Autofocus AF CS 178 Spring 2011 Begun 4 19 11 Finished 4 21 Marc Levoy Computer Science Department Stanford University Outline viewfinders and manual focusing view cameras and tilt shift lenses passive autofocusing phase detection contrast detection autofocus modes lens actuators active autofocusing time of flight triangulation 2 Marc Levoy Single lens reflex SLR with autofocus 3 Nikon F4 film camera Marc Levoy Single lens reflex SLR with autofocus 4 with mirror down some light is shunted to AF remainder reflected up to form image on diffuse focusing screen autoexposure AE viewfinder focusing screen seen upright in viewfinder with same perspective as main sensor AE light meter also sees focusing screen mirror rotates to expose main sensor when taking picture autofocus AF Marc Levoy Viewfinder coverage magnification coverage is fraction of sensor image covered by the viewfinder i e h1 h2 area1 or sometimes area2 magnification is apparent size of objects in viewfinder relative to unaided eye i e 1 tan 1 2 tan 2 with a 50mm lens on camera 5 example Canon 5D II coverage 98 magnification 0 71 2 1 h1 Although I didn t mention this in class to save time one must be careful of sensor crop factors when interpreting viewfinder magnification figures that one sees in reviews of cameras h2 For example the published specs for a Canon 7D which uses an APS C sized sensor as opposed to the full frame sized sensor of the Canon 5DII are coverage 100 magnification 1 0x However a 50mm lens on the 7D acts like an 80mm lens on a fullframe camera due to the 1 6x crop factor of the 7D The way to compensate for this crop factor is to multiply the claimed magnification by 1 1 6x In other words the 7D s 1 0x is comparable to 0 62x on the 5DII But the 5DII s magnification is 0 71x Thus after this correction the 7D s viewfinder isn t so magnificant See http www neocamera comfeature viewfinder sizes php Marc Levoy Electronic viewfinders point and shoot SLR live view electronic viewfinder pros same view as lens without need for reflex mirror can tone map to show effect of chosen exposure cons poor resolution and low dynamic range relative to optical Is the display being tone mapped Will the shot look like this 6 Marc Levoy Non thru the lens optical viewfinder point and shoot Fuji X100 new and its hybrid viewfinder pro infinite resolution and dynamic range like SLR viewfinder retro cool con different perspective view than main lens sees low magnification appears small to eye 7 Marc Levoy Manual rangefinder accurate painstaking different perspective view than main lens sees triangulation concept widely applicable Leica M9 8 digital full frame Adams Marc Levoy Twin lens reflex with focusing screen medium format 2 2 film only no longer manufactured medium format still exists but only in SLRs different perspective view than main lens sees Adams 9 Marc Levoy View camera with focusing screen large format 4 5 or 8 10 Sinar with digital back film or scanned digital ground glass focusing screen dim hard to focus inverted image 10 Sinar 4 5 Adams Marc Levoy View camera movements London As I mentioned in class I forgot to write on the whiteboard that you should read London s chapter 15 on the view camera from which this illustration is taken 11 Marc Levoy Off axis perspective 12 London e in l b a l i a av p o w sh No o t Pho Marc Levoy Tilted focal plane Scheimpflug condition London 13 cannot be done after the photograph is taken Marc Levoy Ansel Adams Railroad Tracks Ansel Adams Monument Valley Tilt shift lenses Canon TS E 90mm lens 16 Marc Levoy Tilt shift lenses Canon TS E 90mm lens 17 Marc Levoy The miniature model look Canon TS E 24mm II 18 simulates a macro lens with a shallow depth of field hence makes any scene look like a miniature model Marc Levoy The miniature model look Canon TS E 24mm II 19 simulates a macro lens with a shallow depth of field hence makes any scene look like a miniature model Marc Levoy SynthCam 4 focusing points single frame 4 points tilted focal plane single frame 4 points tilted focal plane 3 nearly co linear points miniature model effect Not a tilt shift lens http www tiltshiftphotography net 26 gradient blur in Photoshop Marc Levoy Not a tilt shift lens http www tiltshiftphotography net 27 gradient blur in Photoshop original Marc Levoy Not a tilt shift lens http www tiltshiftphotography net gradient blur in Photoshop Q Is this fake identical to the output of a real tilt shift lens 28 Marc Levoy SynthCam photographically produced fake tilt shift Canon 5DII with TS E 24mm Recap the optical viewfinder in a single lens reflex camera SLR gives you the same perspective as your final photograph has high resolution and no limit on dynamic range can t tone map to show effect of exposure view cameras let you eliminate vanishing points view cameras also let you tilt the focal plane 31 this can alternatively be done in Photoshop this cannot be done in Photoshop although you can fake it tilt shift lenses provide both functions for SLRs Que s t ions Marc Levoy Passive autofocus phase detection Flash demo http graphics stanford edu courses cs178 applets autofocuspd html 32 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 disparity between these images is in focus Marc Levoy Passive autofocus contrast detection Flash demo http graphics stanford edu courses cs178 applets autofocuscd html 33 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 gradient of pixel values Marc Levoy Most SLRs use phase detection Canon Canon 7D distance between subimages allows lens to jump directly into focus without hunting 34 equivalent to depth from stereo in computer vision many AF points manual or automatic way to choose among them closest scene feature is often the best but also consider position in FOV Marc Levoy Phase detection is like depth from stereo contents of whiteboard 35 start by finding corresponding features in two views of an object express each correspondance as a pair of pixels dots knowing the separation and aim angles of the two cameras the positions of these two pixels can be projected arrows until