optics2-08apr10-150dpi-med (1)

optics2-08apr10-150dpi-med (1) - Optics II: practical...

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Unformatted text preview: Optics II: practical photographic lenses CS 178, Spring 2010 Marc Levoy Computer Science Department Stanford University ! Marc Levoy Outline ! why study lenses? ! thin lenses graphical constructions, algebraic formulae ! thick lenses lenses and perspective transformations ! depth of Feld ! aberrations & distortion ! vignetting, glare, and other lens artifacts ! diffraction and lens quality ! special lenses telephoto, zoom 2 ! Marc Levoy Lens aberrations ! chromatic aberrations ! Seidel aberrations, a.k.a. 3 rd order aberrations arise because of error in our 1 st order approximation spherical aberration oblique aberrations Feld curvature distortion 3 sin ! " ! # ! 3 3! + ! 5 5! # ! 7 7! + ... $ % & ' ( ) ! Marc Levoy Dispersion ! index of refraction varies with wavelength higher dispersion means more variation amount of variation depends on material index is typically higher for blue than red so blue light bends more 4 (wikipedia) ! Marc Levoy Chromatic aberration ! dispersion causes focal length to vary with wavelength for convex lens, blue focal length is shorter ! correct using achromatic doublet strong positive lens + weak negative lens = weak positive compound lens by adjusting dispersions, can correct at two wavelengths 5 (wikipedia) red and blue have the same focal length ! Marc Levoy The chromatic aberrations ! change in focus with wavelength called longitudinal (axial) chromatic aberration appears everywhere in the image ! if blue image is closer to lens, it will also be smaller called lateral (transverse) chromatic aberration only appears at edges of images, not in the center ! can reduce longitudinal by closing down the aperture 6 (Smith) Comment on closing down the aperture fixed 5/1/10. 2nd comment on lateral aberration edited on 5/9/10. ! Marc Levoy Examples ! other possible causes demosiacing algorithm per-pixel microlenses lens fare 7 (wikipedia) (toothwalker.org) lateral longitudinal correctable in soFtware not ! Marc Levoy Software correction of lateral chromatic aberration ! Panasonic GF1 corrects for chromatic aberration in the camera (or in Adobe Camera Raw) need focal length of lens, and focus setting 8 Q . W h y d o n t h u m a n s s e e c h r o m a t i c a b e r r a t i o n ? ! Marc Levoy Spherical aberration ! focus varies with ray height (distance from optical axis) ! can reduce by stopping down the aperture ! can correct using an aspherical lens ! can correct for this and chromatic aberration by combining with a concave lens of a different index 9 (wikipedia) ! Marc Levoy Examples 10 Canon 135mm f/2.8 soft focus lens sharp soft focus (Canon) ! Marc Levoy Hubble telescope 11 before correction after correction Hubble Space Telescope Eagle Nebula (NASA) ! Marc Levoy Focus shift ! Canon 50mm f/1.2 L 13 (wikipedia) (diglloyd.com) focused at f/1.2 !!...
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optics2-08apr10-150dpi-med (1) - Optics II: practical...

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