Cameras
Digital Visual Effects g Yung-Yu Chuang
with slides by Fredo Durand, Brian Curless, Steve Seitz and Alexei Efros
Camera trial #1
scene film
Put a piece of film in front of an object Put a piece of film in front of an object.
Pinhole camera
pinhole camera pinhole camera
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
Why not making the aperture as small as possible?
• Less light gets through
• Diffraction effect
Shrinking the aperture High-end commercial pinhole cameras
$200 $700
$200~$700
Adding a lens
scene lens film
Lenses
Thin lens equation:
Thin lens formula
Similar triangles everywhere!
y’/y = D’/D
D’ D f D’ D f
y y’
Frédo Durand’s slide
Thin lens formula
y’/y = D’/D
Similar triangles everywhere!
D’ D
y’/y = (D’-f)/f f
D’ D f
y y’
Frédo Durand’s slide
Thin lens formula D’ D 1 1 1
+ = f
The focal length f determines the lens’s ability to bend (refract)D’ D
D D f
light. It is a function of the shape y ( ) and index of refraction of the lens.f D’ D f
Frédo Durand’s slide
Adding a lens
“circle of confusion”
scene lens film
confusion”
A lens focuses light onto the film
Th i ifi di t t hi h bj t “i f ”
• 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:
http://www.phy.ntnu.edu.tw/java/Lens/lens_e.html
Zoom lens
200mm
28mm
simplified zoom lens Nikon 28-200mm zoom lens. simplified zoom lens
in operation From wikipedia
Field of view vs focal length
i o
Scene
w α
Sensor
f
1 1 1
Gaussian Lens Formula:
f
f o i
Gaussian Lens Formula:
α = 2arctan(w/(2i))
Field of View: ( ( )) ≈ 2arctan(w/(2f)) Example: w = 30mm, f = 50mm => α ≈ 33.4º
( ( ))
Slides from Li Zhang
Focal length in practice
24mm
50mm
135 135mm
Distortion
No distortion Pin cushion Barrel 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
Correcting radial distortion
from Helmut Dersch
Vignetting g g
Vignetting
L1
L2
L3 B
A A
more light from A than B !
Slides from Li Zhang
Vignetting g g
Vignetting
L1
L2
L3 B
A A
more light from A than B !
original corrected
Goldman & Chen ICCV 2005
Slides from Li Zhang
Chromatic Aberration
Lens has different refractive indices for different wavelengths.g
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 of glass with different refractive indexes can
reduce or eliminate this problem. Slides from Li Zhang
Exposure = aperture + shutter speed
F
• Aperture of diameter D restricts the range of rays (aperture may be on either side of the lens)
(aperture may be on either side of the lens)
• Shutter speed is the amount of time that light is allowed to pass through the aperturep g p
Exposure
• Two main parameters:
Aperture (in f stop) – Aperture (in f stop)
– Shutter speed (in fraction of a second)
Effects of shutter speeds
• Slower shutter speed => more light, but more motion blur
• Faster shutter speed freezes motion From Photography, London et al.
Walking people Running people Car Fast train
1/125 1/250 1/500 1/1000
Aperture
• Aperture is the diameter of the lens opening, usually specified by f-stop, f/D, a fraction of the usually specified by f stop, f/D, a fraction of the focal length.
– f/2.0 on a 50mm means that the aperture is 25mmf/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
• When a change in f stop occurs, the light is either doubled or cut in half.
doubled or cut in half.
• Lower f-stop, more light (larger lens opening) (larger lens opening)
• Higher f-stop, less light (smaller lens opening) (smaller lens opening)
Depth of field
Changing the aperture size affects depth of field.
A smaller aperture increases the range in which A smaller aperture increases the range in which the object is approximately in focus
Di h Diaphragm
Point in focus lens
sensor Object with texture
Depth of field
Changing the aperture size affects depth of field.
A smaller aperture increases the range in which A smaller aperture increases the range in which the object is approximately in focus
Di h Diaphragm
Point in focus lens
sensor Object with texture
Depth of field
From Photography, London et al.
Exposure
• Two main parameters:
A t (i f t ) – Aperture (in f stop)
– Shutter speed (in fraction of a second)
• Reciprocity
The same exposure is obtained with
i l d
an exposure twice as long and an aperture area half as big
H t f t g i f
– Hence square root of two progression of f stops vs. power of two progression of shutter speed
shutter speed
– Reciprocity can fail for very long exposuresp
From Photography, London et al.
Reciprocity
• Assume we know how much light we need We have the choice of an infinity of shutter
• We have the choice of an infinity of shutter speed/aperture pairs
Wh t ill id h i f h tt d?
• What will guide our choice of a shutter speed?
– Freeze motion vs. motion blur, camera shake
Wh ill id h i f ?
• What will guide our choice of an aperture?
