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# Focus

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### Transcript

• 1. Focus, Depth of Field and Perspective Digital Photography
• 2. Sharp focus attracts the eye.
• 3. Sharp focus attracts the eye.
• 4. Think of the “plane of critical focus” as a pane of glass stretched from one side of the scene to the other where everything is in focus.
• 5. 1/3 2/3Notice that the depth of field extends about one-third in front of the plane of critical focus and two-thirds behind it.
• 6. The photographer directsyour eye to the point ofinterest by using focus.
• 7. Micha Bar-Am“Yad Vashem Holocaust Memorial,” Jerusalem, 1981
• 8. The “plane of critical focus” is also called the “image plane.”
• 9. What exactly is sharpness,and how much can it be controlled?
• 10. In theory, a lens can only focus on a flat plane atone single distance at a time (the plane of focus) and objects at all otherdistances will be less sharp.
• 11. But, in mostcases, part ofthe scene will be acceptably sharp both in front of and behind the most sharplyfocused plane.
• 12. Light from a single point on the subject reaches the lens in the shape of a cone. Behind the lens, that same light converges again in the shape of a cone, to a corresponding image point.
• 13. If your film or sensor is located at the tip of that cone of light, that image point will be exactly in focus.
• 14. If the recording surface is not at the tip of the coneand instead slices through the cone in front of or behindthe focus point, it will record a small circle instead of a point.
• 15. A circle can be just small enough that you can’t see that it’s a circle instead of a point. This size is called a “circle of confusion.”
• 16. Any points in the recorded image that make a circle this size or smaller will appear to be in focus. Any points within this range are inside the depth of field.
• 17. Reducing the size of the lens aperture makes the baseof each cone of light smaller and its angle narrower.
• 18. The same size circle of confusion can be recorded farther from the focus point.
• 19. So even though a smaller aperture doesn’t change theactual location of the plane of focus, it widens the range of points that appear to be in focus, thus increasing the depth of field.
• 20. There are actually 3 different ways to control the depth of field in a photograph: change the aperture change the focal length change the distance to subject
• 21. We have already learned how toadjust the aperture to change the depth of field.
• 22. In actuality, the aperture works along with the focallength of the lens to define depth of field.
• 23. The longer the focal length, the less light reaches thesensor or film, therefore a long lens will form a dimmerimage than a short lens unless more light is admitted by the aperture.
• 24. Apertures of the same number are notall the exact same size!
• 25. The sizes ofapertures aredetermined sothat at a givenf-stop numberthe same amountof light reachesthe film, nomatter what thefocal length ofthe lens.
• 26. If the focallength of the lensis 100mm, you needa lens opening of25mm to produce anf/4 aperture.
• 27. If the focallength of the lensis 200mm, you needa lens opening of50mm to produce anf/4 aperture.
• 28. What does all this mean to you?
• 29. When adjusting your camera fordepth of field, JUST REMEMBER: The smaller the aperture, the greater the depth of field The shorter the focal length of the lens, the greater the depth of field The greater the distance from the subject the greater the depth of field
• 30. The smaller the aperture, the greaterthe depth of field
• 31. The shorter the focal length of the lens,the greater the depth of field
• 32. The greater the distance from thesubject the greater the depth offield
• 33. Two more techniques let you control depth of field: Zone focusing Focusing on the hyperfocal distance
• 34. For both of these techniques, you need a lens with a depth-of-field scale. depth-of-field scale.
• 35. ZoneFocusingFocusing
• 36. Zone focusing isuseful when you wantto shoot rapidlywithout refocusing,and can predictapproximately where,if not exactly when,action will takeplace.
• 37. To zone focus, use a lens’s depth-of-field scale to findthe f-stop settings that will give you adequate depth of field.
• 38. Suppose the nearest focus point you want sharp is 7 feet away and the farthest is 13 feet away. Turn the focusing ring until thosedistances on the distance scale fall opposite a matched pair of f- stops on the depth-of-field scale. If you set your lens aperture to that f-stop, objects between the two distances will be in focus.
• 39. Suppose the nearest focus point you want sharp is 7 feet away and the farthest is 13 feet away. Turn the focusing ring until thosedistances on the distance scale fall opposite a matched pair of f- stops on the depth-of-field scale. If you set your lens aperture to that f-stop, objects between the two distances will be in focus.
