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 larger b)Become smaller c)Stay about the same Week 10 Quiz

Refraction and dispersion ‘s law: n 1 sin θ 1 = n 2 sin θ 2 Refractive index depends on wavelength of light (color) With different refractive indices, colors are dispersed

Chromatic aberration To 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) 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 Elements Achromatic 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 aberration Light at edges and center of a spherical lens converge at different spots. Aspherical elements help correct spherical aberration

Vignetting Same sky, different brightness

Vignetting Four causes: Mechanical vignetting: physical obstruction of light Optical vignetting: Gradual dropoff due to lens design Natural vignetting: Light entering at steep angle Photosite vignetting (digital sensors): More difficult for photosites to detect light at steep angle Fixes: Filters Smaller aperture (stopping down) Software correction f/1.4f/2.8

Barrel/Pincushion Distortion

Barrel/Pincushion Distortion: Straight stuff is curvy Barrel Increasing magnification near optical axis :S:S Solution: Software correction Pincushion Decreasing magnification near optical axis Normal Constant Magnification

Sharpness “Soft”“Sharp”

Sharpness: stopping down For a given lens, using a smaller aperture tends to produce sharper images to a certain point The “sweet spot” varies from lens to lens, usually ~2 stops smaller than max f/1.4f/2.8f/8

f/2.8f/8f/22 Sharpness: diffraction limit At very small apertures, lenses run into the “diffraction limit”, making images softer For ~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