This Week Exposure (today) The Art, Science and Algorithms Camera - PDF document

This Week • Exposure (today) The Art, Science and Algorithms – Camera Basics – Simple Math of Photography • Metering • Zone System Exposure & Metering 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 Aperture Control : Amount of light falling shake on a unit area of sensor per second. Photo Credit: 3 4 Side-Effect of Shutter Speed Creative Shutter Speeds • Motion Blur Photo Credit: Pasant @Flickr • Halving the shutter doubles the motion blur. 3 seconds 4 seconds Bulb: 3-10 second 1/30 Panning 30 seconds 15 seconds Wikipedia Fast Shu:er Speed Slow Shu:er Speed Photo Credit: Wikipedia, Cooriander & Pasant @flickr 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 Photo Credit: Fredo Durand 7 8 Shutter Shutter • Simple leaf-shutter, typically Simple Leaf Shu3er only one speed 1. Shu:er plate 2. Aperture covered – Disposable cameras 3. Aperture during exposure • Typically long way from 4. Leaf blade 5. Catch mechanism sensor plate 6. Bu:erfly spring – 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 Diagram Credit: London, Stone Upton (2910) Diagram Credit: Wikipedia 9 10 Aperture regulates Light per Unit Area Your Best Friend • Light and Distance adheres to • Use a tripod – it will enhance sharpness the Inverse Square Law: – How Area A (light intensity per – Avoid camera shake unit area), changes with distance. – Light Intensity falls of within the square of the distance. • Inversely proportional to the square of the distance • Lenses regulates the lens opening by the f-number (or f-stop) of the aperture. – At a given f/stop lenses allow the same amount of light • e.g., @ f/4 all lenses allow the same amount of light Photo Credit: 11 12

2 N = f ⎛ f ⎞ Aperture Simple Geometry Area = π ⎜ ⎟ ⎝ 2N ⎠ D • We call this Irradiance: – Light on unit area of sensor per second ) is proportional to size of the area . – Inversely proportional to the square distance to the 2 sensor 1 – Proportional to the square of the diameter of the opening – Inverse square distance to the sensor (~ focal length f) 1 1 • 2 x D (doubling the aperture), its area § Twice the diameter means four times the area. N = f (hence the light that can get through it) § Stop down à higher f-number D increases by 4X (because area) § Fast lens allowing a low f-number à you can • [same focal length] use lower shutter speeds in lower light situation • As the distance to the sensor is doubled , § Light captured by a lens is proportional to the area of the aperture. (circle) the area intersecVng the cone increases 2 2 ⎛ ⎞ ⎛ ⎞ Area = π D f by 4 so the light falling per unit area = π ⎜ ⎟ ⎜ ⎟ ⎝ 2 ⎠ ⎝ 2N ⎠ 13 14 decreases by 4X [ changing focal length ] Wide Open (full) Aperture Aperture f = focal length f = focal length D = f D = f D = diameter of opening D = diameter of opening # # General Idea: • # = f # ∗ D = f Lens opening given by # f-number • A relative aperture size e.g., “#”, called an f-number, To maintain the same f-number a longer lens needs a written f/#, reflects the fact that it is computed by D larger diameter to produce the same illuminance dividing the focal length by the absolute aperture (D). ( lumen/m 2 ) on focal plane (longer lenses has a • Examples: # ∗ D = f magnifying effect) – Aperture of a 100 mm lens at f/2 is a • Circle of diameter 100/2 = 50mm. – Aperture of a 50 mm lens at f/2 is a [example coming too see this better] • Circle of diameter 50/2 = 25mm • At a given f/stop, e.g., @ f/4 all lenses allow the same We will continue to use amount of light. – Aperture of a 100 mm lens at f/2 is a • Greater f-number (smaller hole) • Circle of diameter 100/2 = 50mm. – And less light per unit area reaches the image plane – Aperture of a 50 mm lens at f/2 is a (irradiance), watts/m 2 lower f-number long lenses fat & expensive • Circle of diameter 50/2 = 25mm More ``glass’’ required. h:ps://en.wikipedia.org/wiki/F-number 15 16 N = f N = f-number = (f/#) Allowing Light with Aperture Aperture f = focal length D D = diameter opening • 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) 2x 1x Doubling both the absolute aperture diameter (D) and the focal N = f N = f-number = (f/#) length(f) cancel (b/c reciprocity); leaving the same relative aperture size f = focal length (N). In this example, both lenses are f/2. D D = diameter opening 17 18 [ 50 * (X) / (25 * (X) ]

N = f D = f Aperture Side-Effect of Aperture D N • Doubling D, increases the area by 4X. • Doubling N (two f/stops) doubles depth of – light falling per unit area decreases by 4X field • Doubling N reduces D by 2x , light is – Reduced by 4x – Example: going from f/2.0 to f/4.0 (2 stops) 2 2 • Cuts the light by 4x Area = π D ⎛ ⎞ ⎛ f ⎞ = π ⎜ ⎟ ⎜ ⎟ 2 2N ⎝ ⎠ ⎝ ⎠ • Cut the light (2x) instead of (4x) what should the factor of N be? (has to be smaller than 2, since 2 will cut it by 4 x the light) – (1 < N < 2) – √ 2 = 1.41 ( 1 stop) Photo Credit: London, Stone Upton 19 20 Depth of Field (recall) Exposure ( H ) : = I rradiance * T ime • A point in the scene is focused Determined by : Aperture & Shutter Speed at a point on the sensor • Reciprocity • Moving the sensor in z – Equivalence of Relationship of shutter and aperture. • the depth where this happens • Irradiance (I) – (light per unit area) Aperture is called the depth of focus Controlled (by sensor) • this corresponds in the scene – Controlled by aperture to a depth of field (scene) – Aperture also affect DOF • Halving the aperture diameter • Time (T) (smaller) doubles the depth of field – controlled by the shutter – doubling exposure time doubles motion blur • Exposure Stays Constant by (more next) • Lowering one f/stop (cramp, close) AND doubles Time Diagram Credit: London, Stone Upton 21 22 Stops: Reciprocity Tool Aperture and Shutter Stops • Increase exposure by 1 stop means capturing twice as much light as the previous stop . • Example: • Apertures 1 stops differ by a 1.41 factor (sqrt(2)) • f/1, f/1.4, f/2 (sqrt(2)*sqrt(2), …. h:ps://www.flickr.com/photos/hamed/2476599906/ 23 24

Reciprocity Trading off motion for DOF • Assume we know how much light we need • We have infinite choices 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, diffraction limit • Often we must compromise – Open more to enable faster speed (but shallow DoF) h:p://petapixel.com/2012/06/11/whats-the-f-number-of-the-human-eye/ 25 26 Sensitivity (ISO) • Third variable for exposure • Linear effect (200 ISO needs half the light as 100 ISO) • 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) Diagram Credit: Dpreview next 27 28 Demo Summary Exposure • Aperture (f number) (depth of field control) • Trade-offs – Ratio between focal length &aperture diameter: � diameter = f / <f number> affecting – 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 brightness – A good standard lens has max aperture f/1.8 (less expensive f/3.5) • Flash Demo • Shutter speed (motion blur control) – In fraction of a second (from • Examples: 1/30, 1/60, 1/125, 1/250, 1/500 (factor of 2) – Rule of thumb: Stanford) • 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 (50, 100, 200, 400, 800, 1600) Reciprocity between these three numbers, for a given exposure: � two degrees of freedom (2 things you can change) h:p://graphics.stanford.edu/courses/cs178- 10 /applets/exposure.html 29