Physics 1230: Light and Color. Guest Lecture, Jack again. Lecture 23: More about cameras

Transcription

1 Physics 1230: Light and Color Chuck Rogers, Ryan Henley, Valyria McFarland, Peter Siegfried physicscourses.colorado.edu/phys1230 Guest Lecture, Jack again Lecture 23: More about cameras 1

2 Project Progress Reports Due after break, Friday (in dropbox) 15 points towards the 65 point project total Turn in an outline of your project paper Try to include one picture or figure for your project. Let us know how your observations are going. Are there questions you have? Are there some confusing things that you d like feedback on? This point in the project is a great time to ask us if you need special feedback. 2

3 Last Time: Camera with a lens f An image forms at the back of the camera. The distance between the lens and the back of the camera, x i, is almost always much smaller than x o, and is (of course) determined by the lens equation, by x o and f.

4 Last Time: Camera parts 1. Lens 2. Shutter 3. Diaphragm 4. Focusing screen 5. Film (or digital imaging chip)

5 Camera parts Lens Bellows Sheet film holder Produces the image that you want to record. Focus knob Shutter & Diaphragm

6 Last Time: Question Back of camera You have a camera back and can use two different focal length lenses to produce an image on the camera back. Which lens produces the larger image size? 50 mm normal lens (A) 50mm 85 mm portrait lens (B) 85mm 6

7 Question Back of camera You have a camera back and can use two different focal length lenses to produce an image on the camera back. Which lens produces the larger image size? 50 mm normal lens (A) 50mm 85 mm portrait lens (B) 85mm 7

8 Long lens focal lengths - vary

9 What are apertures, f-numbers, and stops? The aperture is the opening of a lens s diaphragm through which light passes.

10 f-number controls amount of light f-number = focal length diameter of opening = f d 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. 10

11 f-number scales 1 Big opening Factors of 2 apart Small opening Why these numbers? more light less light Diaphragm has variable opening. 11

12 f-number scales 1 Big opening Factors of 2 apart Small opening Why these numbers? more light less light Diaphragm has variable opening. 12

13 f-number scales factors of 2 in light 1 Big opening Factors of 2 apart Small opening Why these numbers? more light less light Diaphragm has variable opening. 13

14 Amount of light f number Amount of light 1 100% % 2 25% or 1/ /8 4 1/ /32 8 1/64 14

15 What are apertures, f-numbers, and stops? The aperture is the opening of a lens s diaphragm through which light passes. Sets lens opening so amount of light though lens. Also sets the depth of field

16 How quickly do images go out of focus? Rays of light diverge on either side of the imaged point. screen F in focus Not perfectly sharp, but acceptable. fuzzy 16

17 What is depth of field? Depth of field refers to how far apart objects can be and still be acceptably in focus. screen F Are not in focus at the same place 17

18 Large versus small aperture Narrow depth of field You can operate some lens either with a large aperture or small aperture. Which has the larger depth of field? A) Large aperture. B) Small aperture. Large depth of field C) They have same DoF.

19 Large aperture has a larger cone of light

20 Large versus small aperture Narrow depth of field You can operate some lens either with a large aperture or small aperture. Which has the larger depth of field? A) Large aperture. B) Small aperture. Large depth of field C) They have same DoF.

21 Large aperture has smaller depth of field

22 Large aperture has smaller depth of field Narrow depth of field Large depth of field

23 Shutters 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 back of the camera? 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. 23

24 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 Faster speeds, better able to stop (freeze) fast motion but give less light to work with 24

25 Demo: Look at a digital camera

26 Demo: Look at a digital camera Focal length: Changed by Zoom lens 18 to 55 mm Object we re photographing much farther away than focal length f5.6 to f36 Exposure determined by shutter Can set various automatic or manual modes To focus: move lens slightly (move lens away from sensor to focus on closer object)

27 Questions For which focal length will the metronomes appear smaller (take up a smaller portion of the image screen)? A)18 mm B) 55 mm C) the same in both cases Which setting will give us a larger depth of field? A)F5.6 B) F36 C) the same in both cases Note depth of field will also change with focal length and object distance

28 Questions For which focal length will the metronomes appear smaller (take up a smaller portion of the image screen)? A)18 mm B) 55 mm C) the same in both cases Which setting will give us a larger depth of field? A)F5.6 B) F36 C) the same in both cases Note depth of field will also change with focal length and object distance

29 Different shutter speed 29

30 Equivalent exposure settings fixed focal length ¼ sec. lets subjects move Greater depth of field 1 / 250 sec. stops motion Background out of focus Snapshot of motion

31 The photo was taken most likely using A) Large f-number & fast shutter speed. B) Large f-number & slow shutter speed. C) Small f-number & fast shutter speed. D) Small f-number & slow shutter speed. E) None of above. Question

32

33 Break for questions

34 Your Eye Also Uses a Lens!

35 Eye parts that you see Pupil Opening to the inside of the eye Iris changes its opening to adjust light the pupil is the opening Sclera the white outer wall 35

36 Internal parts of the human eye Specialized optical instrument and image analysis computer.

37 The Eye: Analogy to the Camera Lens and cornea Iris (diaphram) Ciliary muscle (focus) Retina (FILM??) We will see that the retina does FAR more than film or CCD 37

38 The Lens system: Imaging &Focusing Most converging of light is by the cornea. Assisted to varying degrees by eyelens focus. Ciliary muscles puff up to relax the lens for close focusing 38

39 The Eye as a cool instrument: The eyelens We know that lenses suffer from various aberrations. What happens in the eyelens? Spherical aberration is mostly corrected Cornea is not spherical surface (aspherical) Iris cuts out rays through the edge of the lens Index of refraction is not uniform. Curvature of field retina is curved to correct for this Chromatic aberration: Bluest light is absorbed Many of these tricks are now used in technology. 39

40 The retina: Light sensitive back surface of your eye

41 The Retina: Detecting the light and processing the images The retina and optic nerves are recognized as actually parts of the brain (like your olifactory bulb in the nose). They start development IN the brain and migrate Has 10 8 nerve endings to detect image rods, for high sensitivity (night vision) cones, for color and detail, 7 million optic nerve = 10 6 transmission lines fovea, region of best vision (cones) More nerves in your retina than some creatures have in their entire brains. Processing Power. 41