sensor noise
SLR (Single-Lens Reflex)
•
•
•
•
•
• Reflex (R in SLR) means that we see through
the same lens used to take the image.
• Not the case for compact cameras
Blooming
Diffusion
Dark current
Photon shot noise
Amplifier readout noise
SLR view finder
Prism
Color
Your eye
Mirror
(flipped for exposure)
Film/sensor
So far, we’ve only talked about monochrome
sensors. Color imaging has been implemented in a
number of ways:
• Field sequential
• Multi-chip
• Color filter array
• X3 sensor
Mirror
(when viewing)
Light from scene
lens
Field sequential
Field sequential
Field sequential
Prokudin-Gorskii (early 1900’s)
Lantern
projector
http://www.loc.gov/exhibits/empire/
Prokudin-Gorskii (early 1900’s)
Multi-chip
wavelength
dependent
Embedded color filters
Color filter array
Kodak DCS620x
Color filters can be manufactured directly onto
the photodetectors.
Why CMY CFA might be better
CMY
Color filter arrays (CFAs)/color filter mosaics
Color filter array
Bayer pattern
Color filter arrays (CFAs)/color filter mosaics
Demosaicking CFA’s
Bayer’s pattern
bilinear interpolation
original
Demosaicking CFA’s
input
linear interpolation
Demosaicking CFA’s
Constant hue-based
interpolation (Cok)
Median-based interpolation
(Freeman)
Hue:
Interpolate G first
1. Linear interpolation
2. Median filter on color
differences
Demosaicking CFA’s
Demosaicking CFA’s
Median-based interpolation (Freeman)
Gradient-based interpolation
(LaRoche-Prescott)
1. Interpolation on G
original
input
color difference
(e.g. G-R)
median filter
(kernel size 5)
linear interpolation
Reconstruction
(G=R+filtered
difference)
Demosaicking CFA’s
Demosaicking CFA’s
Gradient-based interpolation
(LaRoche-Prescott)
2. Interpolation of color
differences
bilinear
Cok
Freeman
LaRoche
Demosaicking CFA’s
Foveon X3 sensor
• light penetrates to different depths for different
wavelengths
• multilayer CMOS sensor gets 3 different spectral
sensitivities
Generally, Freeman’s is the best, especially for natural images.
Color filter array
red
green
X3 technology
blue
output
red
green
blue
output
Foveon X3 sensor
Bayer CFA
Sigma SD9 vs Canon D30
Cameras with X3
X3 sensor
Sigma SD10, SD9
Polaroid X530
Color processing
• After color values are recorded, more color
processing usually happens:
– White balance
– Non-linearity to approximate film response or match
TV monitor gamma
White Balance
White Balance
illumination
warmer +3
reflectance
automatic white balance
perception
Color constancy
Color constancy
What color is the dress?
Human vision is complex
Autofocus
• Active
– Sonar
– Infrared
• Passive
Digital camera review website
• A cool video of digital camera illustration
• http://www.dpreview.com/
Camcorder
Interlacing
without interlacing
Deinterlacing
with interlacing
Deinterlacing
Discard
(even field only or
odd filed only)
blend
Hard cases
Progressive scan
weave
Computational cameras
References
• http://www.howstuffworks.com/digital-camera.htm
• http://electronics.howstuffworks.com/autofocus.htm
• Ramanath, Snyder, Bilbro, and Sander. Demosaicking
Methods for Bayer Color Arrays, Journal of Electronic
Imaging, 11(3), pp306-315.
• Rajeev Ramanath, Wesley E. Snyder, Youngjun Yoo,
Mark S. Drew, Color Image Processing Pipeline in Digital
Still Cameras, IEEE Signal Processing Magazine Special
Issue on Color Image Processing, vol. 22, no. 1, pp. 3443, 2005.
• http://www.worldatwar.org/photos/whitebalance/ind
ex.mhtml
• http://www.100fps.com/
More emerging cameras