Teach you how to understand the histogram in Photoshop to make the exposure perfect

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With the continuous development of digital cameras (DC) image processing technology, more and more cameras have built-in histogram functions. Although histograms are still unfamiliar to beginners. But it already exists in our life and work. For example, in the famous image processing software Photoshop, the command for the corresponding histogram is histogram (the Chinese version is "histogram ").

Histogram meaning



In the histogram of an image, the horizontal axis represents the brightness of the image, from left to right, and gradually transits from full black to full white; the vertical axis represents the relative number of pixels in the image within the brightness range. In such a two-dimensional coordinate system, we can have an accurate understanding of the brightness and brightness of an image. In Photoshop, click "image → adjustment → Color Order (shortcut: Ctrl + l)" to open the color order adjustment box and adjust the histogram of the image to control the brightness and shade of the image.

Figure 1 histogram legend

Histogram features

The histogram function in DC is the same as that in Photoshop. When the black color block in the histogram is biased toward the left, it indicates that the overall color of the photo is dark, or it can be understood as the picture is underexposed. When the black color block is concentrated on the right side, it indicates that the overall color of the photo is highlighted. Unless it is necessary for a special picture, we can understand that the photo has been exposed. Next, we will use several photos to intuitively understand the image features reflected by the histogram.

Figure 2 accurate exposure

Figure 2 shows a normally exposed building image and its corresponding histogram. We can see that the Left position in the histogram is relatively high and dense, because the building's back has more dark areas, the left side of the histogram reflects the distribution of the dark area. At the right of the center of the histogram, we can see a higher peak because the brightness corresponding to the yellow area of the large part of the image is here. At the rightmost end of the histogram, we can see a small and prominent wave crest, which corresponds to the strong reflection of the cylinder on the building in the image, because the brightness is too large, beyond the range expressed by the histogram, it is reflected in the rightmost side to form a wave crest.

According to the content expressed by the histogram, the brightness of the image is basically within the range of its performance, and there is not much overflow. More importantly, the brightness of each object in this image is in line with our general understanding. We can think that the exposure of this image is more accurate.

 

Figure 3 overexposure figure 4 underexposure

In the same case, if we increase or decrease the exposure during shooting, we will get the results of 3 and 4 respectively.


In Figure 3 and figure 4, we can see clearly. If the exposure is increased, the hidden part on the left side of the histogram is almost blank. If you reduce the exposure, you can see the opposite result in the histogram. In this way, if we see a histogram, we can have an accurate understanding of its brightness and shade even if we don't look at the image.

It should be pointed out that it is not the best choice for an image that is centered at the top of the histogram and is relatively uniform. It is used to determine whether the impression of an image is accurate, the key is to see if it accurately reflects the photographer's intention. For example, most of the night scenes in the histogram are in the dark area.

Taking Figure 5 as an example, the large black blocks in the histogram are concentrated in the left area, which indicates that there are many shadows in the image, which is also in line with the general concept of night scenes. There is also a relatively small peak that appears on the right side of the histogram, corresponding to the highlighted part caused by light in the image. Although the graph lines in this histogram are not evenly distributed in the center, it is also an example of correct exposure due to the special scenes. This situation is rare in daily photos, but I believe it will be of great help for everyone to understand the concept of histograms.

Figure 5 night view exposure

Practical Histogram

After learning about the basic concepts and information of histograms, how can we apply relevant knowledge to daily photos? After all, DC's EVF and LCD screen are all restricted by their own brightness, color saturation, sharpness, and other display features. There will always be some differences between the image performance and reality, it directly affects our accurate judgment and selection of the photo brightness (exposure. The appearance of the histogram just supplements this deficiency.

Specifically, the application of histograms in DC can be divided into two aspects: on the one hand, the impression of the image can be checked after shooting; on the other hand, before the photographer presses the shutter, it provides an accurate picture shade distribution reference for the photographer. The photographer can adjust the exposure parameters based on actual needs to ensure the photo exposure accuracy.

In this case, we will explain how to use the histogram function more rationally in shooting to obtain photos with accurate exposure.



