Sony DSLR-A100K - alpha; Digital Single Lens Reflex Camera Brochure & Specs page 10

Shooting the digital way
CAMERA SYSTEMS
The lens
"35mm equivalence"
Unfortunately, lens focal lengths are related
to image sensor sizes. And while a 35mm
To the casual user, it's obvious that
film frame is always the same size, digital
camera image sensors vary greatly from
the camera takes the picture. But to
model to model, even within a single
accomplished photo professionals, it's
manufacturer's line! The same 16mm
focal length that would be considered an
really the lens that takes the picture. The
extremely wide "fisheye" lens on a 35mm
camera can actually be a telephoto lens
lens is responsible for so much of what
on a compact digital camera.
defines a great image, including field of view,
To compare "apples to apples," the industry
focus (and the selection of what objects are
uses a "35mm equivalent" specification
that converts every lens to its equivalent
in focus), color, contrast and detail.
35mm angle of view – regardless of image
sensor size.
Focal length
The angle of view that a lens takes in is
most often described by the focal length
Optical zoom lenses are specified by a
(the distance from the image sensor to the
range of focal lengths, such as 24-120mm.
lens's "rear nodal point"). Longer focal
Because the 120mm image is magnified
lengths correspond to narrower angles of
view (telephoto). Shorter focal lengths five times compared to the 24mm image,
this is also called a "5x optical zoom" lens.
correspond to wide angles of view.
In the world of 35mm film lenses, a 50mm
Maximum aperture
lens approximates the angle of view of
natural human vision and is considered
a "normal" lens. A 28mm lens is "wide The lens is like a window, admitting light.
The wider the window, the more light will
angle" and a 200mm lens is "telephoto."
be let in. The width of the window is called
the "maximum aperture" and it's expressed
as an "f" number, the ratio of the focal
length, divided by the aperture diameter.
34mm 50mm
Wide, normal and telephoto views of the same subject taken with 34, 50 and 200mm focal lengths (35mm equivalent).
(Sample photos for illustration purposes.)
16
For example, a 100mm lens with a
maximum aperture of 25mm is an f4.
This generates the same brightness as
a 40mm lens with a maximum aperture
of 10mm (also f4).
The lower the "f" number, the brighter
the lens. Zoom lenses are often brighter
at the wide end than at the telephoto end.
So it's not unusual to see a zoom lens
specification such as 28-200mm, f2-2.8.
Lens and sensor size
The lens and the image sensor work
together as a team. In a fixed-lens camera,
the lens is carefully tailored to the specific
size and resolution of the image sensor.
Small image sensors work with smaller
Optical zoom lenses – ideal for ultra compact cameras.
But if you want the higher performance and
creative control of a large image sensor,
you'll need a larger lens to go with it.
The difference becomes even more dramatic
in telephoto and high magnification zoom
lenses. For example, the Sony DSC-H7
15x optical zoom lens extends from 31 to
465mm (35mm equivalent). On a 35mm
camera, such a lens would be gigantic. Yet
the DSC-H7 is quite compact.
35mm film / 24mm lens
APS-size DSLR /
38.4mm equivalent
Take a lens designed for 35mm. Put it in front of
an APS-size digital image sensor. Result? A focal
length conversion of about 1.5x. (Sample photo
for illustration purposes.)
200mm
CAMERA SYSTEMS
Integral and
interchangeable lenses
Size
Most digital cameras have "integral" lenses,
Zoom range
fixed to the body. Some offer interchangeable
lenses that can be swapped and upgraded.
Both types have their advantages and
Live Preview
Sony makes both types. (See chart for
off the main
image sensor
more information.)
Versatility
Resolution
Migration
When we think of digital camera resolution,
we immediately think of the image sensor.
But an image sensor can only resolve the
detail that the lens presents. A good lens
maintains high contrast at high resolution.
In fact, a special family of graphic curves
called the Modulation Transfer Function
100%
(MTF) describes how well a lens maintains
90%
both resolution and contrast.
80%
70%
60%
Geometric accuracy
50%
The lens is also responsible for rendering
40%
straight lines as straight. While the task
30%
seems simple enough, it is difficult to
20%
achieve at the wide end of zoom lenses,
10%
which tend toward either pincushion or
0%
barrel distortion.
0
Color convergence
When Isaac Newton demonstrated that a
prism could break white light into its
component colors, he also demonstrated
what would become a drawback in lenses.
Without careful correction, lenses cause
colors to break apart. You can see color
fringing or "chromatic aberration" as
unwanted color along the edges of objects
in the picture. Chromatic aberration is
especially noticeable on the edges between
very bright and very dark areas of the scene.
INTEGRAL LENS INTERCHANGEABLE LENSES
Compact. Tailored to the specific size Larger. Typically optimized for 35mm image size.
of the sensor.
Good. Varies by camera. Sony Best. Varies by lens selection. However, it's often difficult
15x optical zoom cameras have to get wide angle because 35mm lenses undergo a 1.5x
phenomenal range from full wide telephoto conversion when used with the smaller,
to full telephoto. APS-size image sensor.
Yes. You see what the camera sees No. The main image sensor is blocked by the mirror
on the LCD monitor, with indication until the moment of exposure. You frame via the optical
of white balance, exposure and viewfinder.
depth of focus.
Good. Zoom lens and conversion Best. A selection of optional lenses gives you maximum
adaptors cover many needs. choice.
No. The body and lens are Yes. As a system evolves, you may be able to keep your
permanently joined. lenses and migrate to compatible new camera bodies.
The a100 works with Minolta Maxxum film camera
lenses from as far back as 1985!
LENS MTF CURVES
KEY
10 Line Pairs/mm Sagittal
10 Line Pairs/mm Meridional
30 Line Pairs/mm Sagittal
30 Line Pairs/mm Meridional
This set of four MTF curves describes
the ability of a lens to maintain contrast
(vertical scale) across the image area
(horizontal scale). As you can see, a
lens can't be described by a single
resolution number.
3 6 9 12 mm
DISTANCE FROM CENTER
Barrel distortion (left) and pincushion
distortion (right) are inaccuracies that
lenses can introduce. (Sample photos
for illustration purposes.)
Chromatic aberration takes the form of
unwanted color fringing, especially on
the edges between very bright and very
dark areas of the scene. (Sample photos
for illustration purposes.)
17