Sony SLT-A77VQ Технічні характеристики - Сторінка 7

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Hoods and filters

Without lens hood (flare, poor contrast)

With lens hood (no flare, high contrast)

Visible ghosts

Enlarged view

No ghosts

How lens hoods work

Extraneous light

Light needed for

image formation

Lens hood

Without circular PL filter

With circular PL filter

(reduced contrast)

(increased contrast and deep saturation)

Without ND filter

With ND filter

(reduced light for slower shutter speed)

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.

Petal hood

Round hood

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.

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.

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 scientific approach

Protar

( 1890- )

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.

The Carl Zeiss lenses that started it all

Protar

Planar

Developed by Dr. Paul Rudolph in 1890, this

Another Paul Rudolph design, developed in 1897.

lens was one of the original Anastigmat series.

Initially this design was called the "Anastigmat

The design was named "Protar" ( from the Latin

Series IA." It features a symmetrical 6-element

"proto," or "first"/"origin" ) in 1900. The front

4-group Gaussian design that facilitates the use

group was a standard achromatic combination

of large apertures. The "Planar" name is derived

of low-refractive-index crown glass and high-

from the flatness of the image. Planar lenses are

refractive-index flint glass, but the rear group

appreciated for their superb image depth and

was an innovative achromatic doublet using

rich color reproduction.

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.

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

Carl Zeiss coated lens

Light source

Image sensor

Reduced reflection

Planar

Tessar

( 1896- )

( 1902- )

The Carl Zeiss traditions of innovative technology

and uncompromising quality are alive in

today's a series lenses as well.

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.

Uncoated lens

Light source

Image sensor

Uncontrolled reflection

Sonnar

( 1929- )