• 40. This is a typicaldigital lens witha DOF scale.Here the apertureis set to f/11.The depth-of-fieldscale shows thatobjects from 1 to2 meters will bein focus at thisaperture.
• 41. Hyperfocal Distance
• 42. Instead of focusing on infinity, (as shownabove), set the distance scale so that theinfinity mark lines up opposite your chosen f-stop on the depth-of-field scale.
• 43. As shown above, the infinity symbol falls just within depth-of-field scale for the chosen f- stop: f/8.
• 44. Perspective:How a photograph shows depth
• 45. Perspectiveis the waythe brainjudgesdepth.
• 46. The brain judges depth in a photograph mostly bycomparing objects in the foreground with those in the background. The greater the size differences perceived, the greater the impression of depth.
• 47. Compare the scale of the bird to the cage in these 3 views.
• 48. When changing the focal lengths only, the scalebetween the bird and the cage remains consistent.
• 49. Expanded perspective seems to resultfrom a very wide lens, but using any lens close to a subject stretches distances because itmagnifies objects near the lens in relation to those that are far from the lens.
• 50. Compressed perspective is usually associatedwith a long focal length lens. It is because the lens is relatively far from both foreground and background thatsize differences between near and far parts of the scene are minimized.
• 51. Making “Close-Ups”
• 52. Close-up Photography
• 53. The best way to take close-upphotographs is with a macro lens.
• 54. There are severaltypes of macro lensesand they are oftendistinguished by theirfocal lengths.
• 55. A 50-60 mm lens is typically usedfor small objects and product shots.
• 56. A 90 - 105 mm range macro lens is versatile and can be usedfor anything from insects to small objects.
• 57. A 120 - 200 mm range lens gives you more working distance and is commonly used for insects and other small animals.
• 58. If a macro lens isnot in yourbudget, areasonably pricedalternative is aclose-up lens.
• 59. A close-up lensattaches to the frontof your camera lens.They come indifferent strengths(measured indiopters). The higherthe diopter number,the closer you canfocus.
• 60. Close-up lenses are relatively inexpensive and small, but imagequality will not be as good as with other close-up methods.
• 61. Still another option for close-upphotography is a bellows which fits between the lens and the camera toincrease the distance from the lens to the sensor.
• 62. Extension tubes work essentially the same way as a bellows. The object of both is to increase the distance from the lens to the sensor.
• 63. Extension tube attachedbetween the camera body and lens.
• 64. How big is a “close-up?”
• 65. A photograph is considered aclose-up if it is 1/10 life size (1:10)to life size (1:1).
• 66. Macro-photography generally refersto an image that is anywhere fromlife size (1:1) to as big as 10 timeslife size (10:1).
• 67. Micro-photography is achieved byphotographing through a microscopeto get an image larger than 10times life size.
• 68. Other terms commonly used are:Photomacrography and Photomicography. .
• 69. Depth of fieldis shallow inclose-ups sofocusing becomescritical.
• 70. A 50 mm lens at a distance of about12 inches from the subject has a DOF of 1/16 inch when the aperture is set to f/4.
• 71. At f/11 the DOF increases - but only to 1/2 inch!
• 72. Here you can see how increasing thedistance between the lens and the sensor increases the size of the image.
• 73. Regardless of the method used to take a close-up,you must move the lens farther from the sensor tofocus closer to your subject.
• 74. And the fartherthe lens isextended, thedimmer the lightthat reaches thelight-sensitivesurface.
• 75. So you must increase theexposure time so the photo will not be underexposed.
• 76. Use a tripod to take close-ups to prevent motion blur.
• 77. Use a cable release if you have one.
• 78. You can also use a flash to “freeze” the motion.
• 79. These photoswere made usinga macro lensand a flash.
• 80. Filters
• 81. A polarizingfilter removes reflections from surfaces such as glass
• 82. This works best at a30 - 40 degree angle.
• 83. A polarizing filter alsomakes distant objects clearer and the sky darker.
• 84. The effect works best when you are shooting at a 90 degree angle from the sun.
• 85. Other populartypes of filters are soft focusand star (crossscreen) filters.