Autofocus AF CS 178 Spring 2009 Marc Levoy Computer Science Department Stanford University Announcements from whiteboard bring cameras and student IDs to section this week 2 so you can get into Green Library 2009 Marc Levoy Outline viewfinders and manual focusing active autofocusing time of flight triangulation passive autofocusing phase detection contrast detection 3 autofocus modes lens actuators 2009 Marc Levoy Large format camera with focusing screen 4 5 or 8 10 formats film or scanned digital ground glass focusing screen dim hard to focus inverted image Adams 4 Sinar 4 5 2009 Marc Levoy Twin lens reflex with focusing screen older medium format cameras 2 2 film different perspective view than main lens sees Adams 5 2009 Marc Levoy Manual rangefinder accurate painstaking 6 different perspective view than main lens sees triangulation concept widely applicable Leica Adams 2009 Marc Levoy Single lens reflex with viewfinder 7 image formed on focusing screen seen upright through viewfinder same view as main lens mirror must be moved quickly to take picture manual or autofocus Nikon F4 2009 Marc Levoy Active autofocus time of flight Goldberg SONAR Sound Navigation and Ranging Polaroid system used ultrasound 50KHz 8 well outside human hearing 20Hz 20KHz limited range stopped by glass hardware salavaged and re used in amateur robotics 2009 Marc Levoy Active autofocus triangulation Goldberg 9 infrared IR LED flash reflects from subject angle of returned reflection depends on distance fails on dark or shiny objects 2009 Marc Levoy Active autofocus triangulation 10 situation from previous slide shown in plan view 2009 Marc Levoy Sidebar laser triangulation rangefinding 11 laser sheet illuminates a curve on the object distance from left edge of image gives distance from laser move object or sweep laser to create range image z x y 2009 Marc Levoy Scanning Michelangelo s David 480 range images 2 billion polygons 22 people 30 nights http graphics stanford edu projects mich 12 2009 Marc Levoy Uses of the 3D model interactive kiosk physical replica 15 2009 Marc Levoy Uses of the 3D model scientific studies 16 2009 Marc Levoy commonly used to digitize physical maquettes to ease creation of 3D CG models 3D model mocap rendered Passive autofocus phase detection Goldberg 19 as the lens moves ray bundles from an object converge to a different point in the camera and change in angle this change in angle causes them to refocus through two lenslets to different positions on a separate AF sensor a certain spacing between these double images is in focus 2009 Marc Levoy Passive autofocus phase detection 20 situations at three focus settings from previous slide drawn in different colors and shown in a single drawing at bottom is depth from stereo in computer vision by finding corresponding object features in two views 2009 Marc Levoy Passive autofocus contrast detection Goldberg To reiterate something I said in class modern point and shoot cameras use contrast detection but they don t use 3 CCDs placed at different depths like this They use only the main sensor and they hunt around more than would be required here for the lens setting that produces the most contrasty image as recorded by that sensor 21 sensors at different image distances will see the same object as contrasty if it s in focus or of low contrast if it s not move the lens until the contrasty subimage falls on the middle sensor which is conjugate to the camera s main sensor compute contrasty ness using local differences of pixel values 2009 Marc Levoy Most SLRs use phase detection Canon distance between subimages allows lens to move directly into focus without hunting 22 equivalent to depth from stereo in computer vision many AF points complicated algorithms for choosing among them generally use closest point but also consider position in FOV 2009 Marc Levoy Most DSCs use contrast detection howstuffworks com uses main camera sensor requires repeated measurements as lens moves which are captured using the main sensor 23 equivalent to depth from focus in computer vision slow requires hunting suffers from overshooting it s ok if still cameras overshoot but video cameras shouldn t 2009 Marc Levoy Autofocus modes AI servo Canon Continuous servo Nikon continues autofocusing as long as shutter is pressed halfway predictive tracking so focus doesn t lag objects moving axially focusing versus metering trap focus find closest and furthest object set focus and N accordingly overriding autofocus 24 trigger a shot if an object comes into focus Nikon depth of field focusing autofocus first then meter on those points manually triggered autofocus AF ON in Canon 2009 Marc Levoy Lens actuators Canon ultrasonic motor USM Canon 25 2009 Marc Levoy Slide credits 26 Goldberg N Camera Technology The Dark Side of the Lens Academic Press 1992 Canon EF Lens Work III The Eyes of EOS Canon Inc 2004 Adams A The Camera Little Brown and Co 1980 Kerr D A Principle of the Split Image Focusing Aid and the Phase Comparison Autofocus Detector in Single Lens Reflect Cameras 2009 Marc Levoy