Lights. Action. Cameras. Shutter/Iris Lens With focal length f. Image Distance. Object. Distance

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Jemimah Matthews
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1 Lights. Action. Phys 1020, Day 17: Cameras, Blm 15.1 Reminders: HW 8 in/hw 9 out Make up lab week straight ater Sp.B. Check scores on CU learn 1 Object Cameras Shutter/Iris Lens With ocal length Dark Box Film Object Distance Image Distance - Understand basics o a camera / imaging system - How lens works (converging, diverging), - Image ormation - Lens equation - Depth o ield - /# 2 1

2 Light A photon particle or a EM wave? When do you need to consider light as PHOTONS? Electronic transition in atom or molecule matches the photon energy that you are interested in: Interaction o light with dye molecules, Causes or sunburn, Color o light emitted by LEDs Also i you have so ew atom light interactions that you cannot consider the average result Isolated atoms or small clusters o atoms, materials thinner than the wavelength o the light. Absorption o photon Emission o photon Moving along 3 When can you consider light as a LIGHT RAY (EM wave)? (raindrops, lenses, particles in paint) 3 Criteria: 1. Atoms in material have no electronic transitions near photon energy o the light. - Same as saying material is transparent at that wavelength. - Glass transparent in visible, not transparent in UV - Water is transparent in visible, not in IR GLASS 2. Physical dimensions o material much bigger than l o light - Many, many oscillations between entrance and exit. 1000s Atoms per l 3. Spacing o atoms much, much smaller than wavelength o light - Can just look at average response o atom light interaction 4 2

3 Cameras Camera collects multiple light rays coming rom each point on object Lens reocuses rays rom each point to a point on ilm orms image Shutter/Iris controls number o rays reaching ilm Dark box ensures that only rays rom object reach ilm Cameras and eyeballs The eye has much in common with a camera. Images are ormed in the same way using: Lens Iris Film/retina Much o what we learn about image ormation is relevant to both 3

4 What does it mean to make an image? Camera creates an image o an object on the ilm Create a pattern o light that exactly reproduces that pattern o light in the original scene 1) Collect multiple rays coming rom each point on object 2) Reocus the rays rom each point on the object back to a point on the screen to orm the image 7 Question about orming images: Create image o two light sources with lens and cover right hal o lens. What will happen on screen? a. Image on let will disappear. b. Image on right will disappear. c. Something else. 8 4

5 How does a lens bend and reocus rays o light? Reraction at the air/glass and glass/air interaces Concrete (air) Sand (glass) Speed o light = wavelength requency Smaller in glass than air Same in air and glass First consider a light ray travelling through a rectangular block o glass Amount o Bend depends on: 1. Change in speed o light at interace (speed = c/n) - Bigger speed change more bend! - For glass, n=1.5 (light travels 50% slower in glass than air) AND 2. Angle (q) at which light hits surace. - Bigger q, more bend! AIR, n=1 GLASS, n=1.5 Small Relection q Bends towards normal Bends away rom normal In a rectangular slab, light ray straightens out again when it comes back out! 10 5

6 Now consider light rays travelling through a lens rom a point source o light Glass Note that lens has curved (not parallel) sides A B C Which ray o light will have changed direction the most upon exiting the glass? 11 Now consider light rays travelling through a lens rom a point source o light Light rays bent towards each other CONVERGING LENS. Rays rom a single point on object converge to a single point on image and then start diverging again. 12 6

7 Rays rom distant objects 1. Point source close to lens - Rays that enter lens have a wide range o angles 2. Point source urther rom lens - Rays that enter lens have a narrower range o angles 3. Point source very ar rom lens - Rays that enter lens have a miniscule range o angles - eectively parallel Focal length Describes power o lens to bend light Enables you to ind where image will be ormed. Parallel rays rom distant light source Focal length () Focal length () A more curved (bulgy) lens: - Has less parallel sides - More bending power - Parallel rays converge at shorter distance rom the lens. - Shorter ocal length 14 7

8 Converging lens Focal length () Bends light rays together Forms a REAL image ater the lens Has a positive ocal length Diverging Lens - Bends light rays apart Forms a virtual image BEFORE the lens Has a NEGATIVE ocal length Virtual image - rays do not pass through image (they only appear to) - you can t put a screen in the image plane Real image - rays really do converge at the image - you can put a screen in the image plane 16 8

