# comm dpreview.cFrom

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## Cameras

### Digital Visual Effects g Yung-Yu Chuang

with slides by Fredo Durand, Brian Curless, Steve Seitz and Alexei Efros

scene film

### Pinhole camera

pinhole camera pinhole camera

scene barrier film

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scene lens film

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### Thin lens formula

Similar triangles everywhere!

### y y’

Frédo Durand’s slide

### y’/y = D’/D

Similar triangles everywhere!

### y y’

Frédo Durand’s slide

### + = f

The focal length f determines the lens’s ability to bend (refract)

### D D f

light. It is a function of the shape y ( ) and index of refraction of the lens.

### fD’Df

Frédo Durand’s slide

“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

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### 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

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

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from Helmut Dersch

### Vignettinggg

Vignetting

L1

L2

L3 B

A A

more light from A than B !

Slides from Li Zhang

### Vignettinggg

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

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### 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

### • 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)

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### 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

### 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.

### • 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.

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### • 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)

### • 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

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### 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

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### SLR view finder

Mirror Mirror (flipped for exposure)

Film/sensor

Mirror (when viewing) Light from scene

lens

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CMY

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### bilinear interpolation

i i l i t li i t l ti

original input linear interpolation

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### Median-based interpolation (Freeman)

original input linear interpolation

color difference median filter Reconstruction (e.g. G-R) (kernel size 5) (G=R+filtered

difference)

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### 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.

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### Cameras with X3

Sigma SD10, SD9 Polaroid X530

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### 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

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### • Active

S – Sonar – Infrared

### CamcorderInterlacing

with interlacing without interlacing

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### Hard casesReferences

• 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/

Updating...

## References

Related subjects :