lenses and depth of field - donald bren school of ...majumder/photo/lensesanddepthoffield.… ·...
TRANSCRIPT
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Lenses and Depth of Field
Prepared by Behzad Sajadi
Borrowed from Frédo Durand’s Lectures at MIT
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3 major type of issues• Diffraction
– ripples when aperture is small
• Third-order/spherical aberrations– Rays don’t focus– Also coma, astigmatism,
field curvature• Chromatic aberration
– Focus depends on wavelength
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References
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Links• http://en.wikipedia.org/wiki/Chromatic_aberration• http://www.dpreview.com/learn/?/key=chromatic+aberration• http://hyperphysics.phy-
astr.gsu.edu/hbase/geoopt/aberrcon.html#c1• http://en.wikipedia.org/wiki/Spherical_aberration• http://en.wikipedia.org/wiki/Lens_(optics)• http://en.wikipedia.org/wiki/Optical_coating• http://www.vanwalree.com/optics.html• http://en.wikipedia.org/wiki/Aberration_in_optical_systems• http://www.imatest.com/docs/iqf.html• http://www.luminous-landscape.com/tutorials/understanding-
series/understanding-mtf.shtml
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Other quality issues
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Flare
From "The Manual of Photography" Jacobson et al
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Example of flare "bug"• Some of the first copies of the Canon 24-105 L had
big flare problems• http://www.the-digital-picture.com/Reviews/Canon-
EF-24-105mm-f-4-L-IS-USM-Lens-Review.aspx•
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Flare and Ghosting
source: canon red book
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Flare/ghosting special to digital
source: canon red book
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Use a hood! (and a good one)
Adapted from Ray's Applied Photographic Optics
Hood is to shortFlare ray
Good hood
Flare
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Lens hood
From Ray's Applied Photographic Optics
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Coating• Use destructive interferences• Optimized for one wavelength
From "The Manual of Photography" Jacobson et al
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Coating for digital
source: canon red book
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Vignetting• The periphery does not get as much light
source: canon red book
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Vignetting• http://www.photozone.de/3Technology/lenstec3.htm
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Lens design
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Optimization software• Has revolutionized lens design• E.g. zooms are good now
From Hecht's Optics
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Lens design, ray tracing
source: canon red book
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Optimization• Free parameters
– Lens curvature, width, position, type of glass– Some can be fixed, other vary with focal length, focus (e.g.
floating elements)– Multiplied by number of lens elements
• Energy/merit function– MTF, etc.– Black art of massaging the merit function
• Optimize for– All image locations– All wavelengths– All apertures– All focusing distances– All focal lengths (zoom only)
• Usually uses simulated annealing
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Floating elements• Move with focus to optimize response
(but are not responsible for focusing)
source: canon red book
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Image stabilization
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Image stabilization
source: canon red book
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Image stabilization
source: canon red book
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Image stabilization
source: canon red book
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1000mm, 1/100s, monopod, IS
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Different versions• Canon, Nikon: in the lens• Panasonic, Konica/Minolta: move sensor
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6.088 Digital and Computational Photography 6.882 Advanced Computational Photography
Focus and Depth of Field
Frédo DurandMIT - EECS
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Focusing
fDD’
1D’ D
1 1f+ =• Move film/sensor
• Thin-lens formula
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In practice, it’s a little more complex• Various lens elements
can move inside the lens– Here in blue
Source: Canon red book.
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Defocus & Depth of field
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Circle of confusion
From Basic Photographic Materials and Processes, Stroebel et al.
circle of confusion
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Depth of focus
From Basic Photographic Materials and Processes, Stroebel et al.
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Size of permissible circle?• Assumption on print size, viewing distance, human
vision– Typically for 35mm film: diameter = 0.02mm
• Film/sensor resolution (8µ photosites for high-end SLR )
• Best lenses are around 60 lp/mm• Diffraction limit
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Depth of field: Object space
lens
sensor
Point in focus
Object with texture
• Simplistic view: double cone– Only tells you about the value of one pixel– Things are in fact a little more complicated to asses
circles of confusion across the image– We're missing the magnification factor
(proportional to 1/distance and focal length)
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Depth of field: more accurate view• Backproject the image onto the plane in focus
– Backproject circle of confusion– Depends on magnification factor
• Depth of field is slightly asymmetrical
lens
Point in focus
Conjugate of circle of confusion
Depth of field
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DoF & aperture• http://www.juzaphoto.com/eng/articles/depth_of_field.htm
f/2.8 f/32
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Depth of field and focusing distance• Quadratic (bad news for macro)
(but careful, our simplifications are not accurate for macro)
f/N
d1 d2
D
CD/f
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Double cone perspective• Seems to say that relationship is linear• But if you add the magnification factor, it's actually
quadratic
lenssensor
Point in focus
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Depth of field & focusing distance
From Photography, London et al.
