What is a straight square and how to treat it?

Histogram is what we use in photos to see how many pixels there are in each value in the image. The value of each pixel in the photo ranges from 0 (black) to 255 (white). The left side of the picture represents the dark tone of the scale, and the right side represents the bright tone.

In color photography, each pixel of each color has its own value (0-255). The histogram in the figure shows the pixel value distribution of each color (red, blue and green), which overlaps with the secondary colors (magenta, cyan and yellow), and finally shows the area containing all three colors and displayed as gray.

Histogram analysis

When the image is well exposed, there will be a large bar chart in the middle of the histogram, but there is almost no information around the edge.

1, histogram is exposed correctly.

The first one showed us the correct exposure image. The ruler will not enter the extremely light tone area, which tells us that there are almost no pure white pixels in the picture. In some shadows (left), the curve drops sharply, which means that several pixels in the photo are pure black, but basically we have obtained all the information of the photo. When a scene contains more contrast than our camera can capture, it is impossible to capture it at once, so we must make a compromise. It is best to make some completely black areas more obvious than strong overexposure.

2. The histogram is underexposed

The photo is seriously underexposed, and pixel distribution appears in the shadow area. In fact, no pixel value is higher than the average gray level. Although we can brighten the image in Photoshop, the problem is that when there are a large number of pixels in the photo with a value of 0, there is no difference between them. What we can do is to increase the value above zero, which will make the image very contrast. It is best to get all the information (i.e. different pixel values) from the scene first.

3, histogram overexposure

Finally, in the overexposed picture, we can see that many pixels have a value of 255 (white). No matter how much darkness you add to Photoshop, you can't separate one white pixel from another, so the details of bright colors will be lost forever.

Get as much information as possible

Through these three photos, we can see that the exposure settings are correct, only a few pixels are pure white, while the first one is pure black, which can capture the biggest scene details in the camera.

There are several techniques to obtain correct exposure, such as bracketing exposure and exposure compensation, but the most important thing is to be able to read the histogram.