Novices must understand the tone of digital photos.

Lead: Which step is inevitable in photo processing? Tone? Every photo has its own tone, and the purpose of many photo processing is to optimize or change the tone of the photo. The richer the tone of a photo, the more room for contrast and color processing. At this time, a very good helper is histogram. It can be displayed not only in the camera, but also in the photo processing software, which can present the tone distribution in the image.

Beginners must look at it to know the tone contrast and contrast range of digital photos.

From the visual aspect of photo processing, the concept of contrast describes the brightness difference between the brightest and darkest parts of an image. That is to say, what we are referring to here is the bright contrast (also called bright-dark contrast) that can be recognized by human eyes in all forms of contrast. The greater the difference between the brightest and darkest parts, the higher the contrast of the image. At the highest contrast, there are only two colors: pure black and pure white? This is the possible limit brightness difference.

Not only the difference between light and dark, but also the so-called contrast range is related to the tone processing of the image. It describes the brightness difference between the brightest and darkest extremes in an image. The larger the contrast range of the image, the easier it is for the viewer to recognize the structure and details, and thus the higher the definition and resolution of this photo.

The number of tone levels is determined by the color depth of the photo. At 8-bit color depth, each color channel can have up to 256 colors, that is, 256 tone levels. In most cases, this is enough to display a good quality photo: the brightness change in the image looks smooth, that is, there is no obvious tone layering in the picture as in the artificial picture. 12 color depth of 4096 color scale, or even 14 color depth of 16384 color scale, has better performance effect and can make photos more convincing.

If the contrast of the object is obviously beyond the contrast range that can be sensed by the county-level sensor, then choosing a higher color depth can save as many details as possible, which provides the possibility for us to adjust the contrast range in post-processing. In fact, the maximum contrast range that a natural landscape can provide is far beyond the range of human eyes and machine sensor East Tower. By constantly adjusting the pupil, the human eye can draw the brightest 10- 12 among the 20 brightness levels, so that the darker tone becomes pure black, or record the darker part, overexpose the brighter part, or lose the darkest and brightest ends. But the key point to know is that photo processing software is better at displaying the details of dark areas than bright image areas. Therefore, it is suggested to underexpose rather than overexpose when necessary, otherwise the dark part will lose a lot of color information.

In addition to consciously tolerating the image areas that can't show more details, there are two other ways to deal with the high contrast of the shooting theme: one is to shoot a series of photos with different exposures of the same theme, add high contrast image information to multiple photos, and then combine these photos into a DRI or HDR photo. The other is to reduce the contrast of the theme by adding or reducing pipes, so that you can brighten or darken it and record more details.

Tone and histogram

Each pixel has a fixed luminance value which is determined by one or more individual values * * *. For example, grayscale photos have only one color channel, RGB photos have three color channels, and CMYK photos have four color channels. Each pixel takes a value between 0 and 255 in each color channel, so it has a dark tone or a bright tone. Therefore, each pixel will have a clear hue value (or brightness value in each color channel), and these values will be displayed in the histogram.

All the tone values in the photo will be visually displayed in the tone map or histogram. For black and white photos, this hue is as mild as the brightness value that can be seen in the image, while for color photos, each color channel has a histogram? In RGB images, there is a histogram for red, green and blue, and there is also a histogram integrated by these channels to show the average brightness value of the image. These histograms are displayed by overlapping, juxtaposition or separation in photo processing software.

The histogram is constructed as follows: there is a brightness scale on the horizontal defense line, with 256 values ranging from pure black (left, with a value of 0) to pure white (right, with a value of 255) * * *, and every hue value existing in the image is assigned to this horizontal axis. The more frequently the same hue appears, the higher the thin line above the corresponding value climbs. In other words, the relative number of tones (not the actual number) is displayed vertically. When a tone value appears disproportionately frequently in an image, its corresponding thin line will rush out of the frame. Even cut off from above. It's not a bad thing, it's just a problem.

Reading histogram

The straight line in the vertical direction forms a mountain-like curve. According to the trend of this curve and its corresponding position on the brightness scale, the brightness distribution and contrast of the image can be inferred. The following histogram is of course an example. Especially the first one is typical? The trend of the curve in the ideal real histogram has a great relationship with its corresponding photos.

Ideally, the trend of the tone curve starts from the so-called black point in the lower left corner of the figure and ends at the so-called white point in the lower right corner. This represents a complete range of brightness values, that is, all 256 brightness levels appear in the photo, and the contrast of the subject does not exceed the dynamic range of the camera sensor. This is a photo with bright colors and sharp contrast? The photo (left) in this example is of medium brightness. In addition, every time you adjust the color scale, you should aim at such a curve.