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Depth of field & focal length• Recall that to get the same image size,
we can double the focal length and the distance• Recall what happens to physical aperture size when
we double the focal length for the same f number?– It is doubled
24mm 50mm
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• Same image size (same magnification), same f number
• Same depth of field!
Depth of field & focal length
Wide-anglelens
Telephotolens (2x f), same aperture
DoF
DoF
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DoF & Focal length• http://www.juzaphoto.com/eng/articles/depth_of_fiel
d.htm
50mm f/4.8 200mm f/4.8(from 4 times farther)
See also http://luminous-landscape.com/tutorials/dof2.shtml
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Important conclusion• For a given image size and a given f number, the
depth of field (in object space) is the same. • Might be counter intuitive.
• Very useful for macro where DoF is critical. You can change your working distance without affecting depth of field
• Now what happens to the background blur far far away?
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Sensor size
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Depth of field• It’s all about the size of the lens aperture
lenssensor
Point in focus
Object with texture
lens
sensor Point in focus
Object with texture
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Equation• Smaller sensor
– smaller C– smaller f
• But the effect of f is quadratic
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Sensor size• http://www.mediachance.com/dvdlab/dof/index.htm
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The coolest depth of field solution• http://www.mediachance.com/dvdlab/dof/index.htm• Use two optical systems
lensdiffuser
Point in focus
Object with texture
lens
sensor
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The coolest depth of field solution• http://www.mediachance.com/dvdlab/dof/index.htm
lensdiffuser
Point in focus
Object with texture
lens
sensor
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Seeing through occlusion
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Seeing beyond occlusion• Photo taken through zoo bars• Telephoto at full aperture• The bars are so blurry
that they are invisible
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Synthetic aperture• Stanford Camera array (Willburn et al.
http://graphics.stanford.edu/papers/CameraArray/)
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Autofocus
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How would you build an Auto Focus?
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Polaroid Ultrasound (Active AF)• Time of flight (sonar principle)• Limited range, stopped by glass• Paved the way for use in robotics• http://www.acroname.com/robotics/info/articles/sonar/sonar.html• http://www.uoxray.uoregon.edu/polamod/• http://electronics.howstuffworks.com/autofocus2.htm
http://www.uoxray.uoregon.edu/polamod/
From Ray’s Applied Photographic Optics
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Infrared (Active AF)• Intensity of reflected IR is assumed to be
proportional to distance• There are a number of obvious limitations• Advantage: works in the dark• This is different from Flash assistant for AF where
the IR only provides enough contrast so that standard passive AF can operate
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Triangulation• Rotating mirror sweeps the
scene until the image is aligned with fixed image from mirror M– pretty much stereovision and
window correlation)
From The Manual of Photography
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Different types of autofocus
From The Manual of Photography
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Contrast • Focus = highest contrast
http://electronics.howstuffworks.com/autofocus3.htm
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Phase detection focusing • Used e.g. in SLRs
From The Manual of Photography From the Canon red book
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Phase detection focusing • Stereo vision from two portions of the lens on the
periphery• Not at the equivalent
film plane but farthercan distinguish
too far and too close• Look at the phase
difference between the two images
From The Manual of Photography
In focus
Too close
Too far
Detector
phase
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compute phase difference, deduce distance
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Fake Depth of Field
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Photoshop• Using layers: • One sharp layer, one blurry layer (using Gaussian
blur)• Layer mask selects focusInput (sharp layer)
Result
Blurred layer
Mask of blurry layer)
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Photoshop• Problem: halo around edges
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Photoshop lens blur• Reverse-engineered algorithm: average over circle• Size of circle depends on pseudo depth • Discard pixels that are too much closer
Input Depth map (painted manually)
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Photoshop lens blur• Filter>Blur>Lens blur
Input
Result
Depth map (painted manually)