Transcript photographic-image-formation-II.ppt

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?