Physics 1230: Light and Color Ivan I. Smalyukh, Instructor Office: Gamow Tower, F Phone: Lectures: Tuesdays & Thursdays, 3:30 PM - 4:45 PM Office hours: Mondays & Fridays, 3:30 PM – 4:30 PM TA: Jhih-An Yang Class # 13 Chapter #4: Cameras & Photography

22 Ch. 4 –The Camera and Photography Camera parts Lenses for cameras Controlling light Photography as art Photography as science We are here 2 Ansel Adams

33 Ansel Adams

44 Siege of Yorktown, 1861 James F. Gibson

5 Early Daguerreotype photos 5 Poe, 1848 Paris, 1838 Subject must remain still for minutes. Mercury droplets on polished silver.

Chapter # 4: Cameras Largely based on the knowledge about lenses; Understanding of lenses & applications of the thin lens equation is important! 6

A pinhole camera works by blocking rays Pinhole Camera Light bulb Image of light bulb blocked rays What is an image? A real image is formed on a screen when rays from each point on the object reach the corresponding points on the screen and no other rays from other points on the object reach those points

Why we do not use such a simple camera? (A) Not very bright images; (B) Focusing is difficult; (C) Because of spherical aberrations; (D) A, B, & C (E) none of the above;

A telephoto effect (enlarged image) for a camera with a LENS requires increasing the distance from lens to camera back AND increasing the focal length of the lens to keep the image in focus As the distance between the lens and the back of the camera is increased the image remains in the same place –It is no longer in focus at the back of the camera –It is in focus at the same place as before –x 0 and f are the same so x i must be at the same place In addition to increasing the distance between the lens and the back of the camera the focal length of the lens must be increased by the right amount! –The new image is larger and in focus f f f new

Review: the lens equation F = focal length X O = object distance X I = image distance Usually, F and X O are given. We want to find X I 10 Distant objects: Let X o be very large, say 1,000,000 meters. Then 1/X o = , which is very small. You can ignore it. Then For distant objects, the image is at the focal point (ask a burnt ant)

Review: application of the lens equation F = focal length X O = object distance X I = image distance Usually, F and X O are given. We want to find X I 11 When are the object distance to the lens and the image distance from the lens equal?? OR

The lens equation gives the same results as ray-tracing but without any rays! 1/x 0 + 1/x i = 1/f f is focal length of lens x o = positive distance from object to center of lens (when object is left of the lens) x i = distance from image to lens center x i is a positive number for (real) image on other side of lens from object. x i is a negative number for (virtual) image on same side as object. Given two, find the third. Can use the lens equation to find image position if know object position or vice versa, without any rays ff xoxo xixi (will be positive number for this case)

Here is one example of how to use the lens eqn with a converging lens Given: f = 10 cm Object is 15 cm in front of lens: x 0 = 15 Find: –Where is image and is it real or virtual? Solve equation for x i : –Substitute numbers for letters –Subtract 1/15 from both sides –Arithmetic on calculator –Multiply by x i /0.033 — = — + — 111 fxoxo xixi — - — = — 111 xixi = — 1 xixi x i = — = 30 cm Image is 30 cm to right of center of lens and is real because x i is positive

Here is a sketch to show the previous result We can verify our result by ray-tracing f = 10 x o = 15 x i = 30

Here is an example of how to use the lens eqn with a diverging lens (see Fig. 3.28) Given: f = —5 cm NOTE, THE FOCAL LENGTH OF A DIVERGING LENS IS NEGATIVE Object is 12 cm in front of lens: x 0 = 12 Find: –Where is image and is it real or virtual? Solve equation for x i : –Substitute numbers for letters –Subtract 1/12 from both sides –Arithmetic on calculator –Multiply by x i /(-0.283) — = — + — 111 fxoxo xixi -512 — - — = — xixi = — 1 xixi x i = — = cm Image is 3.53 cm to left of center of lens and is virtual because x i is negative

Summary of the meaning of negative number in the lens and magnification equations Negative focal length, f, means the lens is diverging. –Otherwise it is converging. Negative image distance, x i, from image to lens means the image is on same side as object and virtual (rays coming from it never really meet) –Otherwise image is real Negative image distance, x I, means the image is on the same side of the lens as the object (a virtual image). Negative magnification, M, means the image is upside down (inverted) relative to the object. 1/f = 1/x i + 1/x o M = -x i /x o

17 Questions: (A)Or (B): The object distance is POSITIVE or NEGATIVE? The image distance is POSITIVE or NEGATIVE? The image height is POSITIVE or NEGATIVE? Always! Same side means… Both above or below?

