Canon PowerShot 350 Service Manual - Page 29

Fixed, Focus

Get Canon PowerShot 350 PDF manuals and user guides View all Canon PowerShot 350 manuals
Add to My Manuals
Save this manual to your list of manuals

Page 29 highlights

Part 1: General information 5.3 FIXED-FOCUS AND AUTO-FOCUS When using a camera to take a picture the position of the lens must be adjusted so that the object is focused on the image plane (CCD). Optically, only one point (or plane) can be in focus, with the image becoming less clear as the distance from that point increases. In other words, the image becomes progressively out of focus. Because no-one likes blurred photographs, the object must be clearly in focus in every picture. Early cameras were focused manually until the image was sharp in the viewfinder. Later, cameras were built to focus automatically by computing the distance to the subject. This "autofocusing" remains the mainstream today. In reality, however, there is some latitude. The sharpness of human vision has its limitations, and some lack of sharpness in the image can be tolerated without being noticeable as blurring of the picture. The limit within which the eye cannot detect blurring is Permissible circle of confusion - - Depth of focus Focal point Permissible Circle of Confussion Blurring known as the "permissible circle of confusion" (Fig. 1-10). The distance between the Fig. 1-10 Circle of Confusion focal point and the permissible circle of con- fusion on either side is known as the "depth of focus", and we can assume that the image will appear in focus if the focal point is within Depth of Field this range. Similarly, the distance either side (Range of object distance that will be within Depth of focus at the image plane) of the object that falls within the depth of Depth of focus focus is known as the "depth of field" (Fig. i- Fig. 1-11 ll). Because the depth of focus is larger for smaller apertures (Fig. 1-12), the depth of Aperture field also increases. Therefore, by selecting the appropriate focal length and aperture, it is possible to, for example, have a range from one meter to infinity within the depth of field. Aperture Depth of focus In normal usage, focussing is not necessary if the depth of field covers from lm to infinity, vastly reducing the complexity and cost of the 41 Depth of focus system. Cameras based on this principle are known as "fixed-focus", or pan-focus, cam- Fig. 1-12 eras. Most low-end and disposable cameras are fixed-focus cameras. However, as we discussed, because the fixed-focus method imposes limitations on the focal length and aperture, it's difficult to make fixed-focus cameras with large aperture or telephoto lenses. Also, close ups are also very difficult. Thus, fixed-focus and autofocus cameras each have advantages and disadvantages, and are selected according to the type of camera and what it will be used for. AF Fixed focus Cost High Low Space Large Small Lens freedom Good(VVide to Tele) Poor(VVide angle only) Picture quality Very Good Fair When actually designing a camera, it's important to clarify such items as the size of the circle of confusion, the minimum focusing distance, the lens angle(focal length), maximum aperture, etc. 1-19

