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Understanding Film-like Photography: From Pixels to Rays

This course explores the principles of film-like photography, focusing on the understanding of how light rays interact with lenses and sensors. Topics covered include focal length, thin lens law, lens flaws, color sensing, polarization, and the limitations of film-like photography methods in digital photography.

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Understanding Film-like Photography: From Pixels to Rays

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  1. Course 15: Computational Photography Course 15: Computational Photography A.3: Understanding Film-like Photography Tumblin

  2. Computational Photography Computational Photography A3:UnderstandingFilm-Like Photographyor ‘from 2D Pixels to 4D Rays’(10 minutes) Jack Tumblin Northwestern University

  3. Naïve, Ideal Film-like Photography Well-Lit 3D Scene: Sensor: a film emulsion, : or a grid of light meters (pixels) Ray ‘Center of Projection’ Position (x,y) Angle(,) 2D Sensor: Pixel Grid,Film,…

  4. Rays and the ‘Thin Lens Law’ • Focal lengthf: where parallel rays converge • Focus at infinity: Adjust for S2=f • Closer Focus ? Larger S2 f Sensor S2 Thin Lens Try it Live! Physlets… http://webphysics.davidson.edu/Applets/Optics/intro.html

  5. Rays and the ‘Thin Lens Law’ • Focal lengthf: where parallel rays converge • Focus at infinity: Adjust for S2=f • Closer Focus ? Larger S2 f f Sensor Scene S2 S2 Thin Lens Try it Live! Physlets… http://webphysics.davidson.edu/Applets/Optics/intro.html

  6. Not One Ray, but a Bundle of Rays Lens Scene Sensor Aperture • BUT Ray model isn’t perfect: ignores diffraction • Lens, aperture set the point-spread-function (PSF) (How? See: Goodman,J.W. ‘An Introduction to Fourier Optics’ 1968)

  7. Basic Ray Optics: Lens Aperture For the same focal length: • Larger lens • Gathers a wider ray bundle: • More light: brighter image • Narrower depth-of-focus • Smaller lens • dimmer image • focus becomes less critical • more depth of focus

  8. Film-like Optics: Thin Lens Flaws • Aberrations: Real lenses don’t converge rays perfectly • Spherical: edge rays  center rays • Coma: diagonal rays focus deeper at edge http://www.nationmaster.com/encyclopedia/Lens-(optics)

  9. Lens Flaws: Chromatic Aberration • Dispersion: wavelength-dependent refractive index • (enables prism to spread white light beam into rainbow) • Modifies ray-bending and lens focal length: f() • color fringes near edges of image • Corrections: add ‘doublet’ lens of flint glass, etc. http://www.swgc.mun.ca/physics/physlets/opticalbench.html

  10. Chromatic Aberration • Lens Design Fix: Multi-element lenses Complex, expensive, many tradeoffs! • Computed Fix: Geometric warp for R,G,B. Near Lens Center Near Lens Outer Edge

  11. Radial Distortion (e.g. ‘Barrel’ and ‘pin-cushion’) straight lines curve around the image center

  12. Vignette Effects Bright at center, dark at edges.Several causes compounded: • Edge pixels span smaller angle and center pixels • Ray path length is longer off-axis • Internal shadowing • Compensation: • Use anti-vignetting filters, (darkest at center) • OR Position-dependent pixel-detector sensitivity. http://homepage.ntlworld.com/j.houghton/vignette.htm

  13. Film-like Color Sensing • Visible Light: narrow band of e’mag. spectrum •   400-700 nm (nm = 10-9 meter wavelength) • (humans:<1 octave honey bees: 3-4 ‘octaves do honey bees sense harmonics, see color ‘chords’ ? • Equiluminant Curvedefines ‘luminance’ vs. wavelength http://www.yorku.ca/eye/photopik.htm

  14. Film-like Color Sensing • Visible Light: narrow band of emag spectrum •   400-700 nm (nm = 10-9 meter wavelength) • At least 3 spectral bands required (e.g. R,G,B) • RGB spectral curves Vaytek CCD camera with Bayer grid www.vaytek.com/specDVC.htm

  15. Color Sensing • 3-chip: vs. 1-chip: quality vs. cost http://www.cooldictionary.com/words/Bayer-filter.wikipedia

  16. 1-Chip Color Sensing: Bayer Grid • Estimate RGBat ‘G’ cels from neighboring values http://www.cooldictionary.com/words/Bayer-filter.wikipedia

  17. Polarization Sunlit haze is often strongly polarized. Polarization filter yieldsmuch richer sky colors

  18. RAYS and PROCESSING • ONE Ray carries doubly infinitesimal power: Ray bundles with finite, measurable power will: • Span a non-zero area • Fill a non-zero solid angle • Everything is Linear: (HUGE win!) Ray reflectance, transmission, absorption, scatter*… • Rays are REVERSIBLE. Helmholtz reciprocity Ray bundles? Not so much: falls quickly with angle,area growth…

  19. Film-like Photography:Many Limitations • Optics: Single focus distance, limited depth-of-field, limited field-of-view, internal reflections/flare/glare • Lighting: Camera has no knowledge of ray source strength, position, direction; little control (e.g. flash) • Sensor: Exposure setting, motion blur, noise, response time, • Processing: • Quantization/color depth, camera shake, scene movement…

  20. Conclusions • Film-like photography methods limit digital photography to film-like results or less. • Broaden, unlock our views of photography: • 4-D, 8-D, even 10-D Ray Space holds the photographic signal. Look for new solutions by creating, gathering, processing RAYS, not focal-plane intensities. • Choose the best, most expressive sets of rays, THEN find the best way to measure them.

  21. Useful links: Interactive Thin Lens Demo (or search ‘physlet optical bench’) www.swgc.mun.ca/physics/physlets/opticalbench.html For more about color: • Prev. SIGGRAPH courses (Stone et al.) • Good: www.cs.rit.edu/~ncs/color/a_spectr.html • Good: www.colourware.co.uk/cpfaq.htm • Good: www.yorku.ca/eye/toc.htm

  22. Course 15: Computational Photography Course 15: Computational Photography

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