How to properly learn to read a histogram and get high-quality photos? Total quality management Histogram what why and how

The order of plotting the histogram is as follows:

1. Collected statistical data - the results of measurements of the parameter of the object. In order for the histogram to make it possible to estimate the type of distribution of a random variable, it is preferable to have at least thirty measurement results.

2. The largest and the smallest value of the indicator among the obtained measurement results is revealed.

3. The width of the range of values ​​of the indicator is determined - from the greatest value the smallest value is subtracted from the indicator.

4. The appropriate number of intervals is selected within which it is necessary to group the measurement results.

5. The boundaries of the intervals are set. The boundaries of the intervals must be set so that the data values ​​do not fall on any of the boundaries of the interval. For example, if you have selected intervals with boundaries from 0.5 to 5.5, from 5.5 to 10.5, etc. then the data value 5.5 will fall into both the first and second intervals. To avoid this problem, you can change the intervals from 0.51 to 5.50, from 5.51 to 10.50, and so on, so that no data value will fall outside the interval.

6. The number of hits of the measurement results in each of the intervals is counted.

7. A histogram is constructed - intervals are marked on the abscissa (horizontal axis), and the frequency of hits of measurement results in each interval is marked on the ordinate (vertical axis). The intervals can be set in natural units (if the scale allows), i.e. in the units in which the measurements were carried out, or each interval can be assigned a serial number and marked on the abscissa axis the numbers of the intervals. The result is the bar chart shown in the figure below.

Tool "Histogram" is included in the collection " Seven quality tools". You can purchase the collection" Seven Quality Tools "in the" Quality Management "online store.

If there is a tolerance band for the controlled parameter, then the histogram can contain the upper and lower limits of the tolerance field. This allows you to see in which direction and how the value of the controlled indicator is shifted relative to the tolerance field. The boundaries are plotted along the abscissa axis.

The histogram shown in the figure above has the shape of a normal distribution, which indicates the stability of the process, but it often happens that the shape of the distribution deviates from the normal. This indicates violations in the process and the need to apply control actions. Some common deviations and their reasons are presented below.

The histogram is shifted to the left (asymmetry to the left):

This can be caused by the process shifting towards the upper tolerance limit, or results from the set of measurements that fall outside the upper tolerance limit are sorted, or the nature of the process physically prohibits any measurements greater than the maximum tolerance values.

The histogram is shifted to the right (asymmetry to the right):

This can be caused by the process shifting towards the lower tolerance limit, or results from the set of measurements that fall outside the lower tolerance limit are sorted, or the nature of the process physically prohibits any measurements less than the minimum tolerance values.

Bimodality:

The histogram displays two combined processes. This situation can occur if the measurement results are obtained from two different devices, two operators, controllers, different measuring instruments, or from different measuring points.

The histogram is truncated:

The distribution is not normal because there is no gradual decrease in the frequency of measurement results from the center to the tolerance limits. This type of histogram occurs if the process is not able to meet the specifications and some measurements are sorted from both sides when approaching the tolerance limits, or too small values ​​of the measurement results are lost.

The histogram has no center:

The distribution center was sorted from the measurement dataset. This situation can arise due to insufficient requirements in the engineering specification.

The histogram contains protrusions at the borders:

Some of the measurements on the distal side of the distribution have been changed to bring the process characteristics into line with the specified tolerance range or measurements outside the tolerance range were recorded as included in the tolerance range.

Strengths histograms, as a quality control tool, lie in its clarity, simplicity, the ability to quickly represent the type of distribution of a large amount of data. Also, the histogram shows the relationship of changes in controlled parameters in relation to engineering specifications.

The disadvantages include- the lack of the ability to quantify the stability of the process, the lack of reference to time, the need for a large amount of data for an accurate assessment of the distribution structure, the possibility of different interpretation of the results, some subjectivity in the presentation of the distribution form.

