Computer Graphics and Imaging UC Berkeley CS184/284A, Spring

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


Pinholes & Lenses Form Image on SensorLondon and Upton


Shutter Exposes Sensor For Precise Duration

The Slow Mo Guys, https://youtu.be/CmjeCchGRQo

https://youtu.be/CmjeCchGRQo


Sensor Accumulates Irradiance During Exposure


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


Image Processing: From Sensor Values to Image

Pinhole Image Formation


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)


Largest Pinhole Photographlegacyphotoproject.com“The Great Picture”

http://legacyphotoproject.com


Largest Pinhole Photographlegacyphotoproject.com












Largest Pinhole Photograph

Field of View


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

◆


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



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


Effect of Sensor Size on FOVObject

Lens

Sensor(s)

35mm Full Frame

APS-C


Sensor Sizes

Credit: lensvid.com

http://lensvid.com


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)


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


Perspective Composition

16 mm (110°)

Up close and zoomed widewith short focal length


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


A Photographer’s Mindset

“Choose your perspective before you choose your lens.”

— Ming Thein, mingthein.com

http://mingthein.com

Exposure


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


Focal Plane Shutter (1/25 Sec Exposure)




Focal Plane Shutter (Fast Exposures)




Electronic Rolling Shutter




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


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.)


Main Side Effect of Shutter Speed

Motion blur Doubling shutter time doubles motion blur

London


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


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


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


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


Fastest Photography Lens F-Stop?

Hari Subramanyam, https://www.flickr.com/photos/dementedjesus/

Leica Noctilux-M 50mm f/0.95 ASPH Lens


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


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


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.


ISO Gain vs Noise in Canon T2iCredit: bobatkins.com

http://bobatkins.com


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


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

http://bobatkins.com

Documents

Computer Graphics and Imaging UC Berkeley CS184/284A, Spring