Autofocus (AF)Marc LevoyComputer Science DepartmentStanford UniversityCS 178, Spring 2011Begun 4/19/11. Finished 4/21.! Marc LevoyOutline!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•triangulation2! Marc LevoySingle lens reflex (SLR) with autofocus3Nikon F4(film camera)! Marc LevoySingle lens reflex (SLR) with autofocus!with mirror down, some light is shunted to AF!remainder reflected up to form image on diffuse 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 picture4viewfinderautofocus (AF)autoexposure (AE)focusing screen! Marc LevoyViewfinder coverage & magnification!coverage is fraction of sensor image covered by the viewfinder, i.e.!magnification is apparent size of objects in viewfinder relative to unaided eye, i.e.with a 50mm lens on camera!example: Canon 5D II•coverage = 98%•magnification = 0.71!5h1h2h1h2or sometimes area1area2!2!1!1!2"tan!1tan!2Although 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.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 = 1.0xHowever, a 50mm lens on the 7D acts like an 80mm lens on a full-frame 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 LevoyElectronic viewfinders!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?6point-and-shootSLR “live view”electronic viewfinder! Marc LevoyNon-thru-the-lens optical 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)7point-and-shootFuji X100 (new) and its hybrid viewfinder! Marc LevoyManual rangefinder !accurate!painstaking!different perspective viewthan main lens sees!triangulation conceptwidely applicable8(Adams)!Leica M9(digital full-frame)! Marc LevoyTwin-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 sees9(Adams)! Marc LevoyView camerawith focusing screen!large format: 4!5” or 8!10”•film or scanned digital!ground glass focusing screen•dim•hard to focus•inverted image10Sinar 4!5(Adams)Sinar with digital back! Marc LevoyView camera movements11(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.! Marc LevoyOff-axis perspective12(London)Now available inPhotoshop !!! Marc LevoyTilted focal plane!cannot be done after the photograph is taken13(London)• Scheimpflug conditionAnsel Adams, Railroad TracksAnsel Adams, Monument Valley! Marc LevoyTilt-shift lenses16Canon TS-E90mm lens! Marc LevoyTilt-shift lenses17Canon TS-E90mm lens! Marc LevoyThe “miniature model” look18Canon TS-E24mm II!simulates a macro lens with a shallow depth of field,hence makes any scene look like a miniature model! Marc LevoyThe “miniature model” look!simulates a macro lens with a shallow depth of field,hence makes any scene look like a miniature model19Canon TS-E24mm IISynthCam, 4 focusing pointssingle frame4 points,tilted focal planesingle frame4 points, tilted focal plane3 nearly co-linear points, miniature-model effect! Marc LevoyNot a tilt-shift lens!gradient blurin Photoshop26(http://www.tiltshiftphotography.net/)! Marc LevoyNot a tilt-shift lens!gradient blurin Photoshop27(http://www.tiltshiftphotography.net/)original! Marc LevoyNot a tilt-shift lens!gradient blurin Photoshop28(http://www.tiltshiftphotography.net/)Q. Is this “fake” identical to the output of a real tilt-shift lens?SynthCam: photographically-produced fake tilt-shiftCanon 5DII with TS-E 24mm! Marc LevoyRecap!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•this can alternatively be done in Photoshop!view cameras also let you tilt the focal plane•this cannot be done in Photoshop, although you can fake it!tilt-shift lenses provide both functions for SLRs31Questions?! Marc LevoyPassive autofocus: phase detection!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”32(Flash demo)http://graphics.stanford.edu/courses/cs178/applets/autofocuspd.html! Marc LevoyPassive autofocus: contrast detection!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 values33(Flash demo)http://graphics.stanford.edu/courses/cs178/applets/autofocuscd.html! Marc LevoyMost SLRs use phase detection!distance between subimages allows lensto jump directly into focus, without hunting•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 FOV34(Canon)Canon 7D! Marc LevoyPhase detection is like depth from stereo(contents of whiteboard)!start by finding corresponding features in two views of an