optics1-06apr10-150dpi-med (1)

optics1-06apr10-150dpi-med (1) - Optics I lenses and...

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Optics I: lenses and apertures CS 178, Spring 2010 Marc Levoy Computer Science Department Stanford University
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! Marc Levoy Outline ! why study lenses? ! thin lenses graphical constructions, algebraic formulae ! thick lenses lenses and perspective transformations ! depth of field ! aberrations & distortion ! vignetting, glare, and other lens artifacts ! diffraction and lens quality ! special lenses telephoto, zoom 2
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! Marc Levoy Cameras and their lenses 3 single lens reflex (SLR) camera digital still camera (DSC), i.e. point-and-shoot
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! Marc Levoy Cutaway view of a real lens 4 Vivitar Series 1 90mm f/2.5 Cover photo, Kingslake, Optics in Photography
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! Marc Levoy Lens quality varies ! Why is this toy so expensive? EF 70-200mm f/2.8L IS USM $1700 ! Why is it better than this toy? EF 70-300mm f/4-5.6 IS USM $550 ! Why is it so complicated? 5 (Canon)
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Stanford Big Dish Panasonic GF1 Leica 90mm/2.8 Elmarit-M prime, at f/4 $2000 Panasonic 45-200/4-5.6 zoom, at 200mm f/4.6 $300
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! Marc Levoy Zoom lens versus prime lens 7 Canon 100-400mm/4.5-5.6 zoom, at 300mm and f/5.6 $1600 Canon 300mm/2.8 prime, at f/5.6 $4300
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! Marc Levoy Physical versus geometrical optics ! light can be modeled as traveling waves ! the perpendiculars to these waves can be drawn as rays ! diffraction causes these rays to bend, e.g. at a slit ! geometrical optics assumes ! ! 0 no diffraction in free space, rays are straight (a.k.a. rectilinear propagation) 8 (Hecht)
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! Marc Levoy Physical versus geometrical optics (contents of whiteboard) ! in geometrical optics, we assume that rays do not bend as they pass through a narrow slit ! this assumption is valid if the slit is much larger than the wavelength ! physical optics is a.k.a. wave optics 9
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! Marc Levoy Snell’s law of refraction ! as waves change speed at an interface, they also change direction ! index of refraction n is defined as the ratio between the speed of light in a vaccum / speed in some medium 10 (Hecht) x i x t = sin ! i sin ! t = n t n i later we will use n sin i = n’ sin i’ (for indices n and n’ and i and i’ in radians) Sudden jump to n sin i notation clarified on 5/1/10.
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! Marc Levoy Typical refractive indices ( n ) ! air = 1.0 ! water = 1.33 ! glass = 1.5 - 1.8 ! when transiting from air to glass, light bends towards the normal ! when transiting from glass to air, light bends away from the normal ! light striking a surface perpendicularly does not bend 11 (Hecht) mirage due to changes in the index of refraction of air with temperature
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! Marc Levoy Q. What shape should an interface be to make parallel rays converge to a point? A. a hyperbola ! so lenses should be hyperbolic! 12 (Hecht)
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! Marc Levoy Spherical lenses ! two roughly fitting curved surfaces ground together will eventually become spherical ! spheres don’t bring parallel rays to a point this is called spherical aberration nearly axial rays (paraxial rays) behave best 13 (Hecht) (wikipedia)
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! Marc Levoy Examples of spherical aberration 14 (gtmerideth) (Canon) As I mentioned in class, a spherically aberrant image can be thought of as a sharp image (formed by the central rays) + a hazy image (formed by the marginal rays). The look is quite different than a misfocused image, where nothing is sharp. Some people compare it to photographing through a silk stocking. I’ve never tried this.
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