1. Physics 1230: Light and Color Ivan I. Smalyukh, Instructor Office: Gamow Tower, F-521 Email: ivan.smalyukh@colorado.edu Phone: 303-492-7277 Lectures: Tuesdays & Thursdays, 3:30 PM - 4:45 PM Office hours: Mondays & Fridays, 3:30 PM – 4:30 PM TA: Jhih-An Yang jhihan.yang@colorado.edu Class # 20

2. Last time: The retinal cells have some interesting collective behavior: 1.We detect relative, not total lightness a)Lightness constancy (correct for total amount of light) b)Weber’s Law (perception of lightness depends on ratio between two shades) 2.When we stimulate one cell, it turns down response of neighbors a)Lateral inhibition b)We detect edges, not the entire field of view 3.We are sensitive to a moving world a)Eye jumps around (“saccades”) to move images across retina b)Negative afterimages caused by desensitization of retina cells as they keep being stimulated c)Images disappear if they are in your view constantly

3. Why do we have a blind spot? A.The fovea isn’t very large B.We don’t see things where the fovea blocks our view C.We don’t see things where the pupil blocks our view D.There are no rods or cones where the optic nerve is E.Really not sure

4. Why do we have a blind spot? A.The fovea isn’t very large B.We don’t see things where the fovea blocks our view C.We don’t see things where the pupil blocks our view D.There are no rods or cones where the optic nerve is E.Really not sure So, the signal from the retina goes to the BRAIN

5. How do we process images? (not just gather the light) A. B. C. Both So, our eyes have certain mechanisms for making sense of what we see (lateral inhibition, lightness constancy…) So does our brain. Today we’ll look at some of brain processes.

6. A Question 6 You have two focusing lenses, each with a focal length of F. You put them close together to make them behave as a single lens. The new ‘doublet lens’ has a focal length of: A)2*F because the diopters add. B) F/2 because the diopters add. C) Still F for this special case. D) Something else happens. Is there an experiment you can try?

7. 7 Are the small gray patches below identical? See fig 7.7 A) YES B) NO Craik O’Brien Illusion Contrast at the edge affects your perception of center = simultaneous lightness contrast

8. Craik O’Brien Illusion Simultaneous lightness contrast 8 These are the patches without the surround.

9. 9 Clicker question A white sheet of paper continues to look white as the light level is reduced. We call this effect: A. Simultaneous lightness contrast B. Lateral inhibition C. Weber’s law D. Lightness constancy E. Edge enhancement

10. 10 Clicker question The bands of gray look lighter on their right side because of: A.Simultaneous lightness contrast B. Lightness constancy C. Weber’s law D. Lateral inhibition E. Both A and D

11. 11 Clicker question The arrow points to: A. Photoreceptors B. Horizontal cells C. Bipolar cells D. Amacrine cells E. Ganglion cells

12. 12 Clicker question The arrow points to: A. Photoreceptors B. Horizontal cells C. Bipolar cells D. Amacrine cells E. Ganglion cells

13. 13 Hermann grid illusion: dark areas are from lateral inhibition The red areas show the receptive field. Lateral inhibition is greater at 1 than at 2. The fovea has a smaller receptive field. So the lateral inhibition is the same everywhere in the white area. 1 2 3

14. Now, the brain…. 14 Brain damage on the left side hurts vision on the right side. See text fig. 7.3

15. Brain anatomy Optic chiasma Where optic nerves cross Left field of view goes to right brain Right field of view goes to left brain from both eyes Visual cortex is where you “see” Brain damage at this location hurts vision. 15

16. 16 Clicker question If the left side of your brain is injured, you might lose vision in your A. left eye B. right eye C. left field of view D. right field of view E. some loss in left and right field of view

17. 17 A B Example: Which is the darker patch, A or B?

18. 18 A B Previous experience effect: Here, the eye is “fooled” into thinking the light is from above. The panel “A” has lots of light, so it must be really dark. But “B” must be lighter because it is in the “shade.”