The lens equation: The land of “One-Over-Everything!” F = focal length X O = object distance X I = image distance Usually, F and X O are given. We want to find X I 18 Eventually you start to use 1/everything with ease. Then… give those things a name! Call 1/F the LENS POWER. The units are DIOPTERS or 1/meters.

A Question 19 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?

A Question 20 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? Physics is always based on the experiment!!

Cameras existed for hundreds of years (pinhole and lens versions) The revolution (early 19 th century) was FILM: The method to store and reproduce the images. 21 But first, let’s understand the camera system.

22 I. Camera parts 1.Lens 2.Shutter 3.Diaphragm 4.Focusing screen 5.Film (or digital imaging chip) 22

23 Camera parts (compare to camera you use) 23 Lens Sheet film holder Bellows Shutter & Diaphragm Focus knob This is an old-fashioned view camera.

24 Two common kinds of cameras Point and shoot One lens, viewfinder or liquid crystal display (LCD) Point and shoot digital Single lens reflex (SLR) Interchangeable lenses Single-lens reflex LCD is on the back.

25 Camera parts 25 Sheet film holder Bellows Shutter & Diaphragm Focus knob Lens Produces the image that you want to record.

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

27 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 Back of camera

28 Some common lens focal lengths 40º 28 mm f. l. Wide angle 50 mm f. l. normal 135 mm f. l. telephoto 15º 65º

29 Zoom lenses have adjustable focal length 25 mm f.l. to 250 mm f.l = 10x zoom Lens may have two fixed groups and two moving groups to hold focus constant while zooming

30 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.

10 40 What are apertures, f-numbers and stops? The diameter of the aperture of a lens can be reduced by means of the diaphragm. –The largest aperture is the full diameter of the lens –Smaller apertures are called stops The aperture is measured by the f- number = focal length of the lens divided by the diameter of the aperture: f-number (or f-stop) = f/d –Hence, a large f-number means a small lens diameter What is the f-number (f-stop) of the same lens when the diaphragm reduces the diameter of the lens from 10 mm to 5 mm? 40 mm focal length lens at full aperture of diameter d = 10 mm 40 mm focal length lens with aperture of diameter d = 5 mm f-number =  f d = 4 ( = f/4) a) f/2, b) f/4, c) f/6, d) f/8, e) f/10 105

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? 32 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.

33 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

Sequence of shutter speeds in which each faster speed lets in half the light energy 1/15 sec 1/30 sec 1/60 sec 1/125 sec 1/250 sec 1/500 sec 1/1000 sec 1/2000 sec Faster speeds, better able to stop (freeze) fast motion but give darker image

35 F number controls the amount of light 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

36 F-number scale = factors of two in light 1 Big opening Factors of √2 apart Small opening Amount of light scales with area Area scales with (diameter) 2 more light less light Diaphragm has variable opening.

37 What f-number scale looks like

38 Amount of light f numberAmount of light 1100% 1.450% 225% or 1/4 2.81/8 41/ /32 81/64

39 Why can images be out of focus? F fuzzy in focus Rays of light diverge on either side of the focal point.

40 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

41 Here are two lenses. One is f-number 8 and the other is f-number 2. The top lens is which one? F fuzzy in focus big blur small blur (A) f-8 (B) f-2

42 f-number 8 has more depth of field (less blur) than f-number 2 F fuzzy in focus big blur small blur More like a pinhole camera

43 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

44 Equivalent exposure settings These combinations give the same light f numbershutter speed 1.41/250 21/ /60 41/ /15 81/8 111/4 Sports events, stops motion. Great depth of field, some motion

45 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.

46 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.

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