The meaning of metering
In two consecutive installments, we introduced the shutter and aperture of the camera. Next, lets take a look at the automatic metering system.In short, a camera with an aperture and shutter has the ability to control the amount of incident light.But how much "light" does it have to come in before it can be under or over exposed?In the past, the stage of electronic photography technology has not yet started. Talking about the aperture and shutter of the three-primary camera is manually adjusted by the user, similar to the current "M" full manual mode.To obtain accurate subject light, photographers must use a hand-held light meter to measure the light in order to achieve accurate exposure.随着电子技术的进步，傻瓜相机、数码相机，甚至高阶的单眼相机皆以具有 CPU 运算能力的测光技术，应用在现代机身上，使其对焦更快，测光更准，*作It is also more user-friendly.
A common term "TTL metering" on the specification sheet?This is a method of measuring light through the lens through the lens (English: Through The Lens), referred to as TTL metering.This technology was developed in 1964, and its main purpose is to replace the light metering tools such as light metering that need to be taken out.在摄影时，使用者半按快门之后，激活 TTL 测光机制，光线先经过镜头的折射，进入机身内的测光感应器，这个有点类似今日 CCD 感光器的原件，会将光讯号转成电子讯号，交于 CPU 运算之后得出适当的光圈和快门值。TTL测光的最大好处就是，所测得的光量，就是标准底片曝光量，特别适用于习惯在镜头前加装滤境，或是使用大型蛇腹相机等，透---过 TTL 就不需要再Increase or decrease the exposure compensation, press the shutter directly to take a picture.
Four photometric standards
Most digital cameras or traditional point-and-shoot cameras will list the following four metering modes in the specification table:
Central average metering
One of the most widely used metering modes is also one of the default metering modes of camera manufacturers.This mode takes into account that most photographers are used to placing the focus part in the middle of the screen, so the photosensitive original responsible for metering will collect light metering data outside the center of the screen according to a certain proportion of the metering value from the center of the screen.，经过 CPU 对数值加权平均之后的比例，取得到拍摄的建议测光数据。For Nikon series cameras, the famous center-weighted metering mode takes 75% of the central part (the range varies according to different camera brands), and the remaining 25% gradually extends to the edge.Under normal normal shooting conditions, center-weighted metering is a very practical metering mode, but if the subject of the picture is not in the center or backlit shooting, center-weighted metering is not applicable.
Center part metering
This mode is different from "central average metering", which measures the average area of the screen (about 3 to 12%) depending on the camera brand.The metering mode of the central part is designed to meet the requirements of more demanding professional photographers. It can be applied to some special harsh shooting environments, which can more ensure the calculation of the exposure required for the main representation of the object in the picture.Applications include: stage, backlighting and other scenes, this mode is most suitable, but due to the rise of the area metering (matrix metering) mode, this mode is now less than the camera.
Spot metering (SPOT)
In order to overcome the shortcomings of the central average metering, manufacturers have developed this spot (SPOT) metering mode (1 to 3%) to avoid the effect of backlighting on the subject metering; the range of spot metering is based onA very small area in the center of the view window is used as the exposure reference point, and the light measured in this area is used as the exposure data.这是一种相当准确的测光方式，但对于新手来说，怎样去区别一个测光点，变成了一个需要学习的技巧，错误的测光点所拍出来的画面不是Under 就是 Over，造成Serious exposure error.Because of the spot metering technique, it can also be used in the increasingly popular digital camera "Macro macro shooting", so beginners must do their best to do this metering method, and initially can choose the middle tone in the main scene as the metering benchmarkpoint.
Area metering (or evaluation metering)
这种测光方式属于近代新开发的技术，约在 15 年前 Nikon 率先开发这种独特的区域测光功能，其余中央重点测光之最大不同点，便是它将画面区域成数个区域，After the calculations are performed independently, they are combined and sorted out to obtain a complete exposure value.Early Nikon models divided the metering area into eight large blocks, and independently calculated the values obtained from each metering area, and the database built in the camera was used to integrate and judge the exposure values.Removal of the boundary values in the picture, such as the location of the OVER, the obtained exposure value not only has an accurate effect, but also drives the development of a new generation of camera automation.At present, Nikon is equipped with a 256 area metering function whether it is a traditional camera or a digital camera. Other brands such as Canon and Minolta have similar designs, but the built-in database and processing capabilities of the camera are different.In other words, the accuracy of area metering is not only due to the hardware capabilities, but also the size and discrimination of the database behind it.In the past, in order to find the accuracy of exposure, Nikon analyzed and summarized the exposure values after taking nearly ten thousand photos when constructing the database, as the basis for database judgment.After the users verification, this mode is suitable for shooting landscapes, group photos, etc. In fact, it is also the mode most often used by many amateurs, even professional photographers, especially when shooting forward light and front side light, Or a large-area scene with uniform brightness is most effective.
Principle of metering
The principle of metering is actually very simple. It is assumed that the reflectance of the metered area is 18% to give the aperture shutter combination parameters.“ 18％”这个数值来源是根据自然景物中中间调（灰色调）的反光表现而定，一般白色表面可以反射近 90％的光线英吋）的卡片，将这张灰卡放置于主景同一At the light metering location, the overall reflectance of the resulting light metering area is 18%. Afterwards, the photo is taken according to the aperture shutter combination given by the camera metering, and the resulting photo will be the accurate exposure.
However, if the overall reflectance of the metering area is greater than 18%, for example, metering against a piece of white paper, and shooting according to the aperture shutter combination given by the cameras automatic metering, the resulting photo will be Under.It is seen as gray paper on the photo.Therefore, when shooting scenes with a reflectance greater than 18%, you need to increase the EV exposure compensation value. Similarly, if the overall reflectance of the metering area is less than 18%, such as metering against a piece of black paper, the resulting photo will be OVER, and the black paper will also be photographed as gray paper (dark gray).Therefore, when shooting scenes with a reflectivity of less than 18%, the exposure needs to be reduced.
However, the actual metering situation is not so simple. Complex natural light and shadow, light and color, etc., often interfere with the accuracy of metering.Even, when do you choose central focus, spot and area metering?When is exposure compensation required?How much compensation?In the end, it is up to the photographer to accumulate experience to judge.Master the basic principles of metering and the area of the metering mode of the camera used. Through comparison and actual *, you can accurately judge the shooting of the main scene.
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