Physics 1230: Light and Color

Transcription

1 Physics 1230: Light and Color Photography Photography F/number Shutter speeds Exposure Film Taking good photographs Lenses Digital cameras

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3 PHOTOGRAPHY Simplest camera - pinhole camera

4 TYPES OF CAMERA 35 mm rangefinder: This is a compact camera that uses a rangefinder to tell you when the image is in focus. 35 mm SLR: SLR stands for single lens reflex. Here you look into a viewfinder which sees the image that is on the film. This is done with a movable mirror. View camera: This is the old-fashioned type of camera that requires a tripod. These are still used by professionals. Large format SLR: This is similar to the 35 mm SLR, but it uses larger film (often 70 mm). Digital camera: These cameras do not use film, but instead record the image with a CCD or charge coupled device, which is an array of sensors. The image is divided into little squares called pixels, and the amount of light in each pixel is stored in a file that can be read by a computer.

5 Parts of a camera Lens: Makes the image. Diaphragm or iris or stop: Controls the intensity of the light. Shutter: Controls the duration of the light. Film or CCD: Records the image

6 Other parts of a camera Focusing knob: As the object gets closer, the lens must be moved farther from the film. Bellows: On older cameras, bellows allow the lens to move without letting in light. Ground glass: Scatters light from all parts of the image into the eye so you can focus camera.

7 Imaging with lenses

8 Parts of a camera - diaphragm or iris or stop i.e., a variable aperture f-stops control how much light is passed through the lens. Like the pupil in a human eye, the aperture on a camera controls light. It does so by closing down to restrict light, and opening up to let it through. Lower f-stop numbers indicate MORE LIGHT. f/number is related to the size of the opening in the iris by - DEFINITION: f/number = focal length of lens diameter of hole in iris Every step in this table represents a ONE STOP change in light.

9 f/number dial on a camera

10 Example of diaphragm or iris or stop DEFINITION: f/number = focal length of lens diameter of hole in iris So f/2 is an opening with diameter equal to 1/2 the focal length of the lens, or (focal length)/2 Every step in this table represents a ONE STOP change in light.

11 Concept question How much more light is f/2 than f/4? A: 2 B: 4 C: 8 DEFINITION: f/number = focal length of lens diameter of hole in iris

12 Concept question QUESTION: If f = 50mm and iris size d = 12.5mm, what is the f/number? A: 2 B: 4 C: 8 DEFINITION: f/number = focal length of lens diameter of hole in iris

13 Why are there funny numbers like 2.8 and 5.6 and 11 on the f/number dial? A. The diameters change by a factor of square root of 2, so that the amount of light let in changes by 2 from one stop to the next. B. The area changes by 2 from ine stop to the next. C. The numbers are picked randomly. DEFINITION: f/number = focal length of lens diameter of hole in iris

14 Aberrations - Spherical Spherical aberration happens because a sphere does not have a perfect focus for light rays Rays at the edge focus closer than rays from the center Want to restrict the rays at the edge to get a good image

15 Effect of f-number on your picture quality All lenses have residual aberrations, which can be reduced by stopping down However, there is another important reason to use a variable stop - it can improve the DEPTH OF FIELD The more the lens is stopped down, the greater the depth of field The depth of field also depends on the focal length of the lens

16 Review: finding images using the Thin Lens formula Say that I know F and the distance of the object from the lens Can I find out where the image will be? Yes from - 1/x o + 1/x i = 1/F => 1/x i = 1/F - 1/x o where x i = distance to image, x o = distance to object, and F = focal length

17 Depth of Field All lenses have residual aberrations, which can be reduced by stopping down However, there is another important reason to use a variable stop - it can improve the DEPTH OF FIELD, which is a measure of how much things can be separated and still be in focus. The more the lens is stopped down, the greater the depth of field The depth of field also depends on the focal length of the lens

18 The more the lens is stopped down, the greater the depth of field in this example, neither object a or b is in focus at the film plane: both are slightly out of focus closing down the aperture reduces the angular divergence of the beams behind the focal point

