computer graphics and imaging uc berkeley cs184/284a, spring
TRANSCRIPT
Computer Graphics and Imaging UC Berkeley CS184/284A, Spring 2016
Lecture 16:
Cameras & Lenses I
Image Capture Overview
Ren Ng, Spring 2016CS184/284A, Lecture 16
What’s Happening Inside the Camera?
Cross-section of Nikon D3, 14-24mm F2.8 lens
Ren Ng, Spring 2016CS184/284A, Lecture 16
Pinholes & Lenses Form Image on SensorLondon and Upton
Ren Ng, Spring 2016CS184/284A, Lecture 16
Shutter Exposes Sensor For Precise Duration
The Slow Mo Guys, https://youtu.be/CmjeCchGRQo
Ren Ng, Spring 2016CS184/284A, Lecture 16
Sensor Accumulates Irradiance During Exposure
Ren Ng, Spring 2016CS184/284A, Lecture 16
Why Not Sensors Without Lenses?
Each sensor point would integrate light from all points on the object, so all pixel values would be similar
Lond
on an
d Up
ton
Ren Ng, Spring 2016CS184/284A, Lecture 16
Image Processing: From Sensor Values to Image
Pinhole Image Formation
Ren Ng, Spring 2016CS184/284A, Lecture 3
A. H. Zewail, Phil. Trans. R. Soc. A 2010;368:1191-1204
Mo Tzu (c. 470–c. 390 BC)
Aristotle (384–322 BC)
Ibn al-Haytham (965–1040)
Shen Kuo (1031–1095)
Roger Bacon (c. 1214–1294)
Johannes Kepler (1571–1630)
Recall: Pinhole Camera (Camera Obscura)
Ren Ng, Spring 2016CS184/284A, Lecture 3
Largest Pinhole Photographlegacyphotoproject.com“The Great Picture”
Ren Ng, Spring 2016CS184/284A, Lecture 16
Largest Pinhole Photographlegacyphotoproject.com
Ren Ng, Spring 2016CS184/284A, Lecture 16
Largest Pinhole Photographlegacyphotoproject.com
Ren Ng, Spring 2016CS184/284A, Lecture 16
Largest Pinhole Photographlegacyphotoproject.com
Ren Ng, Spring 2016CS184/284A, Lecture 16
Largest Pinhole Photographlegacyphotoproject.com
Ren Ng, Spring 2016CS184/284A, Lecture 16
Largest Pinhole Photograph
Field of View
Ren Ng, Spring 2016CS184/284A, Lecture 16
Effect of Focal Length on FOV
For a fixed sensor size, decreasing the focal length increases the field of view.
FOV = 2 arctan
✓h
2f
◆
Lens
Sensor
Focal lengthf
h
FOV = 2 arctan
✓h
2f
◆
h
f
FOV = 2 arctan
✓h
2f
◆
Ren Ng, Spring 2016CS184/284A, Lecture 16
Focal Length v. Field of View
• For historical reasons, it is common to refer to angular field of view by focal length of a lens used on a 35mm-format film (36 x 24mm)
• Examples of focal lengths on 35mm format:
• 17mm is wide angle 104°
• 50mm is a “normal” lens 47°
• 200mm is telephoto lens 12°
• Careful! When we say current cell phones have approximately 28mm “equivalent” focal length, this uses the above convention. The physical focal length is often 5-6 times shorter, because the sensor is correspondingly smaller
Ren Ng, Spring 2016CS184/284A, Lecture 16
Focal Length v. Field of View
From London and Upton, and Canon EF Lens Work III
Ren Ng, Spring 2016CS184/284A, Lecture 16
Focal Length v. Field of View
From London and Upton, and Canon EF Lens Work III
Ren Ng, Spring 2016CS184/284A, Lecture 16
Focal Length v. Field of View
From London and Upton, and Canon EF Lens Work III
Ren Ng, Spring 2016CS184/284A, Lecture 16
Focal Length v. Field of View
From London and Upton, and Canon EF Lens Work III
Wide angle: 18mm, 1/750, f/8
Ren Ng, Spring 2016CS184/284A, Lecture 16Normal: 50mm, 1/80, f/1.4
Ren Ng, Spring 2016CS184/284A, Lecture 16Normal: 64mm, 1/3200, f/2.8
Ren Ng, Spring 2016CS184/284A, Lecture 16Telephoto: 150mm, 1/640, f/1.8
Telephoto: 200mm, 1/200, f/2.8
Telephoto: 420mm, 1/1600, f/4
Ren Ng, Spring 2016CS184/284A, Lecture 16Telephoto: 420mm, 1.0s, f/4
Ren Ng, Spring 2016CS184/284A, Lecture 16Telephoto: 420mm, 4.0s, f/4
Ren Ng, Spring 2016CS184/284A, Lecture 16
Effect of Sensor Size on FOVObject
Lens
Sensor(s)
35mm Full Frame
APS-C
Ren Ng, Spring 2016CS184/284A, Lecture 16
Maintain FOV on Smaller Sensor?
