What does the brightness bar on the camera stand for? What's the use? And RGB straight?

The brightness and hue distribution of correctly exposed photos should be relatively uniform, showing that the curve shape looks smooth and full on the histogram, starting from the left end position 0, gradually changing and smoothly transitioning to the right end position 255. All brightness levels have pixels, and there is no overflow at the left end (the darkest place) and the right end (the brightest place), which preserves the detail level of each brightness.

2. Right slope type-underexposed

The histogram curve waveform of underexposed photos is biased to the left, most pixels are concentrated on the left, and the right side of the waveform is obviously decreased. There is a blank from the right side to 255 (the brightest place) with few or no pixels. This kind of photo looks too dim, with many dark parts and insufficient brightness, which can be adjusted by increasing exposure compensation, increasing aperture or reducing shutter speed. When using Photoshop for post-processing, you can click "Image → Adjustment → Dark Tone/Highlight" and drag the slider of "Dark Tone" item to correct underexposure.

3. Left Slope Type-Overexposure

The histogram of overexposed photos is just the opposite of underexposed photos. Pixels are concentrated on the right, and there are few pixels on the left. From 0 (the darkest place) to the beginning of the curve waveform is a blank, with few or no pixels. The photos are brightly colored, or there is a large area of anti-light source. When shooting, you can adjust the exposure setting by reducing the exposure compensation, narrowing the aperture or increasing the shutter speed. When using Photoshop for post-processing, you can click "Image → Adjustment → Dim/Highlight" and drag the slider of "Highlight" item to correct overexposure.

4. Convex-low contrast

The pixels on the histogram are concentrated in the middle of the curve, the middle of the waveform is convex, and the two sides are descending. There are no pixels near 0 and 255, lacking light and dark tones and insufficient contrast. The photo looks blurred and gray. This kind of histogram is very common, which is mainly caused by environmental factors such as weather when shooting, such as fog, dust, too strong sunlight and so on. And can be adjusted by using deflection mirrors and other equipment. When using Photoshop for post-processing, you can click "Image → Adjustment → Color Scale" and move the black-and-white field sliders at both ends to the positions slightly inward from the starting points on both sides of the histogram, so that the color tone of the photo can be adjusted to some extent.

5. Concave type-excessive contrast

The histogram curve waveform of this photo is high on both sides and concave in the middle. Pixels are mainly concentrated on the left and right sides, with few in the middle. There are obvious dark parts and bright parts in the photo, but there is no middle bright part, so the contrast between light and dark is great. Apart from silhouette or Gao Fancha's creation, this histogram mainly fails to grasp the part and method of photometry, which can be adjusted by positioning the photometry point at the junction of light and dark. When using Photoshop for post-processing, you can click "Image → Adjustment → Dark Tone/Highlight" and drag the sliders of "Dark Tone" and "Highlight" respectively to make corrections.

Therefore, when using histogram to guide exposure, we should follow our own shooting needs and the shooting effect we want to achieve. A photo with completely accurate exposure is not necessarily the best photo.

In addition, in order to reduce the difficulty of post-processing, you should not deviate from your own exposure requirements when shooting. It is best to master the following two exposure principles when shooting:

First of all, when the dynamic range of the shot scene exceeds that of the digital camera, at least one of the bright or dark parts of the histogram should not overflow. At this time, the best way is to reduce the contrast with the subtracter of medium gray scale, or you can use the bright or dark part as the exposure benchmark according to your own shooting needs. In addition, you can take a few photos with bracketing exposure on a tripod, and then superimpose them with image processing software such as Photoshop to expand the dynamic range of photos.

Secondly, the exposure rule of "white plus black minus" is not only applicable to traditional cameras, but also to digital cameras. When the bright part or dark part occupies a large area in the picture, the exposure should be increased or decreased appropriately. As for how much exposure to increase or decrease, it depends on the proportion in the shooting scene.

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It is a practical method to use histogram to guide exposure, of course, according to your shooting theme. In order to accurately apply histogram to judge the accuracy of exposure, it needs to be displayed in RGB mode with multiple shots and multiple exercises, and the information of three color channels can be viewed, in addition to a brightness information.

RGB is red, green and blue respectively.