Sony Alpha SLT-A77V Spezifikationen - Seite 6

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Focal length, angle of view and perspective

Focal plane (image sensor plane)

Angle of view

(measured diagonally)

Focal length

Secondary principal point of lens

Focal length vs. angle of view

With 35mm full-frame image sensor

With APS-C type image sensor

16 mm

Fisheye

16 mm

(24 mm)

18 mm

(27 mm)

24 mm

(36 mm)

35 mm

70 mm

(105 mm)

100 mm

(150 mm)

135 mm

(205.5 mm)

250 mm

(375 mm)

400 mm

(600 mm)

* Focal length in ( ): equivalent focal length when mounted on

interchangeable-lens digital cameras with 35mm full-frame sensors.

Focal length

Focal length, or focal length range in the case of zooms, will usually be the

foremost consideration when choosing a lens for a specific photograph or

type of photography. The focal length of a lens determines two characteristics

that are very important to photographers: magnification and angle of view.

Longer focal lengths correspond to higher magnification, and vice-versa.

Wide-angle lenses with short focal lengths have low magnification, which

means you have to get physically close to an average-size subject to fill the

frame. But that also means you can fit large subjects in the frame without

having to shoot from a distance. Telephoto lenses with long focal lengths

have high magnification, so you can fill the frame with subjects that are

further away from the camera.

A technical definition of focal length

The focal length of a lens is defined as the distance from its secondary principal

point to its rear focal point when focus is set to infinity. The secondary principal

point is one of six "cardinal points" that are used as points of reference in an

optical lens (front and rear focal points, primary and secondary nodal points

and primary and secondary principal points). There's no predefined location

for the secondary principal point in a compound lens—it could be somewhere

inside the lens barrel or at some point outside the barrel, depending on the

design of the lens—so there's no easy way to accurately measure the focal

length of a lens yourself.

Focal length and angle of view

"Angle of view" describes how much of the scene in front of the camera will

be captured by the camera's sensor. In slightly more technical terms, it is the

angular extent of the scene captured on the sensor, measured diagonally.

It is important to remember that angle of view is entirely determined by both

the focal length of the lens and the format of the camera's sensor, so the

angle of view you get from any given lens will be different on 35mm full frame

and APS-C format cameras. Different lenses of equal focal length will always

have the same angle of view when used with the same-size sensor.

The "Focal length vs. angle of view" comparison to the left illustrates this

relationship for both 35mm full frame and APS-C format cameras.

Perspective

With long focal lengths, foreground and background objects will often appear

to be closer together in the final image. This effect is sometimes called "telephoto

compression," although it is not actually caused by the lens itself. What really

happens is that when using a telephoto lens, you will need to be further away

from your subjects. As such, the distance of the subject from the background

relative to the subject's distance from the camera lens becomes smaller and

smaller the further away the photographer stands. From that perspective

they actually are closer together! Another way of saying this is that since both

the foreground and background objects are at a considerable distance

from the camera, their relative sizes in the final image will be closer to reality.

When shooting with a wide-angle lens you normally need to get close to the

foreground subject so that it is sufficiently large in the frame, which is why

more distant objects look comparatively smaller. The difference in apparent

perspective is actually a result of how far you are from your subject.

24mm focal length,*

300mm focal length,*

84° angle of view

8° angle of view

* 35mm format equivalent

Macro photography

Maximum magnification ratio

As mentioned on the previous page, the magnification of any lens is

determined by its focal length. For macro photography we are also concerned

with how close we can get to our subject. These two factors, focal length and

minimum focusing distance, determine the lens's maximum magnification

ratio, sometimes referred to as "reproduction ratio." The closer you can get to

your subject with a lens of a given focal length, the higher the magnification

ratio you'll achieve.

The classic definition of a macro lens is one that has a maximum magnification

ratio of at least 1:1, or "1x" in lens specifications. This means that a subject can

be reproduced at full size on the camera's image sensor: a 10mm object can

be projected onto the sensor as a 10mm image when the lens is sufficiently

close to the subject. A maximum magnification ratio of 1:2 or "0.5x" would

mean that the maximum size that an image of the same 10mm object could

be projected onto the sensor would be 5mm, or just half its true size.

Other macro lens characteristics you should know about

Macro lenses are specifically designed to deliver optimum optical performance

at very short focusing distances, and will usually be sharpest at close range,

but that doesn't mean that you can only use them for macro photography.

Many macro lenses are also capable of excellent performance when shooting

normal subjects at normal distances as well.

Another important characteristic of macro lenses used at short range is that

they have very narrow depth of field. That means they have to be focused very

carefully to get the desired details in perfect focus. A tripod can make focusing

easier in some situations. You might have to stop the aperture down quite a bit

to achieve sufficient depth of field with some subjects. But shallow depth of

field can be an advantage, emphasizing the essential in-focus detail while

defocusing and de-emphasizing distracting background.

30mm Macro lens (SAL30M28)

Working distance (approx. 2 cm/0.8 in. at 1x magnification)

Minimum focusing distance (approx. 13 cm/5.1 in. at 1x magnification)

Working distance (approx. 16 cm/6.3 in. at 1x magnification)

Minimum focusing distance (approx. 35 cm/13.8 in. at 1x magnification)

Minimum focus and working distance

The "minimum focusing distance" lens specification can be confusing.

Minimum focusing distance is measured from the subject to the rear focal

point of the lens, which is at the image sensor plane in the camera body.

The term "working distance" is used to describe the distance between the

subject and the front element of the lens.

If a lens is specified as having an 0.2 meter ( 20 centimeter ) minimum

focusing distance, for example, depending on the thickness of the

camera body and the length of the lens, you might only have a few

Image sensor plane

100mm Macro lens (SAL100M28)

Image sensor plane

centimeters of working distance when focused at the minimum focusing

distance in order to take a 1:1 macro shot. Being that close to your

subject can make lighting difficult ( special macro flashes and ring lights

are available to overcome this type of lighting problem ) , focusing can

be difficult if the subject or camera moves even slightly, and you're likely

to scare away living subjects at such close distances. If any of those

problems occur, you need to choose a macro lens that has a longer focal

length for more working distance.

0.35x

1.0x