LensSmall Sensor
Lens
Large Sensor
To maintain FOV, decrease focal length of lens in proportion to width/height of sensor
Focal length
Shorterfocal
length
Perspective Composition (Photographer’s Mindset)
Ren Ng, Spring 2016CS184/284A, Lecture 16
Perspective Composition – Camera Position / Focal Length
In this sequence, distance from subject increases with focal length to maintain image size of human subject.
Notice the dramatic change in background perspective.
From Canon EF Lens Work III
Ren Ng, Spring 2016CS184/284A, Lecture 16
Perspective Composition
16 mm (110°)
Up close and zoomed widewith short focal length
Ren Ng, Spring 2016CS184/284A, Lecture 16
Perspective Composition
200 mm (12°)
Walk back and zoom inwith long focal length
Dolly-Zoom Cinema Technique – “Vertigo Effect”
First used by Alfred Hitchcock in “Vertigo” 1958
Dolly-Zoom Cinema Technique – a.k.a. “Vertigo Effect”
By Steven Spielberg in “Jaws” 1975
Ren Ng, Spring 2016CS184/284A, Lecture 16
A Photographer’s Mindset
“Choose your perspective before you choose your lens.”
— Ming Thein, mingthein.com
Exposure
Ren Ng, Spring 2016CS184/284A, Lecture 16
Exposure
• H = T x E
• Exposure = time x irradiance
• Exposure time (T)
• Controlled by shutter
• Irradiance (E)
• Power of light falling on a unit area of sensor
• Controlled by lens aperture and focal length
Ren Ng, Spring 2016CS184/284A, Lecture 16
Focal Plane Shutter (1/25 Sec Exposure)
The Slow Mo Guys, https://youtu.be/CmjeCchGRQo
Ren Ng, Spring 2016CS184/284A, Lecture 16
Focal Plane Shutter (Fast Exposures)
The Slow Mo Guys, https://youtu.be/CmjeCchGRQo
Ren Ng, Spring 2016CS184/284A, Lecture 16
Electronic Rolling Shutter
The Slow Mo Guys, https://youtu.be/CmjeCchGRQo
Ren Ng, Spring 2016CS184/284A, Lecture 16
Other Shutter Systems
Also have leaf shutters
• Circular iris that closes
Global electronic shutter
• Different circuit design that exposes all pixels with the same time duration
Mixtures of physical and electronic shutter
• E.g. Electronic reset starts exposure, physical shutter closing ends exposure
London
Ren Ng, Spring 2016CS184/284A, Lecture 16
Shutter Speed
Controls how long the sensor is exposed to light
• Linear effect on exposure until sensor saturates Denoted in fractions of a second:
• 1/4000, 1/2000, 1/1000, 1/500, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1, 2, 4, 8, 15, 30s
Blur due to hand-shake is a concern for hand-held shots:
• Rule of thumb: longest hand-held exposure = 1 / f
• e.g. 1/180 second for a 180mm lens (35 mm equiv.)