  • 1
  • 2
  • 3
  • 4
  • 5
  • 6
  • 7
  • 8
  • 9
  • 10
  • 11
  • 12
  • 13
  • 14
  • 15
  • 16
  • 17
  • 18
  • 19
  • 20
  • 21
  • 22
  • 23
  • 24
  • 25
  • 26
  • 27
  • 28
  • 29
  • 30
  • 31
  • 32
  • 33
  • 34
  • 35
  • 36
  • 37
  • 38
  • 39
  • 40
  • 41
  • 42
  • 43
  • 44
  • 45
  • 46
  • 47
  • 48
  • 49
  • 50
  • 51
  • 52
  • 53
  • 54
  • 55
  • 56
  • 57
  • 58
  • 59
  • 60
  • 61
  • 62
  • 63
  • 64
  • 65
  • 66
  • 67
  • 68
  • 69
  • 70
  • 71
  • 72
  • 73
  • 74
  • 75
  • 76
  • 77
  • 78
  • 79
  • 80
  • 81
  • 82
  • 83
  • 84
  • 85
  • 86
  • 87
  • 88
  • 89
  • 90
  • 91
  • 92
  • 93
  • 94
  • 95
  • 96
  • 97
  • 98
  • 99
  • 100
  • 101
  • 102
  • 103
  • 104
  • 105
  • 106
  • 107
  • 108
  • 109
  • 110
  • 111
  • 112
  • 113
  • 114
  • 115
  • 116
  • 117
  • 118
  • 119
  • 120
  • 121
  • 122
  • 123
  • 124
  • 125
  • 126
  • 127
  • 128
  • 129
  • 130
  • 131
  • 132
  • 133
  • 134
  • 135
  • 136
  • 137
  • 138
  • 139
Part
1:
General
information
5.3
FIXED
-FOCUS
AND
AUTO
-FOCUS
When
using
a
camera
to
take
a
picture
the
position
of
the
lens
must
be
adjusted
so
that
the
object
is
focused
on
the
image
plane
(CCD).
Optically,
only
one
point
(or
plane)
can
be
in
focus,
with
the
image
becoming
less
clear
as
the
distance
from
that
point
increases.
In
other
words,
the
image
becomes
progressively
out
of
focus.
Because
no-one
likes
blurred
photographs,
the
object
must
be
clearly
in
focus
in
every
picture.
Early
cameras
were
focused
manually
until
the
image
was
sharp
in
the
viewfinder.
Later,
cameras
were
built
to
focus
automatically
by
computing
the
distance
to
the
subject.
This
"autofocus-
ing"
remains
the
mainstream
today.
In
reality,
however,
there
is
some
latitude.
The
sharpness
of
human
vision
has
its
limi-
tations,
and
some
lack
of
sharpness
in
the
image
can
be
tolerated
without
being
notice-
able
as
blurring
of
the
picture.
The
limit
with-
in
which
the
eye
cannot
detect
blurring
is
known
as
the
"permissible
circle
of
confu-
sion"
(Fig.
1-10).
The
distance
between
the
focal
point
and
the
permissible
circle
of
con-
fusion
on
either
side
is
known
as
the
"depth
of
focus",
and
we
can
assume
that
the
image
will
appear
in
focus
if
the
focal
point
is
within
this
range.
Similarly,
the
distance
either
side
of
the
object
that
falls
within
the
depth
of
focus
is
known
as
the
"depth
of
field"
(Fig.
i-
ll).
Because
the
depth
of
focus
is
larger
for
smaller
apertures
(Fig.
1-12),
the
depth
of
Aperture
field
also
increases.
Therefore,
by
selecting
the
appropriate
focal
length
and
aperture,
it
is
possible
to,
for
example,
have
a
range
from
one
meter
to
infinity
within
the
depth
of
fi
eld.
In
normal
usage,
focussing
is
not
necessary
if
the
depth
of
field
covers
from
lm
to
infinity,
vastly
reducing
the
complexity
and
cost
of
the
system.
Cameras
based
on
this
principle
are
known
as
"fixed
-focus",
or
pan
-focus,
cam-
eras.
Most
low
-end
and
disposable
cameras
are
fixed
-focus
cameras.
However,
as
we
discussed,
because
the
fixed
-focus
method
imposes
limitations
on
the
focal
length
and
aperture,
it's
difficult
to
make
fixed
-focus
cameras
with
large
aperture
or
telephoto
lenses.
Also,
close
ups
are
also
very
difficult.
Thus,
fixed
-focus
and
auto
-
focus
cameras
each
have
advantages
and
disadvantages,
and
are
selected
according
to
the
type
of
camera
and
what
it
will
be
used
for.
Permissible
circle
of
confusion
-
-
Depth
of
focus
Focal
point
Permissible
Circle
of
Confussion
Blurring
Fig.
1-10
Circle
of
Confusion
Depth
of
focus
Depth
of
focus
Aperture
Depth
of
Field
(Range
of
object
distance
that
will
be
within
Depth
of
focus
at
the
image
plane)
Fig.
1-11
41
Depth
of
focus
Fig.
1-12
Cost
Space
Lens
freedom
Picture
quality
AF
High
Large
Good(VVide
to
Tele)
Very
Good
Fixed
focus
Low
Small
Poor(VVide
angle
only)
Fair
When
actually
designing
a
camera,
it's
important
to
clarify
such
items
as
the
size
of
the
circle
of
confusion,
the
minimum
focusing
distance,
the
lens
angle(focal
length),
maxi-
mum
aperture,
etc.
1-19