很精彩的笔记，辛苦了！感谢分享！严重支持！

呵呵 谢谢楼上二位的严重鼓励

Some systems go further. For example, some Nikon cameras
have a database of historic images stored in the camera’s
computer together with a light pattern and the actual exposure
used to record the image. When the light meter is
activated, the camera reads the light levels in the scene to
produce a current-scene light pattern, which it then matches
against the image database. Once a match is found, it makes
a note of the exposure used for the historic image and applies
the same exposure value for the current image. Pretty clever
stuff , when you think about it.

有些系统做得更先进，比如，尼康相机含有一个内建的数据库，包含了一些经典图像的案例中光线的样式与精确的曝光参数。当测光器被激活时，相机会分析场景光线的亮度来确定当前场景的光线样式（依据数据库）。直到找到一条匹配的图像。这时相机就会将经典图像的曝光参数，应用到当前图像中去。想想看，这的确是个相当智能的功能。
（译者注：真的有这回事吗？）

Despite its accuracy, this system has some limitations. First,
when scene dynamic range exceeds the camera’s ability to
record detail in highlights and shadows simultaneously, clipping
of pixels (i.e., pixels having no measurable digital value)
may result. Additionally, the system is the least suitable for
capturing images with a degree of creativity, such as photographing
silhouettes or rim lighting. (For details of when to
use multisegment metering mode, see Habit 4.)

除了上述这些，这种测光模式也有它的局限性。首先，当场景的动态范围超过了相机所能达到的动态范围时，相机可能会丢失亮部或者暗部的细节（或者说，相机没办法找到合适的曝光尺度）。当然，我们也可以利用这套系统来拍摄一些富有创造性的图像。譬如剪影与逆光拍摄时的（更多详细的讨论详见习惯四）。

Center-Weighted Metering
When center-weighted metering mode is selected, the camera
measures the light for the full picture frame, but weights
the exposure value to the level of light present in the center
of the viewfinder.

中央重点测光
当选择中央重点测光模式时，相机会分析画面中所有位置的光线，但是会以取景器中间位置的光线作为重点进行测量。

The size of the center area to which the weighting is applied
can be adjusted in some cameras, ranging from a circle
with a diameter between 6 and 12 mm. Although a slightly
older form of metering, it is accurate when matched
with the right subject, typically portraiture (of people or
animals). (For details of when to use center-weighted metering
mode, see Habit 4.)

不同的相机有不同的测量直径，直径在6到12mm之间不等。尽管这是一种比较老式的测光模式，但当拍摄肖像类图像时，它可以精确地曝光。（更多关于中央重点测光模式的讨论详见习惯4）

Spot Metering
When metering mode is set to spot, light from a tiny portion
of the frame (around 3%) only is assessed. This makes
it possible to assess the brightness of an area of the scene in
isolation from surrounding areas. For example, when photographing
for a silhouette, a spot meter reading can be taken
for the bright tones, without shadow areas influencing the
resulting EV.

点测光
当我们设定测光模式为点测光时，在取景器中只有大约3%的位置参与测光。我们可以通过这种测光模式，来将场景中明亮的部分独立出来。比如当拍摄剪影时，点测光模式可以将高光细节表现出来而不受暗部区域的任何影响。


PHOTO © CHRIS WESTON
Center-weighted metering is ideal for portraits of animals. (left)
中央重点测光模式是拍摄动物肖像的理想模式（左图）。
PHOTO © CHRIS WESTON
Creative exposures are easier to manage when metering mode is set to spot
metering. (right)

当我们将测光模式设置成点测光模式时，我们可以很容易地创造出一些富有创造性的图像。（右图）

Spot metering is also the most accurate mode to use when
manually assessing scene dynamic range, which is often done
in landscape photography, (for example, when calculating the
strength of a graduated neutral-density filter required to balance
highlight and shadow tones in a high-contrast scene).
(For details of when to use spot metering mode, see Habit 4.)

点测光模式也是最精确的测光模式，尤其是在拍摄风光片时，经常被用作评估场景动态范围的工具。（例如：为平衡场景中高光与阴影的光度。我们可以用点测光模式来评估该使用何种等级的中灰滤镜。）（详细介绍参见习惯4）
EXPOSURE MODE
Exposure mode determines how much control the photographer
has over the two main tools of exposure—lens aperture
and shutter speed. Most current DSLR cameras have four
main exposure modes: program, aperture priority, shutter
priority, and manual. All modes work slightly differently, but
they all achieve the same end, which is to control exposure.
(Manufacturers refer to these modes in different ways.
Typically, they will be labeled P for program mode, A or Av
for aperture-priority mode, S or Tv for shutter-priority mode,
and M for manual mode.)


曝光模式
曝光模式决定了我们将以何种方式来控制曝光中最重要的两个工具---光圈与快门。绝大多数的现代单反有四种主要的曝光模式：程序；光圈优先；快门优先；手动。每种模式都略有不同，但是他们最终的目的都是一致的，即控制曝光。
（不同的厂家有不同的定义方式。比较典型的定义方式是：他们会用P来代表程序曝光，A或者AV来代表光圈优先，S或TV来代表快门优先，以及M代表手动模式。）

Program Mode
In program mode, the camera determines both lens aperture
and shutter speed, based on the meter reading. While this
has advantages for basic point-and-shoot photography, it is
of no use for anyone serious about mastering the skills of
photography. Another disadvantage of program mode is that
in this setting many existing cameras also set ISO and white
balance automatically, leaving no control to the photographer
over four of the five most important camera controls.

程序模式
在程序模式下，相机基于曝光尺度可以同时决定光圈与快门大小。这是一种“傻瓜式”的拍摄方式，但对于那些重视拍摄技巧的人来说是不会考虑的。程序模式的其它缺点是，许多相机的程序模式会自动设置ISO与白平衡，因此有四五个最重要的相机控制是处于失控状态的。

PHOTO © NIKON UK
Some DSLR cameras provide a range of variable program modes suitable for
photographing different types of subject. Even so, they should be used with caution,
if at all.

一些单反相机会提供多种多样的程序模式来适应不同的拍摄物。尽管如此，在使用时仍需加以小心。