Optics II: Digital Photography, Lecture notes of Optics

Download Optics II: Digital Photography and more Lecture notes Optics in PDF only on Docsity! Digital Photography - Optics II Jae-Hyun Jung, Ph.D Schepens Eye Research Institute, Mass. Eye and Ear Department of Ophthalmology, Harvard Medical School Dalian Maritime University Summer Lecture Course 2019 Optics II: practical photographic lenses Lectures on Digital Photography Spring 2016 Marc Levoy Principal Engineer Google Research Professor, Emeritus Computer Science Department Stanford University Acknowledgement This lecture is based on  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) Chromatic aberration ✦ dispersion causes focal length to vary with wavelength • for convex lens, blue focal length is shorter ✦ correct using achromaticdoublet • strong positive lens + weak negative lens = weak positive compound lens • by adjusting dispersions, can correct at two wavelengths  MarcLevoy5 (wikipedia) red and blue have the same focal length  Marc Levoy The chromatic aberrations ✦ longitudinal (axial) chromatic aberration • different colors focus at different depths, creating colorful bokehs • appears everywhere in the image ✦ lateral (transverse) chromatic aberration • if blue image is closer to lens, it will also be smaller • only appears at edges of images, not in the center ✦ can reduce longitudinal by closing down the aperture 6 (Smith)  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 different properties9 (wikipedia) hyperboloidal lens spherical lens  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  Marc Levoy Coma ✦ magnification varies with ray height (distance from optical axis) 14 (ryokosha.com) (Hecht) Contrary to what I said in class, Florian Kainz has observed that coma is a noticeable problem in many lenses, especially wide-angle lenses, when doing astrophotography, i.e. stars (which are sharp points) in the corners of the field of view look like my illustrative image on this slide.  Marc Levoy Astigmatism ✦ transverse and sagittal rays focus at different depths ✦ my full eyeglass prescription • right: -0.75 -1.00 axis 135, left: -1.00 -0.75 axis 180 15 focus of sagittal rays focus of transverse rays (Pluta) Correcting astigmatism using a cylindrical lens (contents of whiteboard) ✦ ✦ for myopia + astigmatism, one needs a spherical lens + cylindrical lens, i.e. a lens with different radii of curvature in two perpendicular directions • in my right eye, first direction has focal length -1 /0.75 = -1.33 meters, and second direction has focal length -1 / 1.00 = -1.00 meters lens is then rotated around the optical axis before mounting in frame • in my case extrusion axis of second curvature is 135º (10:30 - 4:30 on the clock)  MarcLevoy16 19 uncorrected lens (toothwalker.org) anastigmat no astigmatism no astigmatism ✦ spherical lenses image planes onto curved surfaces T = tangential focus surface S = sagittal focussurface P = Petzval surface (field curvature only, if there were no astigmatism) ✦ anastigmat makes T = S at axis and one other rayheight  MarcLevoy Correcting for astigmatism and field curvature  Marc Levoy A spherical focus surface camera 20 [Son 2011]  Marc Levoy Distortion ✦ change in magnification with image position (a) pincushion (b) barrel ✦ closing down the aperture does not improve this 21 (Smith) (Kingslake) pincushion distortion • correctable in software  Marc Levoy Veiling glare ✦ contrast reduction caused by stray reflections ✦ can be reduced by anti-reflection coatings 24 in the outermost lens we don’t care about killing this reflection but we do care about killing this one Veiling glare ✦ contrast reduction caused by stray reflections ✦ can be reduced by anti-reflection coatings • based on interference, so optimized for one wavelength • to cover more wavelengths, use multiple coatings  MarcLevoy25 (wikipedia) Camera array with too much glare ✦ ✦ ✦ 12 ×8 array of 600 ×800 pixel webcams = 7,200 ×6,400 pixels goal was highest-resolution movie camera in the world failed because glare in inexpensive lenses led to poor contrast  MarcLevoy26 Stanford Multi-Camera Array  Marc Levoy Vignetting (a.k.a. natural vignetting) ✦ ✦ ✦ irradiance is proportional to projected area of aperture as seen from pixel on sensor, which drops as cos θ irradiance is proportional to projected area of pixel as seen from aperture, which also drops as cos θ irradiance is proportional to distance2 