1. How to Use histograms

Most mainstream cameras currently have the histogram function, taking Sony f828 digital cameras as an example. Before shooting, the camera LCD
The corresponding histogram information can be instantly displayed on the screen (see figure 6 ). If the current histogram shows too much dark information, it indicates that the image exposure is insufficient. The photographer can appropriately increase the exposure so that the curve in the Histogram can be fully translated to the right to obtain the correct exposure. You can also determine whether the appropriate exposure parameters have been adjusted based on the changing histogram. And vice versa.

Figure 6 Histogram

In this way, we can learn the impression size of the image to be obtained based on the built-in histogram function of the camera. How can we use further adjustments to see a histogram with even color order distribution?

2. Adjust the exposure volume

We can use various methods to change the exposure parameters so that the color distribution of the image displayed by the Histogram can achieve our shooting purpose. The following three examples show how to use different shooting techniques to achieve accurate exposure (Note: to better express the histogram, in this example, we use a histogram in Photoshop ).

Method 1: Use the exposure compensation function to directly adjust the exposure volume

Figure 7 insufficient exposure

Figure 7 is the lively night market light in Causeway Bay, Hong Kong. The entire screen is dominated by various light box signs and neon lights. The two curve peaks of the histogram are concentrated on both sides and overflow, which indicates that the photo's brightness and shade contrast is too high, and the details of the bright and dark pictures in the image will produce irreversible losses. By analyzing the current situation, it is easy to see that many scenes are "hidden" in the hidden area of the histogram, and are not displayed.

Figure 8 moderate exposure

In this case, we increase the number by 0.3 ~ The effects of figure 8 can be obtained after 0.7 exposures are compensated (+ eV. We can see that the dark scene just now has added a lot of details, and the performance of the entire image is close to that of the naked eye.

In this case, if you increase the number by 0.3 ~ With 0.7 Exposure Compensation (+ eV), you will get the effect shown in Figure 9: The image is much brighter, but the noise in the dark area increases significantly, and the entire picture has no night scene. Therefore, if we expose the image as shown in Figure 9, the details of the dark part can be fully reflected, but the bright part is exposed too much, and the image effect is poor. This is a far cry. Therefore, if we see a serious problem caused by exposure such as Figure 9 in the histogram, We can subtract 0.3 ~ 0.7 exposures are compensated (-eV) to get 8 results.

Figure 9 exposure transition

Many cameras now provide similar shortcut keys for the exposure compensation function. If not, you can find the corresponding options in the camera menu.

Method 2: flexible use of light measuring methods

The camera's exposure is calculated based on the in-device exposure data after its built-in light measuring program determines the brightness and other data. In general, the default light measurement program focuses on central light measurement. In most cases, the correct exposure parameters can be obtained. However, there are exceptions. In this case, we can use the camera's point metering function and set the brightness to Neutral gray (similar to the center of light and dark) obtain the appropriate exposure parameters in most conditions. Similar optical measurement methods include multiple points of light measurement, which should be applied based on the specific functions of the camera and the use experience.

Method 3: Adjust the exposure parameters directly

Experienced photographers can estimate the exposure volume to be compensated based on the histogram data, and adjust the exposure parameters such as aperture and shutter manually. Not only can we get a suitable exposure, but we can also get a more reasonable exposure parameter based on the actual situation and the depth of field and exposure speed.

There are many ways to change the exposure, but no matter what method is used, the key is to understand the meaning of the histogram, in order to ensure that you are aware of your shooting results before shooting, this allows you to take images with reasonable color levels.

3. Use the true meaning of the Histogram

The true meaning of the exposure parameter determined by reference to the histogram information is to directly view the internal information of the image to obtain the most appropriate exposure parameter, it will not be affected by the differences between the display effect of the evf or LCD and the actual image exposure. Because the physical characteristics of image-related input and output devices are inconsistent, especially in the home environment, it is more difficult to obtain a unified color description in a strict sense, this is why what we see during and after shooting is not what we see when we see and print is not what we get, while the histogram aside from the performance of the image surface, provide us with a quantifiable standard of real and reliable data for later output. Therefore, if you want to become a photography enthusiast with low standards, you must understand the histogram.