The mystery of exposure and photometry system

Lead: Is the scenery too bright or too dark? We will unveil the mystery of exposure and photometry system for you.

The mystery of exposure and metering system The built-in metering system of the camera can handle everything for me, so I only shoot according to the exposure parameters provided by the camera, right?

Aperture, shutter speed and ISO value are the three major factors that determine exposure, and the metering system in the camera matches them with each other. The camera's metering system simply measures the total amount of light reflected by the shooting scene, that is? Reflection photometry? Then, the camera will determine a set of exposure parameters according to the luminosity value to obtain a more balanced exposure. You can choose automatic mode, so that the aperture, shutter speed and ISO value are all determined by the camera. Or you can set this set of parameters manually.

Okay, reflection photometry? Is there anything I need to worry about?

Yes, because in the process of photometry, there are many situations that will lead to measurement errors. Have the metering systems of most cameras been determined? Balance? The standard of exposure is to make 18% neutral gray appear in as many areas as possible in the whole scene. This is because, generally speaking, most scenes will reflect 18% of the incident light. For many scenarios, can this default criterion really get good results? Like green grass under the blue sky. But in fact, our real world is not all composed of middle tones. Dark forests, bright snow scenes and white skies. All these will bring trouble to the existing metering system. This is why we often encounter exposure problems, which make photos look too dark or too bright.

So, the camera's metering system is not? Do you know? What is the subject it faces, right?

The problem lies in the proportion of a certain brightness area in the picture. If you shoot swans in the snow or trees in the pine forest, it is no different from ordinary scenes for the metering system. It still wants to make the exposure of the whole scene as close as possible to the neutral gray tone of 18%. Take Swan in the Snow as an example: because the background is white and the main body is white, this scene is very bright, and a lot of reflected light will enter the lens. But the camera's metering system can't achieve this. It still assumes that the scene itself should be in the middle tone, but it is illuminated by extremely bright light. In this way, it will make a decision to reduce exposure, like this? Too bright alto? The scene is displayed as neutral gray close to 18%. Therefore, the photos will be underexposed and the snow will look gray. And if the scene is almost completely dark, a similar situation will happen, such as the example of the forest we mentioned earlier: such a scene reflects little light, so the metering system still regards it as an intermediate scene? Only the light shining on this scene is weak. Therefore, the light metering system increases the exposure, so that the overall brightness of the picture increases to 18% gray. The end result is that the photos are overexposed and the trees look pale.

So, how should we deal with these exposure errors?

At this time, exposure compensation should appear. Press with ... +/-? Mark the exposure compensation button and move the camera control panel to the left or right at the same time to correct the exposure selected by the metering system.

On the viewfinder and display screen of the camera (LCD screen on the top of the camera or display screen on the back of the camera), there is an electronic analog exposure display, that is, a straight line divided into multiple blocks. There is a sign at the midpoint of the line, indicating that the camera metering system thinks? Is that correct? Exposure rate. When using exposure compensation, while adjusting the compensation value through the dial, you will see that the indicator on the electronic analog exposure display will move positively or negatively. When will it arrive? +? When the direction of the sign moves, the photo will light up; Reverse the dial and the indicator will turn to? -? The movement of digital symbols darkens the photos.

When observing the scene from the optical viewfinder, it is impossible to directly see the influence of this adjustment on the picture effect. However, if you use live view mode or electronic viewfinder (EVF), you can observe the effect of exposure compensation in real time. How much compensation should be applied depends on many factors, such as the hue of the subject, lighting conditions and the metering mode used (see below).

What is metering mode?

Most digital SLR cameras provide the following three basic metering modes: area metering, central focusing metering and spot metering (these three modes will have different names on different cameras). These metering modes will focus on some areas in the scene with different proportions, which makes the exposure results of the camera have a certain tendency. Spot metering mode is the best choice if you want to accurately measure a certain position in the picture. As the name implies, it only measures light in a small area of the picture (generally speaking, the target area of spot metering is the center of the field of view, but many cameras also allow the autofocus point that is currently active to be used as the metering point).

Central focus metering mode is a metering mode inherited from film camera. Although the light of the whole picture will be measured, the position of the subject will be close to the center of the picture by default, so the key area of photometry will be placed in the center area. Nowadays, SLR cameras all have intelligent zoning metering or multipoint metering systems (such as Canon's evaluation metering mode and Nikon's matrix metering mode). These systems will divide the picture into several areas and measure the brightness of these areas respectively. Finally, the system will analyze and integrate the photometric results of each area, and then apply them to exposure.

