This Week!

• Exposure! The Art, Science and Algorithms – Camera Basics! of Photography! – Simple Math! • Zone System! Exposure & Metering! CSCI 4900/6900! Maria Hybinette!

1! 2! Maria Hybinette!

Exposure! Shutter Speed!

• Controls light to digital sensor (or film)! • Controls how long the sensor is exposed to light! • Two main controls (parameters):! • Linear effect on exposure until sensor saturates! – Shutter Speed! • Denoted in fraction of a second: ! – 1/30, 1/60, 1/125, 1/250, 1/500! • Controls amount of time light ‘shines’ on the sensor! – Get the pattern ?! – Aperture! • On a normal lens, hand-hold down to 1/60! • Controls the amount of light falls on a unit area per second! – Rule of thumb: shortest exposure: 1/ focal length! • 1/50 for a 50 mm lens (slower motion blur)! • Exposure = Irradiance x Time! • 1/500 for a 500mm lens, ! – so large lenses needs faster shutter speeds to avoid camera !"#$%&$#'()*%$)+!""#$%&'("%)"*+,-(").**+'," shake! %'"."&'+("./0."%)"10'1%/"20/"103%'45" 3! 4!

Side-Effect of Shutter Speed! Creative Shutter Speeds!

:-%(%";/04+(!""" • Motion Blur! :.1.'("<6*+3=/" • Halving the shutter doubles the motion blur.!

3 seconds 4 seconds Bulb: 3-10 second

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6.1("7-&80/"72004"" 7*%9"7-&80/"72004"" 6! 5! Effect of shutter speed! Shutters!

• Freezing motion! • Central Shutters ! – Rule of thumb! – Mounted within lens assembly (some in-front of lens, early cameras)! Walking people Running people Car Fast train – 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 1/125 1/250 1/500 1/1000 moves to uncover sensor! • Modern are mostly electronic! • Digital cameras typically use a combination of mechanical and electronic timings! 7! 8!

Shutter! Shutter!

• Simple leaf-shutter, typically ,-."+#'/#01',2&3#$' only one speed! >5 7-&80/"2*.(0" C5 #20/(&/0"3%D0/04" – Disposable cameras! @5 #20/(&/0"4&/+',"0E2%1&/0" F5 G0.)"H*.40" • Typically long way from B5 ;.(3-"$03-.'+1$" sensor plate ! I5 J&80/KL"12/+'," – Slow to open & close! • edges less exposure time! – Doesn’t matter for DOF! • Leaf-shutter, typically only one speed! • Near focal plane shutters! – Disposable cameras! – Faster not to affect DOF! – Slit action to ensure all of sensor • Typically long way from sensor plate! get same amount of light! – Slow to open & close = edges less exposure time!

9! 10!

M+40"N20'"O)&**P" Your Best Friend! Aperture!

f R"S")T'&$H0/""S"O)?UP" • Use a tripod – it will enhance sharpness! N = )"""S")%3.*"*0',(-" D Q"S"4+.$0(0/"%20'+'," – Avoid camera shake! • Lens opening! • A relative aperture size e.g,. “#”, called an f- number, written f/#, reflects the fact that it is computed by dividing the focal length by the absolute aperture! (D).! – Example: if the focal length is 4 times the diameter, f-number is f/4, N=4! – Greater f-number (smaller hole), less light per unit area reaches the image plane (irradiance), watts/m2! – To maintain the same f-number a longer lens needs a larger diameter to produce the same illuminance ( lumen/m2) on focal plane (longer lenses has a magnifying effect) [example coming]! 11! 12! *%90/")T'&$H0/"*%',"*0'101").("V"0E20'1+D0" 7(%22+',"Q%9'" f R"S")T'&$H0/""S"O)?UP" f R"S")T'&$H0/""S"O)?UP" N = )"""S")%3.*"*0',(-" N = )"""S")%3.*"*0',(-" D Aperture!Q"S"4+.$0(0/"%20'+'," D Aperture!Q"S"4+.$0(0/"%20'+'," • f/2 on a 50 mm lens (N=2) 2 = 50/D (D=25 mm)! • 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)! • f/2 on a 100 mm lens (N=2) 2 = 100/D (D=50 mm)! ! !

• Doubling both the absolute aperture diameter (D) and the focal length (f) cancel; leaving the same relative 13! 14! aperture size (N). In this example, both lenses are f/2.!

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

• Irradiance (light on unit area of sensor per • Doubling D, increases the area by 4X.! second ) is proportional to the area of the ! ! – light falling per unit area decreases by 4X! aperture:! ! ! – Square of the aperture diameter D! • Doubling N reduces D by 2x, light by 4x! – Inverse square distance to the sensor (~ focal – Example: going from f/2.0 to f/4.0 ! length f)! • C"E"Q""O4%&H*+',"(-0".20/(&/0PW"+(1"./0." • Cuts the light by 4x! O-0'30"(-0"*+,-("(-.("3.'",0("(-/%&,-"+(P" – Cut the light by 2, increase N by "2! +'3/0.101"HL"FX5" • #1"(-0"4+1(.'30"(%"(-0"10'1%/"+1"4%&H*04W" (-0"./0."+'(0/103Y',"(-0"3%'0"+'3/0.101" HL"F""1%"(-0"*+,-(").**+',"20/"&'+("./0."

15! 4#5$#06#6'HL"FX! 16!

Main Side-Effect of Aperture! Depth of Field (preview)! • A point in the scene is • Depth of field (more on that later)! focused at a point on the sensor! • Doubling N (two f/stops) doubles depth of • Moving the sensor in z! field! • the depth where this happens is called the depth of focus! • this corresponds in the scene to a depth of field! • halving the aperture diameter (smaller) doubles the depth of field!

17! 18! Exposure ! Reciprocity!

• Exposure = Irradiance x Time! • Assume we know how much light we need ! – Aperture (f=stop) & Shutter Speed! • We have the choice of an infinity of shutter speed/ aperture pairs! • Reciprocity! 72#'60.#'#8")6&$#'-6')9%0-*#4':-%2'0*' #8")6&$#'%:-5#'06'+)*;'0*4'0*'0"#$%&$#' !"#!'20+1'06'9-;" • Irradiance (I)! – Controlled by aperture! – Lowering one f/stop doubles exposure! • What will guide our choice of a shutter speed?! – Freeze motion vs. motion blur, camera shake! – Lowering by two f/stop doubles DOF! • What will guide our choice of an aperture?! • Exposure time (T)! – Depth of field! – controlled by the shutter ! • Often we must compromise! – – doubling exposure time doubles H! Open more to enable faster speed (but shallow DoF)!

19! – doubling exposure time doubles motion blur! 20!

Trading off motion for DOF! 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!

21! 22!

Demo! Single Reflex Camera!

• Trade-offs affecting brightness! • Flash Demo (from Stanford)!

Nikon F4 ! (film camera)!

-82!??,/.2-+3151(.')%/4504&?3%&/101?31>Z[T23! >A?.22*0(1?0E2%1&/05-($*" 24! Single Reflex Camera! Summary Exposure !

• Aperture (f number) (depth of field control)! – Ratio between focal length &aperture diameter: # !diameter = f / ! – 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)! Nikon F4 ! – Gain applied to sensor! – In ISO, bigger number, more sensitive (100, 200, 400, 800, 1600)! (film camera)! Reciprocity between these three numbers, for a given exposure:# two degrees of freedom (2 things you can change)! 25!

Slide Credits/Resources!

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

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