This Week ! Exposure ! The Art, Science and Algorithms Camera - - PDF document

The Art, Science and Algorithms

• f Photography!

Exposure & Metering! CSCI 4900/6900! Maria Hybinette!

1!

This Week!

• Exposure!

– Camera Basics! – Simple Math!

• Zone System!

2! Maria Hybinette!

Exposure!

• Controls light to digital sensor (or film)!
• Two main controls (parameters):!

– Shutter Speed!

• Controls amount of time light ‘shines’ on the sensor!

– Aperture!

• Controls the amount of light falls on a unit area per

second!

• Exposure = Irradiance x Time!

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Shutter Speed!

• Controls how long the sensor is exposed to light!
• Linear effect on exposure until sensor saturates!
• Denoted in fraction of a second: !

– 1/30, 1/60, 1/125, 1/250, 1/500! – Get the pattern ?!

• On a normal lens, hand-hold down to 1/60!

– Rule of thumb: shortest exposure: 1/ focal length!

• 1/50 for a 50 mm lens (slower motion blur)!
• 1/500 for a 500mm lens, !

– so large lenses needs faster shutter speeds to avoid camera shake!

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Side-Effect of Shutter Speed!

• Motion Blur!
• Halving the shutter doubles the motion blur.!

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Wikipedia

Creative Shutter Speeds!

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Effect of shutter speed!

• Freezing motion!

– Rule of thumb!

7!

Walking people Running people Car Fast train 1/125 1/250 1/500 1/1000

Shutters!

• Central Shutters !

– Mounted within lens assembly (some in-front of lens, early cameras)! – Leaf mechanism generally used for this (see next slide for a simple version) 1/125th second! – Diaphragm shutter (thin blades) 1/500th second!

• Focal plane shutters near the focal plane and

moves to uncover sensor!

• Modern are mostly electronic!
• Digital cameras typically use a combination of

mechanical and electronic timings!

8!

Shutter!

• Simple leaf-shutter, typically
• nly one speed!

– Disposable cameras!

• Typically long way from

sensor plate !

– Slow to open & close!

• edges less exposure time!

– Doesn’t matter for DOF!

• Near focal plane shutters!

– Faster not to affect DOF! – Slit action to ensure all of sensor get same amount of light!

9!

Shutter!

• Leaf-shutter, typically only one speed!

– Disposable cameras!

• Typically long way from sensor plate!

– Slow to open & close = edges less exposure time!

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Your Best Friend!

• Use a tripod – it will enhance sharpness!

– Avoid camera shake!

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Aperture!

• Lens opening!
• A relative aperture size e.g,. “#”, called an f-

number, written f/#, reflects the fact that it is computed by dividing the focal length by the absolute aperture (D).!

– Example: if the focal length is 4 times the diameter, f-number is f/4, N=4! – Greater f-number (smaller hole), less light per unit area reaches the image plane (irradiance), watts/m2! – To maintain the same f-number a longer lens needs a larger diameter to produce the same illuminance ( lumen/m2) on focal plane (longer lenses has a magnifying effect) [example coming]!

12!

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Aperture!

• f/2 on a 50 mm lens (N=2) 2 = 50/D (D=25 mm)!
• f/2 on a 100 mm lens (N=2) 2 = 100/D (D=50 mm)!

13!

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Aperture!

• f/2 on a 50 mm lens (N=2) 2 = 50/D (D=25 mm)!
• f/2 on a 100 mm lens (N=2) 2 = 100/D (D=50 mm)!

14!

N = f D

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• Doubling both the absolute aperture diameter (D) and

the focal length (f) cancel; leaving the same relative aperture size (N). In this example, both lenses are f/2.!

Aperture!

• Irradiance (light on unit area of sensor per

second ) is proportional to the area of the aperture:!

– Square of the aperture diameter D! – Inverse square distance to the sensor (~ focal length f)!

15!

Area = " f 2N # $ % & ' (

2

N = f D

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Aperture !

• Doubling D, increases the area by 4X.!

– light falling per unit area decreases by 4X!

• Doubling N reduces D by 2x, light by 4x!

– Example: going from f/2.0 to f/4.0 !

• Cuts the light by 4x!

– Cut the light by 2, increase N by "2!

16!

Area = " f 2N # $ % & ' (

2

N = f D

Main Side-Effect of Aperture!

• Depth of field (more on that later)!
• Doubling N (two f/stops) doubles depth of

field!

17!

Depth of Field (preview)!

• A point in the scene is

focused at a point on the sensor!

• Moving the sensor in z!
• the depth where this

happens is called the depth

• f focus!
• this corresponds in the

scene to a depth of field!

• halving the aperture

diameter (smaller) doubles the depth of field!

18!

Exposure !

• Exposure = Irradiance x Time!

– Aperture (f=stop) & Shutter Speed!

• Reciprocity!
• Irradiance (I)!

– Controlled by aperture! – Lowering one f/stop doubles exposure! – Lowering by two f/stop doubles DOF!

• Exposure time (T)!

– controlled by the shutter ! – doubling exposure time doubles H! – doubling exposure time doubles motion blur!

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Reciprocity!

• Assume we know how much light we need !
• We have the choice of an infinity of shutter speed/

aperture pairs!

• 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!

• Often we must compromise!

– Open more to enable faster speed (but shallow DoF)!

20!

Trading off motion for DOF!

21!

Sensitivity (ISO)!

• third variable for exposure!
• film: trade sensitivity for grain!
• digital: trade sensitivity for noise !

– multiply signal before analog-to-digital conversion ! – linear effect (200 ISO needs half the light as 100 ISO)!

more in noise lecture!

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Demo!

• Trade-offs

affecting brightness!

• Flash Demo

(from Stanford)!

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Single Reflex Camera!

Nikon F4 ! (film camera)!

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Single Reflex Camera!

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Nikon F4 ! (film camera)!

Summary Exposure !

• Aperture (f number) (depth of field control)!

– Ratio between focal length &aperture diameter: # !diameter = f / <f number>! – Small f number means large aperture (factors of " 2)!

• Examples: f/2.0, f/2.8, f/4.0, f/5.6, f/8.0, f/11, f/16!

– A good standard lens has max aperture f/1.8 (less expensive f/3.5)!

• Shutter speed (motion blur control)!

– In fraction of a second!

• Examples: 1/30, 1/60, 1/125, 1/250, 1/500 (factor of 2)!

– Rule of thumb:!

• Hand-hold up to 1/f seconds, where f is focal length without camera

shake!

• Sensitivity (sensor noise control)!

– Gain applied to sensor! – In ISO, bigger number, more sensitive (100, 200, 400, 800, 1600)! Reciprocity between these three numbers, for a given exposure:# two degrees of freedom (2 things you can change)!

Slide Credits/Resources!

• Prof. Fredo Durand!
• Prof. Marc Levoy!
• London, Stone, Upton “Photography”!
• Wikipedia!

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