Sony SLT-A77VM Технические характеристики - Страница 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
16 mm
Fisheye
Fisheye
16 mm
16 mm
(24 mm)
18 mm
18 mm
(27 mm)
24 mm
24 mm
(36 mm)
35 mm
35 mm
70 mm
70 mm
(105 mm)
100 mm
100 mm
(150 mm)
135 mm
135 mm
(205.5 mm)
250 mm
250 mm
(375 mm)
400 mm
400 mm
(600 mm)
* Focal length in ( ): equivalent focal length when mounted on
interchangeable-lens digital cameras with 35mm full-frame sensors.
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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
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