1. What is depth of field?

1. 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 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: Become larger Become smaller Stay about the same

2. Refraction and dispersion ‘s law: n1sinθ1 = n2sinθ2 Refractive index depends on wavelength of light (color) With different refractive indices, colors are dispersed

3. Chromatic aberration To generate a sharp image, light must converge at the same point. Dispersion causes different wavelengths (colors) to converge at different points.

4. 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

5. Chromatic aberration: “Purple fringing”

6. Red shift (outward) Blue shift (inward) Blue shift (inward) Red shift (outward)

7. Chromatic aberration Longitudinal CA At the image plane, light is misfocused Results in “purple fringing” If we know red is magnified and blue is shrunken, simply contract red and expand blue Lateral chromatic aberration At the image plane, magnification is different for each wavelength Results in color shifts, especially at corners With digital, we can do this by manipulating color channels independently.

8. 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.

9. Special Elements Focus shift vs. light wavelength

10. Spherical aberration Light at edges and center of a spherical lens converge at different spots. Asphericalelements help correct spherical aberration

11. Vignetting Same sky, different brightness

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

13. Barrel/Pincushion Distortion

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

15. Sharpness “Soft” “Sharp”

16. 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/1.4 f/2.8 f/8

17. 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. f/2.8 f/8 f/22

18. Sharpness: diffraction limit Photozone.de review of Tamron 90mm on Rebel XT

19. Sharpness: diffraction limit f/8 f/2.8, sharpened

20. Perspective Distortion