The presence of a liquid crystal screen in a digital camera has made the life of a modern photographer much easier. The ability to immediately view the captured frame and, if necessary, make a take is one of the most convenient features of the “number”. Unfortunately, the screen does not allow you to correctly assess the quality of the resulting image. A more accurate tool for controlling the exposure of a photo is the histogram, the display function of which is available in most digital cameras.

A histogram is a graph of the distribution of tones in an image. The gradations of brightness are plotted horizontally: the leftmost point corresponds to black, and the rightmost point to white. The number of pixels of the corresponding brightness is displayed vertically. By evaluating the amount of highlights and shadows in the resulting image using the histogram, you can determine if the exposure is correct and, if necessary, make corrections.

Let's look at specific examples of how to use the histogram.

How to read a histogram of a photo: examining examples

Photo 1

It can be seen that the image has no areas where details are lost. The histogram confirms this: a fairly flat, without sharp spikes, the graph looks like a relatively flat "hill" that stretches from the left edge of the lower axis to the right. This type of histogram means both the presence of bright and dark areas in the image, and a wide range of midtones. This photo does not require correction. What happens if we adjust the exposure settings? The histogram will move to the right or left, illustrating that either light or dark tones are lost in the picture.

The histogram of this image also occupies the entire coordinate grid, but the graph is different from the previous one. There are few halftones in the picture. The peaks related to the light and dark areas, and the gap between them, characterize the image as contrasting. But, since the graph gives only a quantitative distribution of tones, without displaying the distribution of pixels in a photograph in any way, it is rather difficult to determine the contrast of an image using a histogram. Of course, as in the previous case, changes in exposure will degrade the photo.

Consider this photo. There are no very bright or very dark areas in the picture. The brightness range of such a frame is rather small, and the histogram will look like a horizontally compressed "hill", which does not significantly reach the edges of the graph field. Such a histogram is typical for low-contrast images. This type of diagram does not mean that there will be areas with lost details in the image, because the camera matrix in this case is capable of capturing the entire range of brightness. However, you should be careful about where the histogram will be on the chart. For objects that weakly reflect light (for example, black clothes, deep shadows), the histogram should be shifted closer to the left edge of the coordinate grid, for medium-gray ( typical example: foliage, grass) - be located in the center area and, finally, for objects with good reflectivity (for example, a white bride's dress), the histogram should gravitate towards the right edge.

If the graph is in the wrong part of the field, you should make the exposure corrections. To shift the histogram to the left, the corrections must be negative, and to shift to the right, they must be positive. It is recommended to make corrections by varying the shutter speed, since when the lens is apertured, the depth of field will change.

Variety of this type a histogram is a narrow "hill" that has partially gone beyond the graticule into a dark or light area. With the introduction of the appropriate amendments, the histogram will take on the proper form, completely fitting into the field of the graph.

Let's take a look at the histogram for this shot. The graph does not fade at the headlands - it extends beyond the graticule. This situation occurs when the brightness range of the subject being shot is greater than the range perceived by the camera sensor. This is the most unpleasant case. You have to choose whether to lose detail in shadows or in light. One way to tackle this problem is to create high dynamic range (HDR) imagery. To do this, you need to take several pictures, differing only in exposure. Then using a special software(The latest versions of Photoshop allow you to do this) the frames are combined into a single shot that does not have gaps in shadows and highlights.

Remember that adjusting the exposure is always possible, but not always necessary - it depends on the artistic intent of the photographer. In addition, note that further correction, for example, contrast, in Photoshop is easier to carry out in photographs, the histogram of which belongs to the first type. Of course, you can edit any image, but the first version of the histogram will allow you to change the view of the graph with less effort.

In conclusion, it is worth noting the fact that there is no "correct" histogram. It all depends on the depicted object. The histogram is just a tool that allows you to get rid of exposure errors during shooting or to minimize them, no more. For example, in Ansel Adams' Moon and Half Dome photograph, the histogram is shifted to the area of ​​dark tones, which is not reflected in any way at the level of the photograph. The same type of histogram can be in both a bad shot and a brilliant one.

