CHAPTER 8 HOW NEW TECH CHANGES PHOTOGRAPHY^121
Theimage circle
that lenses cast on
the sensors of two
cameras are the
same size and
cover the same
field of view—
provided the
lenses are set
to the same
focal length on
both cameras. The
picture on the left was
taken with a traditional
35mm film Minolta Maxxum
7000 using a 100mm lens. Like all 35mm film, the diagonal of
the frame is 43.25mm.(^2) The photo on the right
was taken with a
Minolta D7 digi-
tal using the
same lens. The
Minolta has an
image sensor
with a diago-
nal of 28mm.
On both cam-
eras, the lens cre-
ates image circles of
the same size, but the
D7’s digital image sensor
takes in only a portion of the image circle while 35mm film
takes in more. That’s why the same focal length lens on the
digital camera becomes, in effect, a telephoto lens.
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The value of the multiplier is determined by
the size of the image sensor. To determine
the digital multiplier for a camera, divide
43.25mm—the diagonal distance of a
35mm film frame—by the diagonal size of
the camera’s image sensor. For the D7, that’s
43.25 ÷ 28 = 1.54.
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The Upside-Down Math of Digital
The effective focal length of a lens isn’t the only thing affected by the sizes of
digital image sensors. For example, using the Minolta D7 and a 35mm camera:
◆ The D7 has 1.5 times moredepth of field than that found in a 35mm
camera—provided the cameras’ distances from the subject matter are set
up so that they have an equivalent field of view, which is the vertical
and horizontal expanses that can be seen through the lens.
◆ The D7 has 1.5 less depth of field using the same lens that the 35mm
image would have. Of course, the images would be different because the
fields of view would be different.
◆ Take a picture of the same subject matter at the same distance using the
same lens on a D7 and a 35mm camera. Then crop the 35mm image so
you have the same field of view as in the uncropped D7 image. The depth
of field of both will be identical.
The difference in fields of view is the digital multiplier: the factor by which the focal length of the film camera’s lens would have to
increase to match the same field of view as the lens on a digital camera. If a digital camera has a multiplier of 1.5, a 100mm lens
has the same telephoto powers as a 150mm lens on a standard 35mm camera.
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