from aperture to pixel, which rises as 1/cos θ ✦ combining all these effects, light drops as cos4 θ 29 (Smith)  Marc Levoy Other sources of vignetting ✦ pixel vignetting due to shadowing inside each pixel (we’ll come back to this) 30 f/1.4 f/5.6 axial semifield optical vignetting from multiple lens elements, especially at wide apertures mechanical vignetting from add-on lens hoods (or filters or fingers) (toothwalker.org)  Marc Levoy Examples ✦ ✦ ✦ vignetting causes falloff in brightness towards edges of image vignetting affects the bokeh of out-of-focusfeatures vignetting is correctable in software (except for bokeh effects), but boosting pixel values worsens noise ✦ vignetting can be applied afterwards, for artistic effects 31 (toothwalker.org) (toothwalker.org) (wikipedia)  Marc Levoy Diffraction ✦ as wavelength decreases in the ripple tank (c a), propagation becomes more ray-like 34 illuminated by a (spread-out) laser beam & recorded directly on film varying the wavelength of waves passing through a slit in a ripple tank (Hecht) Airy rings ✦ ✦ if the illumination were a laser, a lens would produce this pattern but considering all wavelengths, the dark rings vanish, leaving a blur  MarcLevoy35 diffraction from a slit diffraction from a circular aperture: Airy rings (Hecht)  Marc Levoy Diffraction in photographic cameras ✦ well-corrected lenses are called diffraction-limited ✦ the smaller the aperture (A) (or the longer the wavelength), the larger the diffraction blur ✦ the longer the distance to the sensor ( f ), the larger the blur when it gets there ✦ thus, the size of the blur varies with N = f /A 36  Marc Levoy The Abbe diffraction limit ✦ where • λ = wavelength • NA = numerical aperture ≈ 1 / 2N ✦ Example: iPhone 6s when looking at green • λ = 550nm • N = f/2.2 • d = 1.45µ • pixels are 1.22µ wide, so the iPhone 6s would be roughly diffraction-limited if its lenses were free of aberrations 39 d  .61 NA  1.2 N   Marc Levoy Recap ✦ all optical systems suffer from veiling glare • anti-reflection coatings help ✦ all optical systems suffer from flare and ghosts • don’t point your camera at bright lights; use lens hoods ✦ vignetting arises from many sources • natural - falloff at the edges of wide sensors • optical - caused by apertures, lens barrels • mechanical - caused by wrong lens hoods, hands, straps • pixel - caused by shadowing inside pixel structures ✦ diffraction - blur that varies with N = f /A • avoid F-numbers above f/13 (for full-frame camera) • subjective image quality depends on both sharpness and contrast 40 Questions?  Marc Levoy Lens design software ✦ uses optimization to make good recipes better 41  Marc Levoy Telephoto lens ✦ the blue lens is replaced with the two green ones, thereby reducing the physical size of the lens assembly, while preserving its focal length (hence magnification) 44  Marc Levoy Lens combinations: telephoto 45 Nikon 500mm telephoto Opteka 500mm non-telephoto  Marc Levoy Lens combinations: zoom 46 Canon FD 24-35mm f/3.5 L manual focus lens ( F l a h demo) https://sites.google.com/a/google.com/ digital-photography/applets/telephoto- zoom-lens ✦ called optically compensated zoom, because the in-focus plane stays (more or less) stationary as you zoom ✦ to change focus, you move both lenses together  Marc Levoy Recap ✦ telephoto lenses separate focal length & back focal distance • for long focal length lenses, to reduce their physical size • for wide-angle lenses, to ensure room for the reflex mirror ✦ most modern zoom lenses are focus-compensated • as you zoom, they stay in focus 47 Questions?  Marc Levoy Slide credits ✦ Steve Marschner ✦ Fredo Durand ✦ Cole, A., Perspective, Dorling Kindersley, 1992. ✦ Kemp, M.,The Science of Art,Yale University Press, 1990. ✦ Hecht, E., Optics (4th ed.), Pearson / Addison-Wesley, 2002. ✦ Renner, E., Pinhole Photography (2nd ed.), Focal Press,2000. ✦ London, Stone, and Upton, Photography (9th ed.), Prentice Hall, 2008. ✦ D'Amelio, J., Perspective Drawing Handbook, Tudor Press, 1964. ✦ Dubery, F., Willats, J., Perspective and other drawing systems, Van Nostrand Reinhold, 1972. ✦ Kingslake, R. Optics in Photography, SPIE Press, 1992. ✦ Pamplona et al., “NETRA: Interactive Display for Estimating Refractive Errors and Focal Range”, Proc. SIGGRAPH2010. ✦ Son, H.S. et al., “Design of a spherical focal surface using close-packed relay optics”, Optics Express 19(17), 2011. ✦ http://dpreview.com 48