19 Sometimes a shallow depth of field is good

20 Sometimes a shallow depth of field is good

21 But sometimes a large depth of field is desirable

22 Sometimes a little blurring of the background is enough to achieve a nice efect

23 Concept question Which image has a better depth of field? A. f/2 B. f/16 C. Neither

24 Concept question How much bigger in area is the f/2 compared with the f/16? A. 64 B. 8 C. 2 D. 16

25 f/16 Photographs with different stop size In which direction is the camera being stopped down (i.e., the aperture made smaller)? A. Top to bottom B. Bottom to top f/2

26 The f-stop characterizes how much light a lens collects Long focal length lens with aperture stopped to f/2 Same lens as above but at f/4. Notice that light meter reads lower. Shorter focal length lens stopped at f/2. Now the image is smaller, but the illumination (intensity) is the same as in top figure.

27 Parts of a camera - shutter Shutter speed determines HOW LONG the shutter stays open. Examples A half second exposure is ONE STOP darker than a one second exposure. A 1/125 exposure is TWO STOPS brighter than a 1/500 exposure. A 1/1000 exposure is THREE STOPS darker than a 1/125 exposure. The longer exposures ( like 1 second ) give much MORE light to the film than a 1/1000 of a second exposure. So even though the number may look bigger, don't be deceived!

28 You Be the Photographer! What settings would you use to take the photo at right?

29 Here s an photograph taken with no blurring and large depth of field:

30 Exposure is about different combinations of shutter and f-stop settings. These combinations can drastically affect the finished picture. For example, the following three pictures have been given an equal amount of light, but the f-stop and shutter combinations make each one unique.

31 Why is the background all blurred in the right picture, and sharpest in the left? Because if the exposure is made with a wide aperture ( like f/2.8 ), then objects farther away from the subject are thrown farther out of focus.

32 So... if the aperture is small (like f/22) then objects in the background (and foreground) will appear sharper. However, since a longer shutter time was required to make the exposure on the left ( 1/4 Second ) in this example the subjects became blurred from MOTION.

33 Larger f/numbers (smaller apertures) give larger depth of field but need longer exposure times and thus may blur. Small f/numbers need short exposure times because of large apertures. They also have limited depth of focus.

34 What do you notice about the light levels (exposure) in all these images? It is the same in all these images because the combination of f/number and shutter speed are equivalent combinations from an exposure point of view.

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36 Taking a photograph If the camera is all set up to take a photo of the tree, and then your friend wants a head portrait, do you need to - A. Move lens away from film B. Move lens towards film C. Leave lens in same place D. Other

37 Taking a photograph If the camera is all set up to take a photo of the tree, and then your friend wants a head portrait, do you need to - A. Move lens away from film B. Move lens towards film C. Leave lens in same place D. Other

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39 Lenses Popular lens focal lengths mm wide angle (covers 65 degrees) - 50 mm normal angle (covers 40 degrees) standard lens mm telephoto (covers 15 degrees) - Zoom lenses have variable focal length The focusing knob moves the lens back and forth so that you can get the object focused. Often there is a scale marked in feet (or meters!) All distant objects are in focus when the distance from the film to the lens is the focal length.

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41 Telephoto camera lenses Telephoto lenses are useful for viewing and enlarging far away objects What is the magnification for a single lens? A single lens would have to stick out much farther in front of the camera than the telephoto combination

42 Concept Question Why does the telephoto lens enlarge the image of far away objects? A. Magnification = Image distance/object Distance and telephoto lens is like a longer focal length lens with longer image distance B. The diverging lens makes the image bigger C. The object distance is shorter

43 Zoom lenses Zoom lenses are lenses of variable focal length, where the focal plane is fixed on the film plane Such a lens can zoom between normal and wide-angle pictures, and easily enlarge the image.

