This Week ! • Exposure ! The Art, Science and Algorithms – Camera Basics ! – Simple Math ! of Photography ! • Zone System ! Exposure & Metering ! CSCI 4900/6900 ! Maria Hybinette ! 1 ! 2 ! Maria Hybinette ! Exposure ! Shutter Speed ! • Controls how long the sensor is exposed to light ! • Controls light to digital sensor (or film) ! • Linear effect on exposure until sensor saturates ! • Two main controls (parameters): ! • Denoted in fraction of a second: ! – Shutter Speed ! – 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 – Rule of thumb: shortest exposure: 1/ focal length ! second ! • 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 ! • Motion Blur ! :-%(%";/04+(!""" :.1.'("<6*+3=/" • Halving the shutter doubles the motion blur. ! 3 seconds 4 seconds Bulb: 3-10 second >?@A":.''+'," @A"103%'41" >B"103%'41" Wikipedia 6.1("7-&80/"72004"" 7*%9"7-&80/"72004"" 6 ! 5 !
Effect of shutter speed ! Shutters ! • Central Shutters ! • Freezing motion ! – Mounted within lens assembly (some in-front of – Rule of thumb ! lens, early cameras) ! Walking people � Running people � Car � Fast train � – Leaf mechanism generally used for this (see next slide for a simple version) 1/125 th second ! – Diaphragm shutter (thin blades) 1/500 th second ! • Focal plane shutters near the focal plane and moves to uncover sensor ! 1/125 � 1/250 � 1/500 � 1/1000 � • 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" • Typically long way from F5 G0.)"H*.40" B5 ;.(3-"$03-.'+1$" sensor plate ! I5 J&80/KL"12/+'," – Slow to open & close ! • edges less exposure time ! • Leaf-shutter, typically only one speed ! – Doesn’t matter for DOF ! • Near focal plane shutters ! – Disposable cameras ! – Faster not to affect DOF ! • Typically long way from sensor plate ! – Slit action to ensure all of sensor get same amount of light ! – Slow to open & close = edges less exposure time ! 9 ! 10 ! M+40"N20'"O)&**P" Your Best Friend ! Aperture ! N = f R"S")T'&$H0/""S"O)?UP" • Use a tripod – it will enhance sharpness ! )"""S")%3.*"*0',(-" D Q"S"4+.$0(0/"%20'+'," • Lens opening ! – Avoid camera shake ! • A relative aperture size e.g,. “#”, called an f- number, written f/#, reflects the fact that it is computed by dividing the focal length by the absolute aperture (D). ! – Example : if the focal length is 4 times the diameter, f-number is f/4, N=4 ! – Greater f-number (smaller hole), less light per unit area reaches the image plane (irradiance), watts/m 2 ! – To maintain the same f-number a longer lens needs a larger diameter to produce the same illuminance ( lumen/m 2 ) on focal plane (longer lenses has a magnifying effect) [example coming] ! *%90/")T'&$H0/"*%',"*0'101").("V"0E20'1+D0" 7(%22+',"Q%9'" 11 ! 12 !
N = f N = f R"S")T'&$H0/""S"O)?UP" R"S")T'&$H0/""S"O)?UP" Aperture ! Aperture ! )"""S")%3.*"*0',(-" )"""S")%3.*"*0',(-" D D Q"S"4+.$0(0/"%20'+'," 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 aperture size (N). In this example, both lenses are f/2. ! 13 ! 14 ! N = f N = f 2 2 # f & # f & Aperture ! Aperture ! Area = " Area = " % ( % ( D D $ 2N ' $ 2N ' • 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: ! • Doubling N reduces D by 2x, light by 4x ! – Square of the aperture diameter D ! – 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." 4#5$#06#6' HL"FX ! 15 ! 16 ! Main Side-Effect of Aperture ! Depth of Field (preview) ! • A point in the scene is • Depth of field (more on that later) ! focused at a point on the sensor ! • Doubling N (two f/stops) doubles depth of • Moving the sensor in z ! field ! • the depth where this happens is called the depth of focus ! • this corresponds in the scene to a depth of field ! • halving the aperture diameter (smaller) doubles the depth of field ! 17 ! 18 !
Exposure ! Reciprocity ! • Assume we know how much light we need ! • Exposure = Irradiance x Time ! • We have the choice of an infinity of shutter speed/ – Aperture (f=stop) & Shutter Speed ! aperture pairs ! • Reciprocity ! 72#'60.#'#8")6&$#'-6')9%0-*#4':-%2'0*' #8")6&$#'%:-5#'06'+)*;'0*4'0*'0"#$%&$#' !"#! '20+1'06'9-; " • Irradiance (I) ! – Controlled by aperture ! • What will guide our choice of a shutter speed? ! – Lowering one f/stop doubles exposure ! – Freeze motion vs. motion blur, camera shake ! – Lowering by two f/stop doubles DOF ! • What will guide our choice of an aperture? ! • Exposure time (T) ! – Depth of field ! • Often we must compromise ! – controlled by the shutter ! – Open more to enable faster speed (but shallow DoF) ! – doubling exposure time doubles H ! – doubling exposure time doubles motion blur ! 19 ! 20 ! Trading off motion for DOF ! Sensitivity (ISO) ! • third variable for exposure ! • film : trade sensitivity for grain ! • digital : trade sensitivity for noise ! – multiply signal before analog-to-digital conversion ! – linear effect (200 ISO needs half the light as 100 ISO) ! more in noise lecture ! 21 ! 22 ! Demo ! Single Reflex Camera ! • Trade-offs affecting brightness ! • Flash Demo (from Stanford) ! Nikon F4 ! (film camera) ! -82!??,/.2-+3151(.')%/4504&?3%&/101?31>Z[T >A ?.22*0(1?0E2%1&/05-($*" 23 ! 24 !
Single Reflex 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) ! 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 ! 27 !
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