When you press the shutter half-way, not only the amount of reflected light, but also many other factors will be recognized, measured and comprehensively calculated by the camera: the position of the autofocus point, the size and distance of the subject, the color of the scene and whether there is backlight, etc.

How much exposure compensation should I apply for?

One of the problems brought by the zoning metering mode is that when the camera displays the final exposure parameter suggestions, you don't know whether the camera system has been adjusted accordingly and to what extent. Let's go back to the scene of the swan in the snow: at this time, the advanced zoning metering system may have recognized that it is facing a white scene, so it directly decided to increase the exposure to ensure the brightness and white of the picture.

However, if you don't understand this and still use positive exposure compensation (because you think the camera's metering system will treat snow as 18% gray scale), then this photo will be overexposed. However, the camera may not increase enough exposure, so if you don't apply any positive exposure compensation, you may be underexposed. This requires you to be familiar with your camera? Habit? To understand the general response of the zoning metering system under different scenes and different lighting conditions, and whether there is any tendency. In this way, it can be predicted in advance whether the current scene will make the metering system. A mistake? .

If a scene has both midtones, bright highlights and very dark shadows, how can I expose it?

There are more than one solution to this problem, but one thing is certain: a dead white overexposed highlight without details is very ugly and will interfere with the overall effect of the picture. As for the shadow part, even if the exposure is insufficient, many details can be omitted in the post-processing process.

Photography course

So, we generally follow? Ensure the correct exposure of highlights? The principle of. That is to say, we need to ensure that the right side of the photo exposure histogram does not appear? Trim? (beyond the display range of histogram). Trust us, right? Dead black? No shadow? Dead white? The highlights are too bad.

Photometric determination

Measuring the light of the scene before the official shooting is called photometry. When the shutter button is gently pressed, the built-in metering system of the camera will be activated, which will provide us with a suggested aperture shutter combination, which will be displayed on the viewfinder or display screen in live view mode.

Although, you can use the zone metering mode to meter the whole scene, in complex lighting conditions, it is best to use the spot metering mode to directly measure the position of the focus or another position with the same light. Then you can press AE-L key for a long time to lock this exposure parameter, or switch to manual exposure mode, which is more suitable for landscape photography.

In this way, you can re-compose the composition and fine-tune and optimize the exposure again without being affected by the camera system.

No matter how good the residential metering system is, it may be misjudged in low light environment. To solve this problem, you can take more test photos and apply different exposure compensation to increase or decrease the exposure when necessary. In addition, the spot metering mode can also be used to obtain more accurate results. If you spot-check the points shown in the above figure, you will get similar photometric results, so as to apply exposure compensation more pertinently.

Detailed histogram

The histogram of the camera provides us with the most reliable indication of exposure. It depicts the distribution of all tones in the photo? From the leftmost shadow to the rightmost highlight. Every photo you take is a mixture of different tones, and the shape of the histogram is a faithful reflection of it. The scene dominated by the middle button? For example, a church surrounded by green hills will have a? Mountain peak? ; Under the background of bright morning light and tree silhouette, the histogram of such a picture will show a gentle and shallow middle section and towering on the left and right sides, because this scene almost contains only dark colors and highlights. If the information peak of the histogram exceeds the boundary of the maximum display range, it is called. Trim? This means that the picture details of this part of the photo are lost.

? The built-in metering system of the camera tries to present this bright scene as a gray tone in the middle, so the peak of the histogram is mainly concentrated in the middle.

After exposure enhancement with positive exposure compensation, the brightness of the scene is corrected and the peak value of the histogram moves to the right (brightens).

Here, the metering system once again wants to present the originally dark scene according to the middle gray tone, so the part that should have maintained the shadow state becomes too bright and overexposed.

After negative exposure compensation, the picture returns to a relatively natural and dark state, and the peak value of the histogram moves to the left (darkens).

Manual exposure

If you use the manual exposure mode of the camera, then you become the controller of the whole exposure process. The metering system is still effective, and it will feed back the current exposure setting through the viewfinder and the electronic analog exposure display on the display screen.

The midpoint of the indicator line of the electronic analog exposure display represents the correct exposure of the middle tone, and there are three gears on the left and right, which also belong to this exposure range. The exposure indicator will move left and right on the indicator line, so as to display the difference between the light reception of the subject and the middle tone according to its photometric results. Please pay attention. Here? Midtone? It's just that the camera determines according to its own metering results, and the camera system doesn't know what you are really pointing at.

Aim the lens at the midtone area, and the exposure indicator should be located near the midpoint of the indicator line. If not, then the exposure parameters should be adjusted accordingly.