44 Compound lenses Compound lenses allow one to design a lens combination that eliminates aberrations

45 View Cameras Not too many photographers start with a 4x5" view camera, but that is rather a shame. View cameras are the most flexible cameras, usually made from a basic design that has not changed for over 100 years. You know the guy in the old time photo studio who photographs with his head under a cloth? He's using a view camera. Ansel Adams? He took most of his best photos with a huge 8x10" view camera. All those luscious ads for food in magazines? Taken with view cameras. A view camera is fundamentally a light-tight box with a slot at one end for a lens and a slot at the other for the film. You compose and focus your image on a ground glass, then displace the glass with a sheet of film four by five inches in size. The negative from a view camera is about the same size as a proof print that you get back from a 1-hour lab. what-camera-should-ibuy.html

46 View Cameras and Perspective Another advantage of using a view camera is that it gives you an understanding of perspective. With a view camera, the lens and film aren't fixed parallel to each other. This opens up a huge range of creative opportunities that are unavailable to most users of 35mm and medium format gear. For example, if you want to take a photo of a building with a Nikon, you have to point the camera up towards the sky. You will then be projecting the vertical exterior of the building onto the angled film surface. The lines of the building will converge towards the top of the frame. With a view camera, you shift the lens up and/or the film down. The film is now "looking up" at the building through the lens, but the film is still parallel to the building exterior so lines don't converge.

47 Problem of converging lines A PC (perspective control) lens allows one to slide the film frame independently of the other parts of the camera

48 35mm SLR camera The 35mm single lens reflex (SLR) is most folks' idea of a serious camera. SLR means that the same lens is used for viewing and taking pictures. A mirror in the body directs the light from the lens up into a prism for viewing, then flips up out of the way just before an exposure is made. These are extremely versatile instruments in the right hands and can take beautiful pictures if used with care. photo/what-camerashould-i-buy.html

49 Fast and Slow Film This isn't about how fast you can take pictures. It's about how much light you need to get clear images. Film speed indicates sensitivity to light. It's expressed as the ISO number you see on your film boxes. A high-speed (fast) film captures light faster than a slow speed film so you don't need as much light to get a properly exposed image. So be sure to select a higher-speed film (ISO 400 or above) when you're shooting indoors or on a cloudy day. Likewise, a low-speed, slower film (like ISO 100) requires more light and should be used in bright, sunny situations. If you have an older camera, the ASA (American Standards Association) number is the same as today's ISO numbers. So if the speed of your film is ISO 200, you should set 200 on the ASA dial of your camera or meter. A 400 speed film is twice as fast as a 200 speed film and needs half of the light. Why should I not take ASA 25 film to a football game?

50 Equivalent Exposures with Film Speed f/2 and a 1/200 of a second shutter speed with ASA 200 film is the same as - f/2 and a 1/400 of a second shutter speed with ASA 400 film and f/2 and a 1/25 of a second shutter speed with ASA 25 film Example Canon EOS 350D 1/200 second F/9.0 ISO mm

51 You Be the Photographer! What settings would you use to take the photo at right?

52 How does black & white film work? In modern film, small silver halide crystals are embedded in a gelatin emulsion. This side faces the lens. Exposure to light breaks the bonds of chemicals containing silver (like silver iodide or silver bromide) and generates silver nuclei. Tiny grains of silver scatter light and look dark. This is the latent image. Once the bonds are broken, other chemicals (developers) can separate the silver, and the places where light hit become even darker. This makes a negative image. The reducing agent works faster where there already are silver nuclei i.e. where film is exposed. To stop the development before the unexposed crystals are reduced to silver, a stop bath is used. (usually an acid, which makes the developer inactive) Fixing then removes the unexposed crystals to prevent them from darkening later. Hypo is used for this step (short for sodium hypochlorite) For fine detailed pictures, film with smaller silver halide crystals is used. This film typically requires longer exposure time i.e. it is slower.

53 Film developing process

54 Digital cameras Digital cameras are the same as a regular camera except that they use an array of sensors instead of film. Often there is an array of 3 sensors, each with a different filter, to capture color images. Digital cameras have many advantages - - Pictures can be transferred directly into computer file - There is no need for film development - One can see the image right away Digital cameras have some disadvantages - - The images are not as detailed as images on film but getting better quickly! - Cameras are more expensive but you save on film!

