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 2/3
Notice that the depth of field extends
about one-third in front of the plane
of critical focus and two-thirds
behind it.

8. The photographer directs
your eye to the point of
interest by using focus.

10. Micha Bar-Am
“Yad Vashem Holocaust Memorial,” Jerusalem, 1981

12. The “plane of critical focus”
is also called
the “image plane.”

13. What exactly is sharpness,
and how much can it be controlled?

14. In theory, a lens can only
focus on a flat plane at
one single distance at a time
(the plane of focus)
and objects at all other
distances will be less sharp.

16. But, in most
cases, part of
the scene will
be acceptably
sharp both in
front of and
behind the
most sharply
focused plane.

17. 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.

18. If your film or sensor is located at the tip of that cone of
light, that image point will be exactly in focus.

19. If the recording surface is not at the tip of the cone
and instead slices through the cone in front of or behind
the focus point, it will record a small circle instead of
a point.

20. 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.”

21. 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.

22. Reducing the size of the lens aperture makes the base
of each cone of light smaller and its angle narrower.

23. The same size circle of confusion can be recorded
farther from the focus point.

24. So even though a smaller aperture doesn’t change the
actual location of the plane of focus, it widens the
range of points that appear to be in focus, thus
increasing the depth of field.

27. 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

28. We have already learned how to
adjust the aperture to change the
depth of field.

29. In actuality, the aperture
works along with the focal
length of the lens to define
depth of field.

30. The longer the focal length, the less light reaches the
sensor or film, therefore a long lens will form a dimmer
image than a short lens unless more light is admitted by
the aperture.

31. Apertures of the
same number are not
all the exact same size!

32. The sizes of
apertures are
determined so
that at a given
f-stop number
the same amount
of light reaches
the film, no
matter what the
focal length of
the lens.

33. If the focal
length of the lens
is 100mm, you need
a lens opening of
25mm to produce an
f/4 aperture.

34. If the focal
length of the lens
is 200mm, you need
a lens opening of
50mm to produce an
f/4 aperture.

36. What does all this mean to you?

37. When adjusting your camera for
depth 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

38. The smaller the aperture, the greater
the depth of field

39. The shorter the focal length of the lens,
the greater the depth of field

40. The greater the distance from the
subject the greater the depth of
field

41. Two more techniques let you
control depth of field:
Zone focusing
Focusing on the hyperfocal distance

42. For both of these techniques,
you need a lens with a
depth-of-field scale.
depth-of-field scale.

44. Zone
Focusing
Focusing

45. Zone focusing is
useful when you want
to shoot rapidly
without refocusing,
and can predict
approximately where,
if not exactly when,
action will take
place.

46. To zone focus, use a lens’s depth-of-field scale to find
the f-stop settings that will give you adequate depth of
field.

47. Suppose the nearest focus point you want sharp is 7 feet away and
the farthest is 13 feet away. Turn the focusing ring until those
distances 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.

48. Suppose the nearest focus point you want sharp is 7 feet away and
the farthest is 13 feet away. Turn the focusing ring until those
distances 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.

49. This is a typical
digital lens with
a DOF scale.
Here the aperture
is set to f/11.
The depth-of-field
scale shows that
objects from 1 to
2 meters will be
in focus at this
aperture.

50. Hyperfocal Distance

51. Instead of focusing on infinity, (as shown
above), set the distance scale so that the
infinity mark lines up opposite your chosen
f-stop on
the depth-of-field scale.

52. As shown above, the infinity
symbol falls just within depth-
of-field scale for the chosen f-
stop: f/8.

56. Perspective:
How a photograph shows depth

57. Perspective
is the way
the brain
judges
depth.

58. The brain judges depth in a photograph mostly by
comparing objects in the foreground with those in
the background. The greater the size differences
perceived, the greater the impression of depth.

60. Compare the scale of the bird to
the cage in these 3 views.

62. When changing the focal
lengths only, the scale
between the bird and the cage
remains consistent.

69. Expanded perspective seems to result
from a very wide lens, but using any lens close
to a subject stretches distances because it
magnifies objects near the lens in relation to
those that are far from the lens.

70. Compressed perspective is usually associated
with a long focal length lens. It is because the lens is
relatively far from both foreground and background that
size differences between near and far parts of the scene
are minimized.

71. Making “Close-Ups”

72. Close-up Photography

73. The best way to take close-up
photographs is with a macro lens.

74. There are several
types of macro lenses
and they are often
distinguished by their
focal lengths.

75. A 50-60 mm lens is typically used
for small objects and product shots.

76. A 90 - 105 mm range macro lens
is versatile and can be used
for anything from insects to small
objects.

77. A 120 - 200 mm range lens gives
you more working distance and
is commonly used for insects
and other small animals.

78. If a macro lens is
not in your
budget, a
reasonably priced
alternative is a
close-up lens.

79. A close-up lens
attaches to the front
of your camera lens.
They come in
different strengths
(measured in
diopters). The higher
the diopter number,
the closer you can
focus.

80. Close-up lenses are relatively
inexpensive and small, but image
quality will not be as good as with
other close-up methods.

81. Still another option for close-up
photography is a bellows which fits
between the lens and the camera to
increase the distance from the lens to
the sensor.

82. 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.

83. Extension tube attached
between the camera body
and lens.

84. How big is a “close-up?”

85. A photograph is considered a
close-up if it is 1/10 life size (1:10)
to life size (1:1).

86. Macro-photography generally refers
to an image that is anywhere from
life size (1:1) to as big as 10 times
life size (10:1).

87. Micro-photography is achieved by
photographing through a microscope
to get an image larger than 10
times life size.

88. Other terms commonly used are:
Photomacrography and Photomicography.
.

89. Depth of field
is shallow in
close-ups so
focusing becomes
critical.

90. A 50 mm lens at a
distance of about
12 inches from the
subject has a DOF
of 1/16 inch when
the aperture is
set to f/4.

91. At f/11 the DOF increases -
but only to 1/2 inch!

92. Here you can see how increasing the
distance between the lens and the sensor
increases the size of the image.

93. Regardless of the method used to take a close-up,
you must move the lens farther from the sensor to
focus closer to your subject.

94. And the farther
the lens is
extended, the
dimmer the light
that reaches the
light-sensitive
surface.

95. So you must increase the
exposure time so the photo
will not be underexposed.

96. Use a tripod to
take close-ups
to prevent
motion blur.

97. Use a cable release if you have one.

98. You can also use a flash to
“freeze” the motion.

99. These photos
were made using
a macro lens
and a flash.

100. Filters

101. A polarizing
filter removes
reflections
from surfaces
such as glass

102. This works best at a
30 - 40 degree angle.

103. A polarizing filter also
makes distant objects clearer
and the sky darker.

104. The effect works best when you are shooting at a
90 degree angle from the sun.

106. Other popular
types of filters
are soft focus
and star (cross
screen) filters.