– Depth of field, diffraction limit
f
• Often we must compromise
– Open more to enable faster speed (but shallow DoF)
Exposure & metering
• The camera metering system measures how bright the scene is
bright the scene is
• In Aperture priority mode, the photographer sets
th t th t th h tt d
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 p p ( or less)
• In Manual mode, the user decides everything In Manual mode, the user decides everything (but can get feedback)
Pros and cons of various modes
• Aperture priority
– Direct depth of field controlDirect depth of field control
– Cons: can require impossible shutter speed (e.g. with f/1.4 for a bright scene)g )
• 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)
• Third variable for exposure
• Linear effect (200 ISO needs half the light as 100 ISO)
• Linear effect (200 ISO needs half the light as 100 ISO)
• Film photography: trade sensitivity for grain
Di it l h t h t d iti it f i
• Digital photography: trade sensitivity for noise
comm dpreview.cFrom
Demo
See http://www.photonhead.com/simcam/
Film camera
aperture
& shutter
scene lens & film
motor
Digital camera
aperture
& shutter
scene lens & sensor
array motor
A digital camera replaces film with a 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
converts photons to electrons
CCD v.s. CMOS
• CCD is less susceptible to noise (special process, higher fill factor))
• CMOS is more flexible, less expensive (standard process), less power consumption
CCD CMOS
CCD CMOS
Sensor noise
• Blooming Diff i
• Diffusion
• Dark current
• Photon shot noise
• Amplifier readout noise
• Amplifier readout noise
SLR (Single-Lens Reflex) ( )
• Reflex (R in SLR) means that we see through the same lens used to take the image
the same lens used to take the image.
• Not the case for compact cameras
SLR view finder
Prism Your eye
Mirror Mirror (flipped for exposure)
Film/sensor
Mirror (when viewing) Light from scene
lens
Color
So far, we’ve only talked about monochrome sensors Color imaging has been implemented in a sensors. Color imaging has been implemented in a number of ways:
Fi ld ti l
• Field sequential
• Multi-chip
• Color filter array
• X3 sensor
• X3 sensor
Field sequential Field sequential
Field sequential Prokudin-Gorskii (early 1900’s) ( )
L
Lantern projector
http://www.loc.gov/exhibits/empire/
Prokudin-Gorskii (early 1990’s) ( ) Multi-chip
wavelength dependent
Embedded color filters
Color filters can be manufactured directly onto th h t d t t
the photodetectors.
Color filter array
Kodak DCS620x
Color filter arrays (CFAs)/color filter mosaics
CMY
Color filter arrays (CFAs)/color filter mosaics
Why CMY CFA might be better Color filter array
Bayer pattern
Color filter arrays (CFAs)/color filter mosaics Color filter arrays (CFAs)/color filter mosaics
Bayer’s pattern Demosaicking CFA’s
bilinear interpolation
i i l i t li i t l ti
original input linear interpolation
Demosaicking CFA’s
Constant hue-based interpolation (Cok) interpolation (Cok)
Hue:
Interpolate G first
Demosaicking CFA’s
Median-based interpolation (Freeman)
(Freeman)
1. Linear interpolation 2. Median filter on color
differences
Demosaicking CFA’s
Median-based interpolation (Freeman)
original input linear interpolation
color difference median filter Reconstruction (e.g. G-R) (kernel size 5) (G=R+filtered
difference)
Demosaicking CFA’s
Gradient-based interpolation (LaRoche-Prescott)
(LaRoche Prescott)
1. Interpolation on G
Demosaicking CFA’s
Gradient-based interpolation (LaRoche-Prescott)
(LaRoche Prescott)
2. Interpolation of color differences
differences
Demosaicking CFA’s
bili C k F L R h
bilinear Cok Freeman LaRoche
Demosaicking CFA’s
G ll F ’ i h b i ll f l i
Generally, Freeman’s is the best, especially for natural images.
Foveon X3 sensor
• light penetrates to different depths for different wavelengths
wavelengths
• multilayer CMOS sensor gets 3 different spectral iti iti
sensitivities
Color filter array
red green blue output
X3 technology
red green blue output
Foveon X3 sensor
X3 sensor Bayer CFA
Cameras with X3
Sigma SD10, SD9 Polaroid X530
Sigma SD9 vs Canon D30 Color processing
• After color values are recorded, more color processing usually happens:
processing usually happens:
– White balance
N li it t i t fil t h
– Non-linearity to approximate film response or match TV monitor gamma
White Balance
automatic white balance warmer +3
Manual white balance
white balance with white balance with the white book the red book
Autofocus
• Active
S – Sonar – Infrared
• Passive
Digital camera review website
• A cool video of digital camera illustration
h // d i /
• http://www.dpreview.com/
Camcorder Interlacing
with interlacing without interlacing
Deinterlacing
bl d
blend weave
Deinterlacing
Discard
(even field only or Progressive scan odd filed only)
Hard cases References
• http://www.howstuffworks.com/digital-camera.htm
• http://electronics howstuffworks com/autofocus htm
• http://electronics.howstuffworks.com/autofocus.htm
• Ramanath, Snyder, Bilbro, and Sander. Demosaicking Methods for Bayer Color Arrays Journal of Electronic Methods for Bayer Color Arrays, Journal of Electronic Imaging, 11(3), pp306-315.
• Rajeev Ramanath, Wesley E. Snyder, Youngjun Yoo, 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. 34- 43, 2005.
htt // ld t / h t / hit b l /i d
• http://www.worldatwar.org/photos/whitebalance/ind ex.mhtml
http://www 100fps com/
• http://www.100fps.com/