55 Digital cameras digital SLR CCD array flash memory Canon PowerShot A95

56 Image taken with 1280 x 960 digital camera

57 Magnified image of sailboat shows individual pixels

58 Magnified image of sunset shows individual pixels

59 Digital Camera Picture (2Mpixel, 6MB)

60 Camera Phone Picture, 640x480 pixels (0.3Mpixel or 0.9MB)

61 Camera Phone Picture, 640x480 pixels (0.3Mpixel or 0.9MB)

62 Camera Phone Picture, 640x480 pixels (0.3Mpixel or 0.9MB)

63 War of the Pixels Image size/resolution Many digital cameras allow you to change image resolution. Resolution relates to the size your image appears on a computer monitor rather then the physical dimensions an image will print. For example, common sizes found on a consumer level two megapixel digital cameras are 1600 x 1200, 1280 x 960, 1024 x 768 and 640 x 480. SIZE RESOLUTION Maximum standard print size: Minimum resolution 4 X 6" 800 x 600 pixels 5 x 7" 1024 x 768 pixels 8 x 10" 1220 x 960 pixels 11x14" 1600 x 1200 pixels

64 What can we see?

65 Capturing Color in Digital Cameras The resolution of a digital camera is often limited by the camera sensor (usually a charge-coupled device (CCD) chip or a CMOS transistor array) that turns light into discrete signals, replacing the job of film in traditional photography. The sensor is made up of millions of "buckets" that collect charge in response to light. Generally, these buckets respond to only a narrow range of light wavelengths, due to a color filter over each. Each one of these buckets is called a pixel, and a de-mosaicing/interpolation algorithm is needed to turn the image with only one wavelength range per pixel into an RGB image where each pixel is assigned three numbers to represent a complete color. CMOS image sensor

66 Image Resolution in Digital Cameras the Pixel count The one attribute most commonly compared on cameras is the pixel count. Due to the ever increasing sizes of sensors, the pixel count is into the millions, and using the SI prefix of mega- (which means 1 million) the pixel counts are given in megapixels. For example, an 8.0 megapixel camera has 8.0 million pixels. The pixel count alone is commonly presumed to indicate the resolution of a camera, but this is a misconception. There are several other factors that impact a sensor's resolution. Some of these factors include sensor size, lens quality, and the organization of the pixels (for example, a monochrome camera without a color filter mosaic has a higher resolution than a typical color camera).

67 Capturing Color in Digital Cameras The Bayer filter pattern is a repeating 2 2 mosaic pattern of light filters, with green ones at opposite corners and red and blue in the other two positions. The high proportion of green takes advantage of properties of the human visual system, which determines brightness mostly from green and is far more sensitive to brightness than to hue or saturation. Sometimes a 4- color filter pattern is used, often involving two different hues of green. This provides potentially more accurate color, but requires a slightly more complicated interpolation process. The color intensity values not captured for each pixel can be interpolated (or guessed) from the values of adjacent pixels which represent the color being calculated Digital_camera#Digital_single_lens_reflex_cameras

68 Pixels per Dollar As the technology has improved, costs have decreased dramatically. Measuring the "pixels per dollar" as a basic measure of value for a digital camera, there has been a continuous and steady increase in the number of pixels each dollar buys in a new camera consistent with the principles of Moore's Law. This predictability of camera prices was was first presented in 1998 at the Australian PMA DIMA conference by Barry Hendy and is since referred to as "Hendy's Law.

69 Image Resolution in Digital Cameras can you have too many Pixels? Many digital compact cameras are criticized for having too many pixels, in that the sensors can be so small that the resolution of the sensor is greater than the lens could possibly deliver. Excessive pixels can even lead to a decrease in image quality. As each pixel sensor gets smaller it is catching fewer photons, and so the signal-to-noise ratio will decrease. This decrease leads to noisy pictures, poor shadow region quality and generally poorer-quality pictures.

