Lecture 22: Cameras & Lenses III Computer Graphics and Imaging UC Berkeley CS184/284A, Spring 2017 Depth of Field Depth of Field From London and Upton From • Depth of field is the range of object depths that are rendered with acceptable sharpness in an image CS184/284A Ren Ng from subjects closer than “infinity” Depth of field circle of confusion by the F number, converge at a point in the focal plane. The area in front of and behind a focused regardless of the lens focal length. With subject in which the photographed image modern autofocus SLR cameras, focusing CircleFigure-15 of ConfusionRelationship Between for the IdealDepth Focal of Fieldappears sharp. In other words, the depth of is performed by detecting the state of Point and the Permissible Circle of sharpness to the front and rear of the Confusion and DepthSet of circle Field of confusion as the maximum focus in the image plane (focal plane) permissible blur spot on the image planesubject that where image blur in the focal using a sensor which is both optically will appear sharp under final viewing conditions • For printed photographs from 35mmplane film, falls within the limits of the equivalent (1:1 magnification) and 0.025mm (on negative) is typical permissible circle of confusion. Depth of positioned out of the focal plane, and • For digital image sensors, 1 pixel is typical (e.g. 1.4 micron for phones) field varies according to the lens’ focal automatically controlling the lens to bring • Larger if intended for viewing at web resolution, or if lens is poor length, aperture value and shooting the subject image within the depth of distance, so III] [Canon, EF Lens Work if these values are known, a focus area. Lens Ideal focal point rough estimate of the depth of field can be Figure-17 Relationship Between Depth of Fr on o t d calculated using the following formulas: Focus and Aperture f fi ep eld th 2 2 Re Front depth of field = d·F·a /(f + d·F·a) 50mm f/1.8 D ar ep of de 2 2 Aperture th fie pt Permissible of ld h Rear depth of field = d·F·a /(f — d·F·a) foc circle of confusion us f: focal length F: F number d: minimum f/1.8 Permissible circle of confusion circle of confusion diameter CS184/284A a:Ren subject Ng distance (distance from the Circle of confusion first principal point to subject) Depth of focus at maximum aperture Since all lenses contain a certain amount Aperture hyperfocal distance × Permissible of spherical aberration and astigmatism, Near point limiting shooting distance circle of confusion = they cannot perfectly converge rays from a distance hyperfocal distance + f/5.6 subject point to form a true image point shooting distance Depth of hyperfocal distance × focus at f/5.6 (i.e., an infinitely small dot with zero area). Far point limiting shooting distance distance = In other words, images are formed from a hyperfocal distance - shooting distance composite of dots (not points) having a (Shooting distance: Distance from focal plane to subject) Hyperfocal distance certain area, or size. Since the image Using the depth of field principle, as a becomes less sharp as the size of these If the hyperfocal distance is known, the lens is gradually focused to farther dots increases, the dots are called “circles following formulas can also be used: subject distances, a point will eventually of confusion.” Thus, one way of indicating In general photography, depth of field is be reached where the far limit of the the quality of a lens is by the smallest dot characterised by the following attributes: rear depth of field will be equivalent to it can form, or its “minimum circle of ቢ Depth of field is deep at short focal “infinity.” The shooting distance at this confusion.” The maximum allowable dot lengths, shallow at long focal lengths. point, i,e., the closest shooting distance size in an image is called the “permissible ባ Depth of field is deep at small at which “infinity” falls within the depth circle of confusion.” apertures, shallow at large apertures. of field, is called the hyperfocal distance. ቤ Depth of field is deep at far shooting The hyperfocal distance can be Permissible circle of confusion distances, shallow at close shooting determined as follows: The largest circle of confusion which still distances. appears as a “point” in the image. Image ብ Front depth of field is shallower Hyperfocal f 2 f: focal length F: F number distance = sharpness as sensed by the human eye is than rear depth of field. d•F number d: minimum circle of confusion closely related to the sharpness of the diameter Figure-16 Depth of Field and Depth of Focus actual image and the “resolution” of Minimum circle of confusion Thus, by presetting the lens to the human