Integral and
interchangeable lenses
Most digital cameras have “integral” lenses,
fixed to the body. Some offer interchangeable
lenses that can be swapped and upgraded.
Both types have their advantages and
Sony makes both types. (See chart for
more information.)
Resolution
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
(MTF) describes how well a lens maintains
both resolution and contrast.
Geometric accuracy
The lens is also responsible for rendering
straight lines as straight. While the task
seems simple enough, it is difficult to
achieve at the wide end of zoom lenses,
which tend toward either pincushion or
barrel distortion.
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.
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.)
DI
S
TANCE FROM CENTER
KEY
10 Line Pairs/mm Sagittal
10 Line Pairs/mm Meridional
30 Line Pairs/mm Sagittal
30 Line Pairs/mm Meridional
0
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
3
6
9
12
mm
LEN
S
MTF CURVE
S
Barrel distortion (left) and pincushion
distortion (right) are inaccuracies that
lenses can introduce. (Sample photos
for illustration purposes.)
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.
Size
Zoom range
Live Preview
off the main
image sensor
Versatility
Migration
17
INTEGRAL LENS
Compact. Tailored to the specific size
of the sensor.
Good. Varies by camera. Sony
15x optical zoom cameras have
phenomenal range from full wide
to full telephoto.
Yes. You see what the camera sees
on the LCD monitor, with indication
of white balance, exposure and
depth of focus.
Good. Zoom lens and conversion
adaptors cover many needs.
No. The body and lens are
permanently joined.
INTERCHANGEABLE LENSES
Larger. Typically optimized for 35mm image size.
Best. Varies by lens selection. However, it’s often difficult
to get wide angle because 35mm lenses undergo a 1.5x
telephoto conversion when used with the smaller,
APS-size image sensor.
No. The main image sensor is blocked by the mirror
until the moment of exposure. You frame via the optical
viewfinder.
Best. A selection of optional lenses gives you maximum
choice.
Yes. As a system evolves, you may be able to keep your
lenses and migrate to compatible new camera bodies.
The
a
100 works with Minolta Maxxum film camera
lenses from as far back as 1985!
To the casual user, it’s obvious that
the camera takes the picture. But to
accomplished photo professionals, it’s
really the lens that takes the picture. The
lens is responsible for so much of what
defines a great image, including field of view,
focus (and the selection of what objects are
in focus), color, contrast and detail.
Focal length
The angle of view that a lens takes in is
most often described by the focal length
(the distance from the image sensor to the
lens’s “rear nodal point”). Longer focal
lengths correspond to narrower angles of
view (telephoto). Shorter focal lengths
correspond to wide angles of view.
In the world of 35mm film lenses, a 50mm
lens approximates the angle of view of
natural human vision and is considered
a “normal” lens. A 28mm lens is “wide
angle” and a 200mm lens is “telephoto.”
The lens
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
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.
Shooting the digital way
“35mm equivalence”
Unfortunately, lens focal lengths are related
to image sensor sizes. And while a 35mm
film frame is always the same size, digital
camera image sensors vary greatly from
model to model, even within a single
manufacturer’s line! The same 16mm
focal length that would be considered an
extremely wide “fisheye” lens on a 35mm
camera can actually be a telephoto lens
on a compact digital camera.
To compare “apples to apples,” the industry
uses a “35mm equivalent” specification
that converts every lens to its equivalent
35mm angle of view – regardless of image
sensor size.
Optical zoom
Optical zoom lenses are specified by a
range of focal lengths, such as 24-120mm.
Because the 120mm image is magnified
five times compared to the 24mm image,
this is also called a “5x optical zoom” lens.
Maximum aperture
The lens is like a window, admitting light.
The wider the window, the more light will
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.
16
34mm
50mm
200mm
Wide, normal and telephoto views of the same subject taken with 34, 50 and 200mm focal lengths (35mm equivalent).
(Sample photos for illustration purposes.)
35mm film / 24mm lens
APS-size DSLR /
38.4mm equivalent
CAMERA SYSTEMS
CAMERA SYSTEMS
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.)
Содержание PICTURE STATION DPP-FP90
Страница 1: ...Guide to Digital Photography Spring 2007 ...
