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The
illustration at left
shows the relative size
differences of different
sensors on the exposure
plane and their
associated areas of
image
capture.
The
outer portions of the
total image are cropped
away in varying amounts
by the smaller
sensors.
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The
cropped capture area has
a reduced field of view,
creating what is often
called a "multiplier
factor".
The
multiplier factor is
used to compare the
practical focal length
of a lens on a Digital
SLR to a full frame
camera. Because of the
cropped view, the listed
focal lengths of all
lenses used on digital
cameras with sensors
smaller than a full
frame must be adjusted
by the "multiplier
factor" to calculate the
longer practical
focal lengths with their
associated reduced
fields of view.
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Camera
Type
|
Sensor
Measurements
|
Horizontal
View Multiplier Factor
(rounded
off)
|
Physical
area (mm2
)
of sensor compared
to a full
frame
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Kodak
Pro 14n, SLR/n, SLR/c,
Canon 1Ds & 1Ds Mk
II, 35 mm Film
Camera
|
36
mm x 24
mm
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N/A
(full
frame)
|
100%
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Kodak
DCS
560, 660,
760
|
27.65
mm x 18.43
mm
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1.3
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60%
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Nikon
D1, D1H, D1X, D2H, D2X,
D100,
D70
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23.7
mm x 15.6
mm
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1.5
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42.8%
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Canon
D30, D60, 10D, 20D,
Kodak DCS 520,
620
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22.5
mm x 15
mm
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1.6
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39.1%
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Olympus
E1, E-300, Kodak DCS
330
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18
mm x 13.5
mm
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2.0
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28.1%
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Kodak
DCS 315
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13.85
mm x 9.25
mm
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2.6
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14.8%
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Calculating
the practical focal
length
Take
the focal length of any given
lens and multiply it by the
multiplier factor to get the
practical (cropped view) focal
length for the camera in
question. For example, an 18 mm
focal length lens produces a
field of view like a 24 mm lens
on a Nikon D70 (18 mm x 1.5
multiplier), while a 200 mm lens
on a Canon 20D has a field of
view like a 320 mm lens (200 x
1.6 multiplier). Some call it a
telephoto-like effect.
Example
of the cropped field of view and
its associated multiplier
factor
The
picture set below shows the
relative differences in the
fields of view between a full
frame camera and a DSLR camera
with a 1.5x multiplier
factor.
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400
mm telephoto
lens view of an
airplane in
flight as seen
through the
viewfinder of a
full frame
camera.
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The
reduced field
of view cropped
by a 1.5x
multiplier
sensor is shown
inside the red
border.
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The
cropped 1.5x
view produces a
600 mm
effective view
(when compared
to a full frame
camera).
Just
as "zooming in"
is a result of
increased
real
focal length,
the cropped
area created by
the multiplier
factor
increases the
perceived
(practical)
focal length of
the lens.
Objects look
closer because
they are in a
cropped viewing
area.
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Why
is the crop factor
(multiplier factor)
important?
Experienced
photographers buy and
use their lenses
according to focal
length (wide angle
through telephoto), and
speed (aperture rating).
Speed refers to "fast"
lenses (low base
aperture numbers)
compared to "slow"
lenses (higher base
aperture numbers). Fast
lenses are higher
quality (more
expensive), and give a
broader range of use
under different lighting
conditions. (Fast is
desirable.)
- The
increased practical
focal length changes
the traditional use
of the lens
application. A wide
angle lens may not be
"wide enough" because
of the multiplier
factor. On the other
hand, a huge, heavy
telephoto lens might
be substituted with a
lighter weight and
cheaper midrange
telephoto because of
the increased
practical focal
length.
- The
speed rating of the
lens doesn't change
with the increased
practical focal
length. For example,
a 400 mm f/5.6 lens
with a 1.5x
multiplier becomes a
practical 600 mm
f/5.6 lens. (Super
telephoto, reasonably
"fast", and
reasonably priced.)
Compare that with the
several thousand
dollar cost of a
"real" 600 mm f/5.6
lens.
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