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Sony DSLR-A100K - alpha; Digital Single Lens Reflex Camera Selection Manual Page 15

Lenses and accessories selection guide.
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Angle of view
smaller apertures, shorter focal
lengths, or longer focusing
distances. Conversely, it shrinks as
How much of a scene fits into your
you open the lens, use a longer
frame is expressed as an angle of
lens, or get closer to your subject.
view, which changes according
to focal length. A "standard" lens
is one with an angle of view of
Focal length
about 7°, which approximates
the scope of human visual
A camera lens is constructed
perception. A wide-angle lens is
from many lens elements, but
a lens that has a wider angle of
in principle it acts like a single
view, whereas a telephoto lens is a
Principal point
lens with a narrower angle of view.
of convex lens
Depth of field
Focal Length
convex lens. Focal length is the
distance (in millimeters) that light
travels from the center of this
idealized lens to where the image
forms on the focal plane. Subjects
appear larger at longer focal
50mm F2.8
lengths, and smaller at shorter
focal lengths. At any given focal
length, the overall scope of the
image will vary in relation to the
size of the image sensor.
50mm F2.8
Depth of field refers to the area
F-numbers (or "stops") represent
around your subject that appears
a ratio of the size of the aperture
to stay in focus. How "shallow" or
relative to the focal length of a lens.
"deep" it is can be independently
Because the aperture regulates
affected by various factors such
the amount of light passing through
as aperture, focal length, and
the lens, each successive F-stop
focusing distance. For example,
represents a halving or doubling
depth of field gets deeper at
of the brightness. Because an
APS-C size image sensor vs. 35 mm format
Image area of
At any given focal length, a digital SLR with an APS-C size image
Image area of DSLR
35mm format
with APS-C size sensor
sensor has a slightly smaller angle of view than a 3 mm format
47˚ angle
32˚ angle
camera, and yields an image that is about two-thirds in size. So
of view
of view
if you are shooting with an APS-C format camera, it is useful to
keep in mind a conversion factor of 1.x. For example, a 0 mm
focal length is equivalent to 7 mm in 3 mm format.
F-number is actually a fraction,
smaller F-stops correspond to
larger apertures. Lenses with
smaller F-numbers are considered
"fast" as they gather more light,
permit the use of faster shutter
speeds and provide a brighter
viewfinder image.
Magnification ratio
Magnification compares the
Focal Plane
actual size of an object with
Focal Point
how big the object appears
on the image sensor. In macro
photography, 1:1 magnification
means that the object is just
as big in the image as it is in
real life. Lens specifications
usually indicate the maximum
magnification of a lens as either
a rate or ratio.
Perspective is the visual effect
of how far apart foregrounds
and backgrounds appear with
respect to each other. It can be
seen clearly in images taken with
a wide-angle lens: foreground
objects are big while background
objects look small, which creates
a sense of distance between
them. This effect diminishes
at longer focal lengths, as the
distance between foreground and
background gets compressed.
Expand the range of what, how and where you can shoot with
system accessories for your camera. Available accessories
include a variety of high-performance flash units, cables, and
connectors, as well as viewfinder optics, carrying cases, and
other essential items.


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