1. what is depth of field?
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Week 10 Quiz. 1. What is depth of field? 2. Everything else equal, what effect will each of the following have on depth of field (larger, smaller?): -Larger aperture (smaller f-number) -Longer subject distance -Longer focal length - PowerPoint PPT PresentationTRANSCRIPT
1. What is depth of field?
2. Everything else equal, what effect will each of the following have on depth of field (larger, smaller?):-Larger aperture (smaller f-number)-Longer subject distance-Longer focal length
3. If I keep the subject same size (same magnification), by stepping further away and zooming in (larger subject distance, longer focal length), depth of field will:
a) Become largerb) Become smallerc) Stay about the same
Week 10 Quiz
Refraction and dispersion
‘s law:n1sinθ1 = n2sinθ2
Refractive index depends on wavelength of light (color)
With different refractive indices, colors are dispersed
Chromatic aberrationTo generate a sharp image, light must converge at the same point.
Dispersion causes different wavelengths (colors) to converge at different points.
Chromatic aberrationLongitudinal CA
At the image plane, light is misfocused
Results in “purple fringing”
Lateral chromatic aberration
At the image plane, magnification is different for each wavelength
Results in color shifts, especially at corners
Chromatic aberration: “Purple fringing”
Red shift (outward)
Blue shift (inward)
Blue shift (inward)
Red shift (outward)
Chromatic aberrationLongitudinal CA
At the image plane, light is misfocused
Results in “purple fringing”
Lateral chromatic aberration
At the image plane, magnification is different for each wavelength
Results in color shifts, especially at corners
If we know red is magnified and blue is shrunken, simply contract red and expand blue
With digital, we can do this by manipulating color channels independently.
Special ElementsAchromatic elements use a second element to correct dispersion, correcting for two wavelengths
Apochromatic elements use three elements and correct for three wavelengths
Super achromatic elements correct for four wavelengths
Special “low dispersion” materials (ED, UD glass) can be used to reduce dispersions.
Focus shift vs. light wavelength
Special Elements
Spherical aberrationLight at edges and center of a spherical lens converge at different spots.
Aspherical elements help correct spherical aberration
VignettingSame sky, different brightness
VignettingFour causes:
Mechanical vignetting: physical obstruction of lightOptical vignetting: Gradual dropoff due to lens designNatural vignetting: Light entering at steep anglePhotosite vignetting (digital sensors): More difficult for photosites to detect light at steep angle
Fixes:
FiltersSmaller aperture (stopping down)Software correction
f/1.4f/2.8
Barrel/Pincushion Distortion
Barrel/Pincushion Distortion: Straight stuff is curvy
BarrelIncreasing magnification near optical axis
:S
Solution: Software correction
PincushionDecreasing magnification near optical axis
NormalConstant Magnification
Sharpness“Soft” “Sharp”
Sharpness: stopping downFor a given lens, using a smaller aperture tends to produce sharper images to a certain pointThe “sweet spot” varies from lens to lens, usually ~2 stops smaller than max
f/1.4 f/2.8 f/8
f/2.8 f/8 f/22
Sharpness: diffraction limitAt very small apertures, lenses run into the “diffraction limit”, making images softerFor ~1.5x crop APS-C cameras, the diffraction limit is reached ~f/11.
Sharpness: diffraction limit
Photozone.de review of Tamron 90mm on Rebel XT
Sharpness: diffraction limitf/8 f/2.8, sharpened
Perspective Distortion