9 Ray tracing to ind where the image orms Put Film Here Three Special Rays (FOR EACH POINT ON OBJECT) a) ray through ocus becomes parallel b) ray through center stays on track c) parallel ray goes through ar ocal point Image orms in plane where special rays intersect (only need 2 o the three) 17 Question about orming images: Back to our image o 2 light sources ormed by a lens: Which is the image o source 1: A or B? A B From above A B 18 9

10 How ar back does the image orm? Object Distance (o) Image Distance (i) Put Film Here What happens to the image i we move the object closer to the lens? a. Image is same size, same place b. Image is same size and urther rom lens Do experiment. c. Image is bigger and urther rom lens d. Image is smaller and closer to lens 19 What happens i we move the object closer to the lens? Lens equation: 1 = 1 o + 1 i o i 10

11 What will happen to image i we increase the ocal length o the lens? a. Image is same size, same place b. Image is same size and urther rom lens c. Image is bigger and urther rom lens d. Image is smaller and closer to lens o i 21 What will happens to brightness o image as we increase the ocal length? a. Less bright b. More bright c. Same brightness o i 22 11

A useul simulation http://phet.colorado.edu/new/simulations/sims.php?sim=geometric_optics Or on course website: used/useul simulations, optics 23 Why is the iris (aperture) important?

12 A useul simulation Or on course website: used/useul simulations, optics 23 Why is the iris (aperture) important? Consider 2 objects at dierent distances rom lens Iris Screen at Focus o 2. What will happen at screen i close down iris in ront o lens? a. Images brighter and object 1 more in ocus b. Images dimmer and object 1 more in ocus c. Images brighter and object 1 more out o ocus d. Images dimmer and object 1 more out o ocus 24 12

13 F-number (/#) controls image brightness F-number (/#) = / d d /# Large: Focal length large (light spread over larger area) Small diameter lens (less light captured) Dimmer image. /# Small: Focal length small (light spread over smaller area) Larger diameter lens (more light captured) Brighter image. 25 Application: Setting up a manual camera OR. so that s what the numbers on my camera mean! OR...thank goodness or automatic cameras! Aperture setting (/# or stops) Shutter speed Exposure How to pick the right combination o the above 13

1. Setting the aperture Standard -numbers (or -stops) For a given lens, /# setting controls aperture diameter Series o

4) Increasing F-stop by 2 decreases aperture area by 2. (Aperture diamater 2 ) Standard -stops /2, /2.8, /4, /5.

14 1. Setting the aperture Standard -numbers (or -stops) For a given lens, /# setting controls aperture diameter Series o standard /# settings called -stops F-stops increase by actor o 2 (~ 1.4) Increasing F-stop by 2 decreases aperture area by 2. (Aperture diamater 2 ) Standard -stops /2, /2.8, /4, /5.6, /8, /11, /16, /22 Smaller /# Larger aperture Brighter image Smaller DOF Larger /# Smaller aperture Dimmer image Larger DOF 35mm lens set to / Setting the shutter speed Length o time shutter is open in seconds Standard shutter speeds increase by a actor o 2.1/2000, 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30,,,,,,s Pinwheel at dierent shutter speeds Fast e.g. 1/4000s Medium e.g. 1/125s Slower e.g. 1/30 s Shutter speed knob 14

15 3. Exposure - The amount o light per area reaching the ilm For a given lens: Exposure aperture area exposure time (shutter speed) 1/(/#) 2 exposure time Under-exposed Correctly exposed Over-exposed Choose exposure settings based on: - Scene lighting - Film speed (light sensitivity o ilm) - Desired appearance o photograph Aperture and shutter speed combinations Exposure 1/(/#) 2 exposure time You can get the same exposure using a range o dierent aperture and shutter speed settings Example: I get the correct exposure i I use an aperture o /11 and a shutter speed o 1/125 s To achieve approximately the same exposure what shutter speed do I need i I use an aperture o /5.6? a) 1/125 s b) 1/250 s c) 1/500s d) 1/60s e) 1/30s 15