19. Human (predator) eyes are in front of our heads: 130 degrees of overlap. Good depth perception! But field of view just 208 degrees. Dog: overlap is 100 degrees. Rabbit (prey) field of view = 360 degrees! Only 24 degrees of overlap. Parallax / Binocular disparity Dominant eye

20. A bundle of parallel rays enter the eye's pupil. No other rays enter. What does the eye see? A) A single pt of light, surrounded by black. B) A uniformly illuminated wall of light, like a white wall. C) Scattered points of light, like stars in the sky. D) None of these.

21. A bundle of parallel rays enter the eye's pupil. No other rays enter. What does the eye see? A) A single pt of light, surrounded by black. B) A uniformly illuminated wall of light, like a white wall. C) Scattered points of light, like stars in the sky. D) None of these.

22. 22 Robert Hooke’s two-lens microscope object lens 1 Nosepiece or Objective lens 2 eyepiece lens 1 image lens 2 image A magnifying glass (the eyepiece) magnifies the first image further. The first lens, the nosepiece, is used as a projection lens. An image of an image!

23. Lens Power (or diopters) Lens power: D = 1/F Units of D are 1/meters, also called diopters Eyeglass lenses are measured in diopters. This is handy because the diopters of lenses add: D (total) = D1 + D2

24. 24 I. Camera parts 1.Lens – focuses light 2.Shutter – times the exposure 3.Diaphragm – controls amount of light 4.Focusing screen – image created here 5.Film (or digital imaging chip) – saves the image 24

25. If you remove the lens, the image on the screen... A) remains the same B) gets a bit dimmer C) becomes fuzzier. D) becomes upright E) disappears E.We need the lens to get an image. This is a simple camera. This is why we care about real images! How can a virtual photon develop film? (It can’t) From last time

26. Digital Image Storage: The CCD In Film, light moves electrons from one atom to another to do CHEMISTRY. 26 In a CCD, light moves electrons from one atom to another, traps the charge, and moves it to an amplifier to MEASURE IT. CCD: “Charge Coupled Device”. Electronic device for storing images

27. If you move the screen towards the lens, the image: A) remains the same B) gets a bit dimmer C) becomes fuzzier. D) becomes upright E) disappears blurry Focal point

28. blurry focused Imagine the image is unfocussed. What do we need to do? Move the screen forward or back. This is how a camera focuses It adjusts the distance between lens and sensor Focal point

29. Photography principles 1.Lens focuses by moving closer/further from film

30. What will happen if the focal point is moved further away from the lens? If you’re mathy, use this: A: Image moves further away. Image is always outside focal point

31. 31 Lens focal length in cameras 50 mm normal lens 85 mm portrait lens You have a camera and can use two different focal length lenses to produce an image on the film. (A) 50mm (B) 85mm (C) Neither Which lens requires the camera screen to be closer to the lens?

32. 32 Lens focal length in cameras 50 mm normal lens 85 mm portrait lens (A) 50mm (B) 85mm You have a camera and can use two different focal length lenses to produce an image on the film. (The film has to be closer to the short focal length lens to make an image.) Which lens makes a bigger image?

33. Focal length of lens determines magnification and field of view Short focal length = wide angle (more things in the image) Long focal length = “telephoto” (larger, but fewer things) 50 mm normal lens 85 mm portrait lens

34. 34 Which is best for taking a picture of a bird in a tree? A.focal length 200 mm B.focal length 50 mm Longer focal length, larger image of a smaller field of view. Telephoto lens.

35. Photography principles 1.Lens focuses by moving closer/further from film 2.Long focal length = larger image (telephoto)

36. 36 What is depth of field? Depth of field is how far objects can be apart and still be nearly in focus. F Are not in focus at the same place, but film is in a fixed spot!

