Sony DSLR-A100K - alpha; Digital Single Lens Reflex Camera Selection Manual page 14

Lens Technology
Circular aperture
specially developed aspherical
lens elements. This advanced
optical technology minimizes
In general, if an aperture uses 7 or
spherical aberration in all types of
9 aperture blades, then the shape
lenses, including large-aperture
of the aperture becomes a 7-sided
lenses. Aspherical lenses maintain
or 9-sided polygon as the aperture
high contrast in images taken
is made smaller. However, this has
with the aperture wide open,
a certain undesirable effect in
and effectively suppress image
that the defocusing of point light
distortion. Also, because less lens
sources appears polygonal and
elements are required, the entire
not circular. lenses overcome
lens stays compact.
this problem through a unique
design that keeps the aperture
almost perfectly circular from ED (Extra-low
its wide-open setting to when it
Dispersion) glass
is closed by 2 stops. Smoother,
more natural defocusing can be
obtained as a result.
Optical glass
Aspherical lens
Spherical lens
Focal
plane
ED glass
Aspherical lens
Focal
plane
As focal lengths get longer,
lenses built with conventional
optical glass have difficulties with
Spherical aberration occurs in
chromatic aberration, and as a
conventional lenses due to a
result, images suffer from lower
misalignment on the focal plane
contrast, lower color quality,
between light coming from the
and lower resolution. To counter
periphery of the lens and light
such problems, ED glass was
coming from the center of the
developed and is included in
lens. Despite the development
select lenses. It dramatically
of various methods to correct
improves chromatic aberration at
this aberration, the flaw remains
telephoto ranges, and provides
apparent particularly in wide-
superior contrast across the
angle lenses with large apertures.
entire image, even at large
Unless, that is, the lens includes
aperture settings.
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Multi-layered coating
Although most of the light that
falls on an optical glass element
passes through directly, some of
it reflects at the surface to cause
flare or ghost images. In order to
avoid this problem, a thin layer
of anti-reflective coating must
be applied to the lens surface.
Lenses use multi-layered coating
to effectively suppress such
problems over a wide spectrum of
wavelengths.
Double telephoto zoom
Through the use of a telephoto-
type lens structure, the length of
a fixed focal length lens can be
made shorter than its actual focal
length. A double telephoto zoom
lens takes this principle of design
Focal
one step further, by combining
plane
two such telephoto-type structures
to produce a telephoto zoom lens
that is extremely compact.
Floating mechanism
Ideally, lens aberrations can
be adequately compensated
Focal
at any shooting distance. In
plane
reality, compensation is quite
difficult at close distances. The
floating mechanism addresses
this problem by allowing groups
of lens elements to move closer
or farther apart, which makes
it possible to correct various
aberrations at practically all
focusing distances. A double
floating mechanism is included in
the SAL0M28 and SAL100M28.
Internal focusing
Only the middle groups of the
optical system are moved to
achieve focusing, which leaves
the total length of the lens intact.
Benefits include fast autofocusing
of the frame, pressing this button
and a short minimum focusing
on the lens barrel will hold focus
distance. Also, the filter thread
if you wish to recompose the shot.
at the front of the lens does not
Alternately, depth of field preview
rotate, which is convenient if
can be assigned to this button
you're using a polarizing filter.
through the camera's custom
setting menu.
Rear focusing
Focus range limiter
By moving only the rear group of
lenses to focus, the lens allows for
speedy AF operation and a shorter
minimum focusing distance. Also,
because the front of the lens does
not rotate, operability is improved
when you're shooting with a
polarizing filter attached.
range. In macro lenses, this limit
can be on either the close or
Aluminum alloy
distant range (as pictured). In
the SAL70200G, the limit is set
lens barrel
on distant ranges only. In the
SAL300F28G, focusing can be
Aluminum alloy is used in the
limited either to a distant range
construction of the G series and
or to a range that you specify
other high-end lenses to ensure
yourself.
high optical performance. This
material is lightweight, durable,
Distance encoder
and highly resistant to the effects
of temperature change.
The distance encoder is a lens
component that directly detects
the position of the focusing
mechanism, and sends a signal
to the CPU in order to measure
distance to the subject. During
flash photography, this data is
very useful in calculating how
much flash output is appropriate
to the scene. The distance
encoder plays an integral part in
ADI flash metering, which delivers
high-precision flash metering that
is unaffected by the reflectance of
subjects or backgrounds.
Focus ADI flash metering
hold button
Advanced Distance Integration
flash metering is available when
Once focus has
the built-in flash, HVL-F6AM
been adjusted on
external flash, or HVL-F36AM
the desired part
external flash is used together with
a lens that has a built-in distance
encoder.* It provides automatic
metering that is virtually
unaffected by the reflectance of
subjects or backgrounds. Precise
distance information is obtained
through the encoder, and this
data is used to compensate the
flash output accordingly. This
yields accurate exposures more
This function
reliably than conventional TTL
saves you a bit
(through-the-lens) flash metering,
of time during
which can be thrown off by overly
AF operation by
reflective or overly dark subjects
setting a limit
and backgrounds.
on the focusing
* For details, please see the specifications
chart on p.2–2
SSM (Super Sonic
wave Motor)
SSM is a piezoelectric motor
that contributes to smooth and
silent AF operation. The motor
produces high torque at slow
rotation, and provides immediate
start and stop responses. It is
also extremely quiet, which helps
keep autofocusing silent. Lenses
that feature SSM also include
a position-sensitive detector to
directly detect the amount of lens
rotation, a factor that improves AF
precision overall.
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