16 Aperture and shutter speed combinations I determine the required exposure (with a lightmeter). How do I pick the aperture and shutter speed combination? Consider: - Motion o subject - Motion o camera - do I have a tripod? Prioritize - Depth o ield Example: I photograph 100m sprinters at a track and ield event. I want to ensure that my pictures are not blurred What would the most suitable shutter speed be? a) 1/2000s b) 1/125s c) 1/4s In summary When setting up a photograph I need to consider A) Exposure - How much light is there (measure with lightmeter) - What ilm speed do I have B) Aperture and shutter speed. Together they achieve the desired exposure Prioritize the ollowing: - Motion o subject or camera - Depth o ield - Special eects For the rest o us. Thank goodness or automatic cameras! 16

at dierent distances cornea eyelens retina What does 20/20 vision mean?

17 How does the eye orm an image? T he eye uses 2 lenses to orm a sharp image on the retina : Cornea Strong lens o ixed ocal length does most o the ocusing Eyelens Weaker lens o variable ocal length - Enables us to ocus on objects at dierent distances cornea eyelens retina What does 20/20 vision mean? Visual acuity is usually measured with a Snellen chart Snellen chart Furthest distance that the test subject can see object 20/30 Furthest distance that a person with normal vision can see object Examples: 20/20 vision - Normal 20/40 vision - Worse than normal. - Can see at 20 eet what someone with normal vision can see at 40 eet - Can only see letters that are twice as large as someone with normal vision 20/15 vision - Better than normal 17

18 Levels o Vision * 20/20 - Normal vision. Fighter pilot minimum. Required to read the stock quotes in the newspaper, or numbers in the telephone book. * 20/40 - Able to pass Driver's License Test in all 50 States. Most printed material is at this level. * 20/80 - Able to read alarm clock at 10 eet. News Headlines are this size. * 20/200 - Legal blindness. Able to see STOP sign letters. * For normal vision, the ar point (or arthest point that can be seen) is usually ininity. * For normal vision, the near point (or closest point that can be seen) is 25 cm. Common Eye problems - Near and ar-sightedness Relaxed eyes viewing a distant object retina Normal Myopic (Near-sighted) Hyperopic (Far-sighted) Primarily determined by: Shape o cornea (crude ocusing) Length o eyeball 18

Myopia short sightedness What the world looks like Facts 1 in 4 Americans Starts in early teens as eyeball lengthens Cause probably genetic Not caused by TV, reading too young, etc

19 Myopia short sightedness What the world looks like Facts 1 in 4 Americans Starts in early teens as eyeball lengthens Cause probably genetic Not caused by TV, reading too young, etc Foreground clear Distant objects blurred Treatments or myopia Glasses Contact lenses Eye surgery latten the cornea, increase ocal length Radial keratotomy Photoreractive keratotomy (PRK) LASIK 19

20 Radial Keratotomy Cut several spoke-like slits in the cornea with a diamond knie Causes central area to latten and ocal length increase. 85% o people achieve 20/40 vision or better Photoreractive keratotomy (PRK) Uses a computer controlled eximer laser Laser emits pulses lasting only billionths o a second Laser vaporises microscopic slivers rom the cornea theoretically making very accurate and speciic modiications to its shape. Procedure takes 1-2 minutes Procedure is pain ree but 10-20% o people eel pain or weeks aterwards 95% achieve 20/40 vision or better LASIK Like PRK but perormed deeper inside the cornea A surace lap is removed and then replaced to give access inside cornea. Generally results in quicker and less painul recovery than PRK Early studies suggest better inal vision than RK or PRK 20

21 Question on myopia To correct or myopia or near-sightedness, your glasses need to contain: A. Positive (converging) lenses B. Negative (diverging) lenses C. Biocals Question on hyperopia To correct or hyperopia, do you need glasses with: A: Convex lens B: Concave lens C: Biocals 21

Midterm Exam. Lasers. Gases and pressure. Lenses so far. Lenses and Cameras 4/9/2017. Office hours

Midterm Exam. Lasers. Gases and pressure. Lenses so far. Lenses and Cameras 4/9/2017. Office hours Lights. Action. Phys 00, Day : Cameras Reminders: HW 9 due NOW and 0pm tonight on DL Lab 8 today/tomorrow Email AB and EH by THURSDAY i you need to do a make up lab MT 3 on THURSDAY Exam Thursday in class