eyesight. In photography, image hyperfocal distance, the depth of field will sharpness is also dependent on the degree extend from a distance equal to half the Depth of field Depth of focus of image enlargement or projection hyperfocal distance to infinity. This distance and the distance from which the Far point Near point method is useful for presetting a large image is viewed. In other words, in depth of field and taking snapshots practical work it is possible to determine Front without having to worry about adjusting Rear Front depth of field depth depth of certain “allowances” for producing images focus the lens focus, especially when using a of field Near point distance which, although actually blurred to a Image Subject distance distance Rear wide-angle Photo-1 Hyperfocal Length Set certain degree, still appear sharp to the depth Condtion Far point distance of focus lens. (For observer. For 35mm single lens reflex Shooting distance example, when cameras, the permissible circle of confusion Focal plane the EF 20mm is about 1/1000~1/1500 the length of the f/2.8 USM is set film diagonal, assuming the image is Depth of focus to f/16 and the enlarged to a 5”×7” (12 cm × 16.5 cm) The area in front of and behind the focal shooting print and viewed from a distance of 25~30 plane in which the image can be distance is set cm/0.8~1 ft. EF lenses are designed to photographed as a sharp image. Depth of to the hyperfocal distance of ap- produce a minimum circle of confusion of focus is the same on both sides of the proximately 0.7m/2.3ft, all subjects within 0.035 mm, a value on which calculations image plane (focal plane) and can be a range of approximately 0.4m/1.3ft from for items such as depth of field are based. determined by multiplying the minimum the camera to infinity will be in focus.) 197 Depth of Field dN dS C A − = d A Depth of field Depth of focus N d d C S − F = dF A Circle of confusion, C f NN == DA 1 1 1 + = f f DF dF f 1 1 1 DF dF + = DS dS f DS dS 1 1 1 DN dN + = DN dN f DOF = D D F − N D f 2 D f 2 D = S D = S F f 2 NC(D f) N f 2 + NC(D f) − S − S − CS184/284A Ren Ng DOF Demonstration http://graphics.stanford.edu/courses/cs178/applets/dof.html CS184/284A Ren Ng Hyperfocal Distance The focus distance that maximizes the depth of field (such that infinity is at limit of acceptable sharpness) Hyperfocal DOF Hyperfocal distance H H/2 ∞ D f 2 D f 2 D = S D = S F f 2 NC(D f) N f 2 + NC(D f) − S − S − f 2 H As D ,D = H = + f, D = (Calculation omitted) F !1 S NC N 2 CS184/284A Ren Ng Ansel Adams, Mount Williamson Clearing Storm Other Focus / DOF Situations to Consider • How does sensor size affect defocus blur and DOF? • E.g. consider cell phone vs 35mm format sensors • For a given lens & f-stop, how does moving closer/ further from the subject (and adjusting focus onto subject) affect defocus / DOF of other objects? • In 1:1 macro, does focal length affect DOF? • What is the lens-sensor separation for hyperfocal condition, for full-resolution viewing vs web- resolution viewing? If you understand these, you understand lenses! CS184/284A Ren Ng Bokeh Bokeh Bokeh is the shape and quality of out-of-focus blur • For small, out-of-focus lights, bokeh takes on the shape of the lens aperture M Yashna, flickr, 40mm f/3.0 flickr, M Yashna, CS184/284A Ren Ng Bokeh diyphotography.net Heart-shaped bokeh? CS184/284A Ren Ng Bokeh Dino Quinzani, Leica Noctilux 50mm, f/0.95 Why does the bokeh vary across the image? CS184/284A Ren Ng The Psychological Effect of Shallow Depth of Field Dr. Joanne Liu, the president of Doctors without Borders, spoke on 10/7/15 in Geneva. Denis Balibouse/Reuters Hillary Clinton spoke during a campaign event at Cornell College in Mount Vernon, Iowa, on 10/7/15. Scott Morgan/Reuters https://www.youtube.com/watch?v=W5cbk0xVnzA Exposure Exposure • H = T x E • Exposure = time x irradiance • Exposure time (T) • Controlled by shutter (discussed last lecture) • Irradiance (E) • Power of light falling on a unit area of sensor • Controlled by f-stop (aperture and focal length) CS184/284A Ren Ng Exposure Controls in Photography Aperture size • Change the f-stop by opening / closing the aperture (if camera has iris control) Shutter speed • Change the duration the sensor pixels integrate light ISO gain • Change the amplification (analog and/or digital) between sensor values and digital image values CS184/284A Ren Ng Constant Exposure: F-Stop vs Shutter Speed Example: these pairs of aperture and shutter speed give equivalent exposure F-Stop 1.4 2.0 2.8 4.0 5.6 8.0 11.0 16.0 22.0 32.0 Shutter 1/500 1/250 1/125 1/60 1/30 1/15 1/8 1/4 1/2 1 If the exposure is too bright/dark, may need to adjust f-stop and/or shutter up/down.