However, if the contrast of the subject is too high, the two ends of the curve will be cut off. That is to say, some areas in the photo are pure black (left clipping) or pure white (right clipping), which can't describe the details. Because theoretically, the brightness areas to the left of the black point and to the right of the white point in the histogram can no longer describe more details. The reason why this color scale overflow occurs (the two ends of the curve are cut off) is either because the contrast of the shooting theme mentioned above is too high, or because of the operation of expanding the color scale, and the saturation of a single color is too high, which may also cause this color scale overflow.

If the contrast of the object is within the dynamic range of the camera, the graph will be located in a certain area in the middle of the brightness scale without touching the black and/or white points. If the brightness range of the photo (that is, the span of the curve) is less than 256 brightness levels, the contrast of the photo will be very low, and the whole photo will look like a gray veil, leaving a dull impression.

Is it too dark or too low-key

If the peak of the histogram is not in the middle, but on the left, then the photo is mainly dark. The less bright colors appear, the more people tend to call this kind of photo a low-key photo. This special composition method is to combine a large number of dark tones and extremely dark tones with a few tones in the shooting theme.

Even in this case, the curve of the histogram should just start from the black point to avoid losing detailed information in the dark. Some people understand a low-key photo as a photo with a lot of black and a little white in sharp contrast, while others define a low-key photo as a photo with no bright tone at all. The so-called low-key photos of the latter always seem too dull, while the low-key photos defined by the former are probably underexposed photos if they are not carefully arranged or post-processed.

Is it too bright or too high-profile

If the curve representing the bright area in the photo obviously climbs to the right, that is, the peak of the histogram goes to the right, in extreme cases, people will call this photo a high-profile photo. There is little or no dark tone in this kind of photos, which is in sharp contrast to many bright tone driving. Overexposed photos will inevitably form color scale overflow in bright areas, and there is no dark tone in the photos. As for whether this is a high-profile definition and whether there is any difference between the corresponding bright-tone image and the overexposed image histogram, here is also a question. We tend to have a complete range of tones in a photo.

Finally, in any case, don't forget to take a look at the image itself to see if there is any incorrect exposure.

Shadows, midtones, highlights

In digital photo processing, you will often encounter the concepts of shadow, midtone and highlight. They refer to the image area corresponding to the brightness value of a certain area on the brightness scale (rather than the clearly defined brightness value). In other words? Shadow? Is it the darkest brightness level? Highlights? Is the brightest brightness level, and? Midtone? Is a brightness level between the two. The countries between these areas are all going well. The more times you use the photo processing tools to adjust these tonal areas individually, the faster you can feel which areas in the photo are shadows, midtones and highlights.

Midtone contrast

The information of midtone contrast can be read from the histogram. This concept is the difference between shadows and highlights, that is, the bright and dark tones in the midtone area. If the curve of the histogram steadily rises to a peak and then falls, this photo will be dominated by medium gray tones. But if the curve forms two peaks, many dark gray tones contrast with as many light gray tones.

Midtone contrast is independent of the tone range or contrast range of the photo. Without moving the black or white dots, that is, without expanding the gradation or generating gradation overflow at all, the midtone contrast can be correspondingly higher or lower.

Gamma value

This concept comes from film photography, which describes the ability of a medium (such as photographic paper) to display tone levels, that is, the light and dark levels or contrast that the medium can achieve. It is displayed as a line in the graph: when the Gamma value is 1, it is at 45? The diagonal of the angle indicates that the contrast of the photo is exactly the same as that of its carrier (such as negative film). When the gamma value is less than 1, the display is black and pure white, the brightness level of the image is obviously reduced, the contrast is low, and the color of the whole photo is often dim. When the Gamma is greater than 1, it will be displayed as a steep broken line, at which time the brightness and contrast of the photo will increase and the color scale will overflow. If the previous dark gray tone is regarded as black and the previous light gray tone is displayed as white, there will be an image area without details.

In the processing of digital photos, the concept of gray value is used to describe the gray value between you and white point (middle tone), which will not involve black points and white points. Here, the diagonal line in the diagram is not a straight line, but a curve. When the Gamma value changes, the two endpoints of the curve will not move, but the slope of the friend curve will change, and the image that changes the contrast limit of the photo is mainly reflected in the brightness of the photo. By changing the gray value, that is, moving the point representing 50% gray level in the histogram, the proportional relationship between light gray and dark gray tones will change accordingly: if this point moves safely to the black point, the trend of the curve will be flat at first and then steep. At this time, there will be more bright tones in the photo, the difference between bright tones will be enlarged, and the photo as a whole will be brighter. But at the same time, the dark color decreases, and the description of the image information of the dark part is limited. If this point moves safely to the white point, the curve tends to be steep first and then balanced. At this time, the photo effect is also the opposite, with more dark tones and less bright tones, and the overall photo is dark.