Sony a Lenses - Manual (Page 7)

Use your lens hood!
The lens hoods provided with most interchangeable-lenses are not just
accessories to be used occasionally. They are an important part of the lens’s
optical system and should always be used in order to ensure optimum
performance. There are exceptions, such as when an on-camera flash is
used and the lens hood casts a shadow, but for most shooting situations
the lens hood should be on the lens, not in your bag. If your lens has a
built-in extending hood, it should be extended when you’re shooting.
Even though a lenses are uncompromisingly designed with multi-coated
elements and other internal features that minimize flare and ghosting, these
problems can still occur if extraneous light is allowed to enter the lens. And
although the effects of flare might not be obvious in all images, it can subtly
degrade contrast and prevent you from capturing the strongest possible
image. Strong backlighting, particularly near the edge of the image, can
cause ghosts even when a lens hood is used. In such situation the only
solution is to reframe the shot so that the problematic light source is excluded.
Lens hoods block extraneous light
Any light entering the lens that does not come directly from the scene being
photographed is extraneous light that needs to be eliminated. Light that
grazes the front element at a steep angle or bounces around inside the lens
barrel will degrade image quality. A lens hood that is properly designed for
the lens on which it is used will effectively block extraneous light that does
not contribute directly to the image, ensuring that the lens will deliver the
highest resolution and contrast it is capable of. Although most lens hoods
for normal to telephoto focal lengths are basic round designs, lens hoods
for wide angle lenses often have a “petal” shape that is designed to block
unwanted light without intruding into the corners of image area.
Visible ghosts
Protar ®
(1890-)
Planar ®
(1896-)
Tessar ®
(1902-)
Sonnar ®
(1929-)
Uncoated lens Carl Zeiss coated lens
Carl Zeiss ® optics
For many photo enthusiasts, Carl Zeiss lenses have long been the ultimate
choice. Many models are available, but the only autofocus Zeiss lenses
currently available for use on interchangeable-lens digital cameras are
those that have been created through close cooperation between Carl Zeiss
AG and Sony for the a series cameras.
The unmatched T* (T-star) coating
The fact that lens coating technology—vapor deposition of a thin, even
coating on the lens surface to reduce reflections and maximize transmission—
was originally a Carl Zeiss patent is well known. The Carl Zeiss company also
developed and proved the efficacy of multi-layer coatings for photographic
lenses, and this is the technology that became the T* coating.
Until the introduction of coated lenses, the lens surface would reflect a large
percentage of the incoming light, thus reducing transmission and making
it difficult to use multiple elements in lens designs. Effective coatings made
it possible to design more complex optics that delivered significantly
improved performance. Reduced internal reflection contributed to minimum
flare and high contrast.
The Carl Zeiss T* coating is not simply applied to any lens. The T* symbol
only appears on multi-element lenses in which the required performance
has been achieved throughout the entire optical path, and it is therefore a
guarantee of the highest quality.
The scientific approach
It was Ernst Abbe of Carl Zeiss AG who first
applied scientific principles to lens design, rather
than relying on trial-and-error experience. A
significant portion of the history of photographic
lens development centers on the Protar, Planar
and Sonnar designs that featured advanced
optical paths based on those principles. In many
ways the history of Carl Zeiss AG is the history
of photographic lenses.
Hoods and filters
Neutral density filters
Sometimes the light is so bright that you’re forced to use smaller apertures or
faster shutter speeds than you want to. Neutral density (ND) filters reduce the
amount of light entering the lens without affecting the color or tonal balance
in any way, and can be very useful in this type of situation. Suppose you
want to shoot a waterfall using a shutter speed that’s slow enough to blur the
moving water and create a sense of motion, but the lighting at the scene is
too bright. An ND filter will reduce the light intensity so that you can use the
relatively slow shutter speed required to achieve the desired effect.
Circular polarizing filters for
improved contrast and color
Circular polarizing (PL) filters can be used to eliminate reflections and glare
from reflective surfaces such as glass and water, but landscape photographers
find them most useful for increasing contrast and saturation in skies, foliage and
other icons of the landscape genre. In all cases the filter works by eliminating
reflections, but in the latter, it is eliminating reflections from airborne dust and
water vapor, thus removing a veil of glare and allowing the true colors of the
scene to come through.
Light source
Light source
Reduced reflection
Uncontrolled reflection
Image sensor
Image sensor
Without lens hood (flare, poor contrast)
Extraneous light
Lens hood
Without ND filter With ND filter
(reduced light for slower shutter speed)
Enlarged view No ghosts
With lens hood (no flare, high contrast)
Petal hood Round hood
Without circular PL filter
(reduced contrast)
With circular PL filter
(increased contrast and deep saturation)
Light needed for
image formation
How lens hoods work
The Carl Zeiss lenses that started it all
Protar
Developed by Dr. Paul Rudolph in 1890, this
lens was one of the original Anastigmat series.
The design was named “Protar” (from the Latin
“proto,” or “first”/”origin”) in 1900. The front
group was a standard achromatic combination
of low-refractive-index crown glass and high-
refractive-index flint glass, but the rear group
was an innovative achromatic doublet using
Jena glass, with high-refractive-index crown
glass and low-refractive-index flint glass. The front
and rear elements were located on either side
of the diaphragm,
effectively suppressing
chromatic aberration.
This design evolved to
become the Unar lens
and later the Tessar.
Planar
Another Paul Rudolph design, developed in 1897.
Initially this design was called the “Anastigmat
Series IA.” It features a symmetrical 6-element
4-group Gaussian design that facilitates the use
of large apertures. The “Planar” name is derived
from the flatness of the image. Planar lenses are
appreciated for their superb image depth and
rich color reproduction. The Carl Zeiss traditions of innovative technology
and uncompromising quality are alive in
today’s a series lenses as well.
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