What will be on the midterm?Marc LevoyComputer Science DepartmentStanford UniversityCS 178, Spring 2011© Marc LevoyGeneral information✦Monday, 7-9pm, Hewlett 200 (across the hall from us)✦closed book, no notes✦calculators ok, but you won’t need them✦on lectures and assigned chapters in London✦list of formulas will be provided on exam sheets✦practice problems in weekly assgns and sections this week✦attached are some review slides to help you study;treat these as a non-exhaustive summary of the course✦look also at the applets and the recap slides in each lecture✦emphasis will be on the concepts behind the formulas, and on the tradeoffs they imply for the photographer2© Marc LevoyImage formation✦the laws of perspective•especially natural perspective versus linear perspective✦pinhole imaging•tradeoff between aperture size and blur✦imaging uses lenses•Gauss’s ray tracing construction (be able to draw it)•tradeoffs between focal length, sensor size, and FOV•changing the focal length vrs changing the viewpoint✦exposure•tradeoffs between aperture, shutter speed, motion blur, and depth of field (study Eddy’s diagrams!)•tradeoffs that include ISO and noise covered later3© Marc LevoyLenses and apertures✦qualitative understanding of the approximations we make•geometrical optics instead of physical optics•spherical lenses instead of hyperbolic lenses•thin lens representation of thick optical systems*•paraxial approximation of ray angles*✦the Gaussian lens formula (know it and be able to use it)•changing the focal length vrs changing the subject distance •understand lens power and transverse magnification✦center of perspective (ignore the other thick lens terms),convex vrs concave lenses, real vrs virtual images✦depth of field formula•know its parts, how they vary, and the tradeoffs they imply•hyperfocal distance and how to use it4orange lecture slides and items starred (*) here are fairgame for extra-credit Q’s© Marc LevoyPractical photographic lenses✦aberrations (without the algebra)•be able to recognize them by a name or sketch•how is each one fixed? which are correctable in software?which are reducible by stopping down the aperture?✦other lens artifacts•be able to recognize them by a name or sketch•understand the geometry of vignetting, cos4 falloff*✦diffraction, sharpness, and MTF (qualitatively)•what are they, and what factors do they depend on?(some of this was covered in the sampling & pixels lecture)✦special-purpose lenses•principles (not detailed derivations) of telephoto, zoom5© Marc LevoySampling and pixels✦frequency representations of images*✦resolution and human perception•be able to manipulate FOV, dpi, retinal arc, cycles / degree✦sampling and aliasing•what is aliasing? when does it happen? (especially in a camera)•how can aliasing be avoided? prefiltering vrs postfiltering✦definition and uses of spatial convolution•understand the integral and summation forms of this equation•be able to work out a simple convolution, like two rects•no calculus manipulations will be required on the exam✦sampling versus quantization•understand how aliasing differs from quantization artifacts6© Marc LevoyAutofocus (AF)✦view cameras•understand eliminating vanishing points•understanding tilting the focal plane•understand real versus fake tilt-shift effects✦passive autofocus techniques•understand the principle of phase detection•understand the principle of contrast detection•when are they used? what are the tradeoffs?•don’t worry about the details of lenslets, ray geometry, etc.✦active autofocus techniques•tradeoffs between time of flight and triangulation•be able to manipulate the geometry of triangulation,at least for right-angle triangles7© Marc LevoyAutomatic exposure metering (AE)✦what makes metering hard?•understand (qualitatively) the dynamic range problem✦gamma transforms•what is it? when is it applied? what effect does it have?•when can you compare intensity levels in image files?✦metering technologies•what problems are caused by having few metering zones?•tradeoffs between typical shooting modes (A,P,Av,Tv,M)8© Marc LevoyImage stabilization (IS)✦what are the causes of camera shake?•and how can you avoid it (without having an IS system)?✦treating camera shake as a 2D convolution of the image•understand the geometry of this approximation✦image stabilization systems•be able to define mechnical, optical, electronic IS•understand the principles of lens-shift vrs sensor-shift IS•understanding the ray geometry in detail is not required•how much does stabilization help?•what is lucky imaging, and how can a photographer use it?9© Marc LevoyPhotons and sensors✦basic concepts (qualitatively)•photons, quantum efficiency, blooming, smearing•analog to digital conversion•relationship of gamma transforms to # of bits required•don’t worry about specific circuits✦how does aliasing and filtering apply to a digital camera?•fill factor, per-pixel microlenses, antialiasing filters•be able to explain how exposure time is a temporal prefilter✦color sensing technologies•be able to recognize them from a name or sketch•tradeoffs between the technologies (qualitatively)•what is demosaicing?10© Marc LevoyNoise and ISO✦what are the sources of noise in digital cameras?•be able to recognize them by a name or description•which ones grow with exposure time, or with temperature?•which ones can be fixed in software?•benefit of downsizing an image or averaging multiple shots✦signal-to-noise ratio and dynamic range•be able to apply the formulas correctly (we’ll give you a list)✦ISO•what is it, and how is it implemented in digital cameras?•tradeoffs between ISO and noise (study Eddy’s diagram from the image formation lecture!)11© Marc LevoyList of important formulas(will be replicated on exam sheets)12N =fAxixt=sinθisinθt=ntni1so+1si=1f MTyiyo= −sisoFOV = 2 arctan (h / 2 f )DTOT≈2NCU2f2 U ≥f2NC HSNR =µσ=P QetP Qet + D t + Nr2DR =saturation level - D tD t + Nr2SNR (dB) = 20