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 QuizWeek 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 aberration

Longitudinal 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)Blue shift (inward)

Red shift (outward)Red shift (outward)

Chromatic aberration

Longitudinal 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 brightnessSame sky, different brightness

Vignetting

Four 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/2.8f/2.8 f/8f/8 f/22f/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/8f/8 f/2.8, sharpenedf/2.8, sharpened

Perspective Distortion