70 Digital video cameras technology similar to digital still cameras

71 How do I choose a Digital Camera? Judging digital camera quality The quality of a digital camera depends on its resolution, color depth, capacity, optics and convenience features. Here's what to look for: Resolution. The CCD on a digital camera stores the image in a grid of dots, or pixels, that translate into digital ones and zeros that a computer can understand. The more pixels the camera can record, the larger the image and the finer the detail. Generally, you'll pay more money for higher resolution. Most affordable digital cameras can record images in a 640 by 480 pixel grid. That's enough to fill a good chunk of a Web page or produce a decent 4-by-3-inch printed image. With some cameras, you can choose between two resolutions. At the lower resolution, you can store more images, but they won't be as detailed. Color capability. This is the number of colors a digital camera can reproduce, and it's generally measured in something called "bit depth." The bit depth determines how many digital ones and zeros the camera uses to record color information for each pixel. Better cameras can handle 24-bit color, which means they can recognize up to 16 million hues. This approximates the range of the human eye. Capacity. The number of pictures a digital camera can take is determined by its resolution and memory. Low-end digital cameras typically store between 16 and 40 pictures. The actual number may depend on the resolution and compression ratio you choose. Most cameras come with snap-in memory modules. When you fill one, you can replace it with another. But these memory cards aren't cheap -- expect to pay about $100 for a card that can store 15 to 30 full-resolution photos. More sophisticated cameras may have internal floppy disk drives or even tiny hard disks. The best can store a hundred pictures or more and even record sound bytes to go with each shot. Optics. Inexpensive digital cameras come with fixed-focus lenses that produce the equivalent of a moderate wideangle view on a 35mm film camera. They're fine for small groups or landscapes, but tend to exaggerate noses and chins on closeups. Better models have zoom lenses that range from wide-angle to short telephoto. They'll produce better portraits. Most cameras come with a built-in automatic flash. Convenience features. The most important convenience feature is a liquid crystal display screen on the back of the camera. On some models, the LCD takes the place of the viewfinder, so you can see exactly what you're shooting. On other cameras, you'll find an optical viewfinder and an LCD that displays shots after you've taken them. Either way, better cameras will allow you to flip through the photos you've taken and erase shots you don't want. This frees up memory for more pictures.

72 Guide to decent photographs 1. Take 3 times as many photos as you want to keep 2. Soft lighting (sunrise, sunset, slightly overcast) makes better pictures than direct lighting (sun). Direct sun creates shadows which will be underexposed. 3. Film cannot record an extreme range of brightness. Proper exposure of bright objects such as snow or light rock will leave dim objects too black. 4. Fast shutter speeds (1/250, 1/500 etc.) will freeze motion such as waterfalls, sports, traffic, etc., but will require bigger lens opening (e.g. f/2) and give less depth of field. 5. Slow shutter speeds will cause moving objects such as cars to leave streaks. 6. Purposefully blur the background to to call attention to the main object. Small f/numbers (e.g. f/1.4) will blur the background. 7. Big f/numbers (e.g. f/22) will give large depth of field and keep background in focus.

73 Guide to decent photographs ctd. 8. Wide-angle lenses may give a distorted view. It is best not to have any object too close to a wide angle lens, or noses come out too big! 9. Telephoto lenses should be held very still. The larger image formed moves more if the camera is not kept still. A good rule of thumb is: a 200mm lens should not be used with a shutter speed below 1/200 sec; a 400mm lens should not be used with a shutter speed below 1/400 sec. If you must break this rule, use a tripod or friend to steady yourself. 10. Walk around your subject to see what gives the best background. 11. Have your back to the source of light when at all possible. Otherwise shadows will be a problem. Also avoid having the sun shine into the lens, or internal reflections will be created. 12. Consider photographing a part of the subject instead of the whole thing (e.g. leaf of a tree, hands of a person, etc.) 13. Consider editing your subject (move a weed from in front of a flower). 14. Avoid having distractions like power lines in your pictures. 15. For wildlife, a 200 mm or 400 mm telephoto lens is useful. These lenses are usually f/4 to f/22, and do not let in a lot of light. So you will need fast film and a tripod. 16. Keep your camera loaded!