Photographic Image Formation II Szymon Rusinkiewicz, Tim Weyrich: Technology in Art and Cultural Heritage. Princeton Freshman Seminar 2006 Acknowledgment: some figures by B. Curless, E. Hecht, W.J. Smith, B.K.P. Horn, and A. Overview • Filters and photographic lenses • Darkroom development • Digital darkroom • Camera Artifacts • Employing artifacts Filters and Lenses • Color filters • Polarizers • Photographic Lenses Color Filters • Color filters for B/W photography • Color correction filters • IR filters • UV filters • Graduated filters Polarization • Light is a magnetic wave that can be polarized Polarization • Light is a magnetic wave that can be polarized • Linear polarization caused by – Specular reflection – Scattering (e.g. in the atmosphere) – Polarization filters (as e.g. in LC-displays) • Polarization filters let a certain polarization pass Polarized Specularities Sky Polarization Photographic Lenses • Wide angle • Zoom • Fixed-focus • Distortion lens Filters: Digital vs. Film • Contrast reduction (e.g. polarizers) more important with digital cameras (low dynamic range) • Digital cameras less UV sensitive, so UV filters make less sense • IR filters easier to use with digital cameras Darkroom Development Creation of image prints from negatives 1. How to get the negatives? – Developer stop bath fixing bath washing 2. Exposure – Enlarger (format, scale, dodging & burning) 3. Fixation Digital Darkroom • Instead of negatives: direct pixel intensity measurements • Enlarger/Processing Digital image processing • Image processing software – Adobe Photoshop – Corel Paint Shop Pro – Gimp –… Camera Artifacts • Technical limitations lead to image artifacts • Artifacts can appear within any stage of image formation: Real world Optics Sensor Dark Room Monochromatic Aberrations • Real lenses do not follow thin lens approximation because surfaces are spherical (manufacturing constraints) Spherical Aberration • Results in blurring of image, focus shifts when aperture is stopped down • Can vary with the way lenses are oriented Distortion • Pincushion or barrel radial distortion Distortion Chromatic Aberration • Due to dispersion in glass (focal length varies with the wavelength of light) • Result: color fringes • Worst at edges of image • Correct by building lens systems with multiple kinds of glass Correcting for Aberrations • High-quality compound lenses use multiple lens elements to “cancel out” distortion and aberration • Often 5-10 elements, more for extreme wide angle Other Limitations of Lenses • Optical vignetting: less power per unit area transferred for light at an oblique angle – Transferred power falls off as cos4 – Result: darkening of edges of image • Mechanical vignetting: due to apertures Other Limitations • Flare: light reflecting (often multiple times) from glass-air interface • Bloom: Overflow of charge in CCD buckets Sabatier Effect • Also known as “solarization” • On over-exposure, intensities on film negative revert Sabatier Effect Sabatier Effect • Popular since beginning of photography • Does not occur in digital photography • But: Photo editing tools simulate solarization Errors in Digital Images • What are some sources of error in this image? Sources of Error • Geometric (focus, distortion) • Color (1-chip artifacts, chromatic aberration) • Radiometric (cosine falloff, vignetting) • Bright areas (flare, bloom, clamping) • Signal processing (gamma, compression) • Noise But is it all bad? • We learned to expect these artifacts in images • Artifacts can be used as perceptual hints • Not only in photography – Mimicked by painters (superrealism) – Used in computer generated images • Certain artifacts used for artistic purposes Depth-of-Field • Guides the viewer’s focus Lense Distortion • Fish-eye lens effect to hint proximity Bloom / Flares • Used to denote very bright light sources Bloom / Flares • Used to denote very bright light sources Bloom / Flares • Used to denote very bright light sources Using Blur Using Blur Composition Superimposition Sabatier Effect • Also known as “solarization” • On over-exposure, intensities on film negative revert • Used by various artists Man Ray, Jacqueline Goddard as a Nun Tricks With Polaroids • Illustratypes by Davis Freeman • Taking images from the backside of Polaroids • Image appears partly negative, partly positive • Often display Sabatier effect too Illustratype Illustratype Illustratype Illustratype Illustratype Photographic Collages • Rearrange image cut-outs • Today: digital image tools replace scissors Collage Using Blending Summary • Cameras provide us with an image of the world, but can not replace the artist’s interpretation • Characteristic artifacts and limitations • Artifacts can be used for artistic expression • Artist can manipulate imaging process • Are camera images bound to be perspective projections?