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
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A lecture course on digital photography and optics II. The lecture covers topics such as dispersion, chromatic aberration, spherical aberration, astigmatism, field curvature, distortion, and veiling glare. The lecture also discusses practical photographic lenses and how to correct for various aberrations. The lecture is based on a course taught by Marc Levoy, Professor Emeritus at Stanford University, and is part of the curriculum for a summer lecture course at Dalian Maritime University in 2019.
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2018/2019
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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 Lens aberrations ✦ chromatic aberrations ✦ Seidel aberrations, a.k.a. 3 rd orderaberrations
- arise because we use spherical lenses instead of hyperboloidal
- can be modeled by adding 3 rd order terms to Taylor series
- oblique aberrations
- field curvature
- distortion 3 3 5 7
sin
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 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 Examples 7 (wikipedia) (^) (toothwalker.org) lateral (^) longitudinal (cropped from edge of image) ✦ other possible causes of color fringing
- demosiacking algorithm
- per-pixel microlenses
- lens flare
- partly correctable in software - not easily correctable
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 9 by^ combining^ with^ a^ concave^ lens^ of^ different^ properties (wikipedia) hyperboloidal lens spherical lens
Marc Levoy Examples 10 Canon 135mm f/2.8 soft focus lens sharp (^) soft focus (Canon)
Hubble Space Telescope
Eagle Nebula (NASA)
Aberrations and their properties ✦ lateral chromatic aberrations do not appear in center of field
- they get worse with increasing distance from the optical axis
- cannot reduce by closing down the aperture ✦ longitudinal chromatic & spherical aberrations occur everywhere in the field of view
- on and off the optical axis
- can reduce by closing down the aperture ✦ oblique aberrations do not appear in center of field
- they get worse with increasing distance from the optical axis
- can reduce by closing down the aperture
- coma and astigmatism 13 ^ Marc^ Levoy
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) 16 Marc Levoy
Marc Levoy Field curvature 18 (Hecht) ✦ spherical lenses focus a curved surface in object space onto a curved surface in image space ✦ so a plane in object space cannot be everywhere in focus when imaged by a planar sensor
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 ray height Marc Levoy Correcting for astigmatism and field curvature