37. 37 Large lenses have smaller depth of field than small lenses F fuzzy in focus big blur small blur More like a pinhole camera

38. Large lenses have smaller depth of field than small lenses That goes for smaller apperature too Narrow depth of field Large depth of field

39. Photography principles 1.Lens focuses by moving closer/further from film 2.Long focal length = larger image (telephoto) 3.Bigger lenses / aperature = narrow depth of field /focus

40. 40 Camera parts 1.Lens, focal length 2.Shutter (on the lens) 3.Diaphragm (on the lens) 4.Focusing screen or LCD 5.Film (or CCD chip) We are here Controlling the light that enters.

41. The Shutter: Opens the camera lens for a controlled amount of time You can open the shutter on your camera for 1 sec. or for ½ sec. Which one lets more light energy hit the film? 41 A)1 sec. because the light power is on longer B)½ sec because the light power is on less C)The same, because the power hitting the lens is the same in both cases. D)None of these.

42. 42 Shutter speed The shutter controls the duration of the light. Common shutter speeds 1/15 sec 1/30 sec 1/60 sec 1/125 sec 1/250 sec 1/500 sec Each change lets in half the light. FACTORS OF 2 in light energy entering the camera

43. Photography principles 1.Lens focuses by moving closer/further from film 2.Long focal length = larger image (telephoto) 3.Bigger lenses / aperature = narrow depth of field /focus 4.Faster shutter speed = less light

44. 44 The lens itself also controls the amount of light: F-Number Example: 200 mm focal length / 25 mm opening = f-number 8 Small F-number = Bright image If you have a long focal length and want a small f-number, for bright image, do you need large or small diameter lens? Big. That’s why telephoto lenses are so big.

45. 45 The lens itself also controls the amount of light: F-Number Big diameter lens = low f-number, like f-number 1.4 Heavy. Cost more! Works in low light and indoors. Small diameter lens = big f-number, like f-number 3.5. For outdoors, weigh less. Example: 200 mm focal length / 25 mm opening = f-number 8 Small F-number = Bright image

46. 46 Change the diaphragm size = change “d” so change F-number 1 Big opening 1.4 2 2.8 4 5.6 Factors of √2 apart 8 11 16 Small opening Amount of light scales with area Area scales with (diameter) 2 Diaphragm has variable opening. F-numbers go up in factors of 2

47. 47 Which gives more light? A.f-number 4 B.f-number 22 f= F/D so SMALLER f-number is a larger D and therefore, more light.

48. Photography principles 1.Lens focuses by moving closer/further from film 2.Long focal length = larger image (telephoto) 3.Bigger lenses / aperature = narrow depth of field /focus 4.Faster shutter speed = less light 5.F-number = focal length / diameter. Small F- number = bright image. Large F-number = dim image. Change F-number with lens and/or aperature.

49. Photography principles 1.Lens focuses by moving closer/further from film 2.Long focal length = larger image (telephoto) 3.Bigger lenses / aperature = narrow depth of field /focus 4.Faster shutter speed = less light 5.F-number = focal length / diameter. Small F-number = bright image. Large F-number = dim image. Change F-number with lens and/or aperature. If you have a big F-number (dim image) but want a bright image, what do you need to do to the aperature? A.Open it more B.Close it a bit What does that do to the depth of field? A.Increases it B.Decreases it

50. You can get a lot of light in two ways: Low F-number (large aperature) Long shutter speed This determines film’s EXPOSURE But why not just use a large aperature and long shutter all the time?

51. 51 Equivalent exposure settings (same light) ¼ sec. lets subjects move Background in focus (good depth of field) 1 / 250 sec. stops motion Background out of focus

52. 52 Which gives more light? A.shutter speed 1/4 sec B.shutter speed 1/16 sec Longer time open is more light.

53. 53 Which f-number gives half the light of f-4? A.f-2 B.f-2.8 C.f-4 D.f-5.6 E.f-8 f = F/D, so you need a LARGER f-number for less light. 5.6 is sqrt(2) smaller than 4, so twice the light.

54. 54 Which pair gives the same amount of light? 1/30 sec. and f number 4 and a)1/120 sec. and f-1 b)1/120 sec. and f-2 c)1/120 sec. and f-4 4x more light for ¼ the time.