Ren Ng, Spring 2016CS184/284A, Lecture 16
Main Side Effect of Shutter Speed
Motion blur Doubling shutter time doubles motion blur
London
Ren Ng, Spring 2016CS184/284A, Lecture 16
Sensor Irradiance
• As the diameter D of the aperture doubles, its area (hence the light that can get through it) increases by 4x.
• Each point on the lens emanates light angularly (cone in 2nd drawing), and its contribution to sensor irradiance falls off as the square of the distance (radiometry review)
• If the distance to the sensor is doubled, the irradiance on the sensor decreases by 4×.
N =f
D
Area = ⇡(D/2)2
Sensor
Lens + Aperture
Ren Ng, Spring 2016CS184/284A, Lecture 16
Aperture Value (a.k.a. F-Number, F-Stop)
• Irradiance on sensor is proportional to
• Square of lens aperture diameter D
• Inverse square of distance from lens to sensor (~focal length f)
• So that aperture values give irradiance regardless of focal length, aperture number N (a.k.a. f-number) is defined relative to focal length:
• An f-stop of 2 is sometimes written f/2, reflecting the fact that the absolute aperture (A) can be computed by dividing focal length (f) by the relative aperture (N).
• F-stops: 1.4, 2, 2.8, 4.0, 5.6, 8, 11, 16, 22, 32, 45, 64,
• 1 stop doubles exposure
N =f
D
Ren Ng, Spring 2016CS184/284A, Lecture 16
Example F-Stop CalculationsD = 50mm
f = 100mm
N = f/D = 2
D = 100mm
f = 200mm
N = f/D = 2
D = 100mm
f = 400mm
N = f/D = 4
Ren Ng, Spring 2016CS184/284A, Lecture 16
Camera Exposure Recap
• H = T x E
• Shutter
• Doubling the open time doubles H
• Increases motion blur
• Aperture
• Increasing one f-stop doubles H
• Decreases depth of field
Ren Ng, Spring 2016CS184/284A, Lecture 16
Fastest Photography Lens F-Stop?
Hari Subramanyam, https://www.flickr.com/photos/dementedjesus/
Leica Noctilux-M 50mm f/0.95 ASPH Lens
Ren Ng, Spring 2016CS184/284A, Lecture 16
Main Side-Effect of Aperture
Depth of field (range of object depths that are sharp) Increasing 2 f-stops doubles the depth of field More on this in a later lecture
Ren Ng, Spring 2016CS184/284A, Lecture 16
f / 40.01 sec
f / 110.1 sec
f / 320.8 sec
Constant Exposure: Depth of Field vs Shutter Speed
• Photographers must trade off depth of field and motion blur for moving subjects
Shallow Depth of Field Can Create a Stronger Image
From Peterson, Understanding Exposure 200mm, f/4, 1/1000 (left) and f/11, 1/125 (right)
Motion Blur Can Help Tell The Story
From Peterson, Understanding Exposure 1/60, f/5.6, 180mm
Ren Ng, Spring 2016CS184/284A, Lecture 16
ISO (Gain)
Third variable for exposure Film: trade sensitivity for grain Digital: trade sensitivity for noise
• Multiply signal before analog-to-digital conversion
• Linear effect (ISO 200 needs half the light as ISO 100)
More on this in a later lecture.
Ren Ng, Spring 2016CS184/284A, Lecture 16
ISO Gain vs Noise in Canon T2iCredit: bobatkins.com
Ren Ng, Spring 2016CS184/284A, Lecture 16
Things to Remember
Effect Cause Field of view Sensor size, focal length Depth of field Aperture, focal length, object dist. Exposure Aperture, shutter, ISOMotion blur ShutterGrain/noise ISO
Pinholes and lenses form perspective images Perspective composition, dolly zoom
Ren Ng, Spring 2016CS184/284A, Lecture 16
Acknowledgments
Many thanks to Marc Levoy, who created many of these slides, and Pat Hanrahan.
• London, Stone, and Upton, Photography (9th ed.), Prentice Hall, 2008.
• Peterson, Understanding Exposure, AMPHOTO 1990.
• The Slow Mo Guys
• bobatkins.com
• Hari Subramanyan
• Canon EF Lens Work III