Modern digital cameras greatly facilitated the work of the photographer, making it more efficient, speeding up the process of obtaining a finished photograph by hundreds of times. In fact, we immediately see an almost finished photo on the camera display. Another question is whether the quality of photographs has grown from this ... But this is not the point now.

One of the ways to preliminary estimate the quality of a photo is to analyze the histogram on the camera screen.

bar graph is a graph of the brightness distribution of image pixels. The camera can display both the overall RGB composite channel histogram and the individual channel red, green, and blue histograms. The horizontal axis is the pixel brightness values, from black to white, through intermediate gradations (for color channels, respectively, from black to the most saturated color). The vertical axis is the number of pixels corresponding to a specific brightness, expressed in relative units.

How to use a histogram

Let's take a look at examples of what information can be obtained from analyzing the histogram, and how you need to adjust the camera settings to get best quality Images. Indeed, in many articles there is a phrase "follow the histogram" or "be guided by the histogram". Let's figure out what, specifically, you need to follow and then what to do with these "orientations".

Normally exposed shot

If the scene you are shooting has a wide enough range of brightness, then the histogram of a normally exposed image will look something like the picture above. The histogram occupies the entire tonal range, while on the right, in the area of ​​highlights, it should not be clipped (there should be a small area). Although, in some cases, the histogram may be clipped. This happens when there are very bright light sources or highlights in the frame. If the picture is normally exposed, no adjustments are required to the settings.

Important note: on the screen of the camera you see an image specially created for viewing, the so-called embedded thumbnail. It has already been processed by the camera's processor, that is, the contrast has been raised, the gamma has been adjusted, etc. Thus, if you shoot inRAW, then you have some margin for highlights, that is, you can allow a slight clipping (cropping of the histogram) in the light areas. With a further decrease in the exposure in the converter, the information will be restored, and the noise in the image will slightly decrease. This method of shooting is calledETTR (ExposureToTheRight).

Under-exposed photo

Otherwise, this picture is called underexposed or underexposed.

In this case, the image visually looks darker than it really should be. At the same time, the histogram is cropped to the left, in the area of ​​shadows, and an extended area is formed in the area of ​​light tones. If the shooting is made in the JPEG format, then with the subsequent post-processing of such a picture, we get ourselves a lot of problems. These are the lost details in the shadows, an increase in the noise level when the image is brightened, the need for color correction. When shooting in RAW format, the situation is not so tragic, but the image quality will still be lower than if we used a photo with a normal exposure. The noise in the image will inevitably increase, therefore, it will be necessary to deal with them. And the fight against noise is always a compromise between noise and small details. There is no program yet that would perfectly remove noise while retaining the same detail as in a picture with a lower ISO and normal exposure.

In order to get a normally exposed photo, if the test one was underexposed, you should do one or two of the following:

1. Open the aperture by 1-2 (the values ​​are approximate, in each case they are determined individually) steps. In this case, you will get a shallower depth of field.
2. If you cannot open the aperture, or you need to maintain depth of field, you can increase the exposure time by 1-2 stops. It should be borne in mind that moving objects may be blurred, or there may be a "wiggle" effect due to hand shake.
3. If you need to keep the depth of field and shutter speed constant, increase the ISO value by 1 to 2 stops. This will inevitably increase the noise level, however, it is better to get a sharp, clear picture with some noise than a blurry one with a low noise level.

Photo with overexposure.

Otherwise, this picture is called overexposed or overexposed.

In this case, the image looks lighter visually than it really should be. In this case, the histogram is cropped to the right, in the area of ​​highlights, and in the area of ​​shadows, on the left, an extended area is formed. If the shooting is made in the JPEG format, then the lost information cannot be restored, that is, the details in the bright areas are lost irretrievably. Sometimes overexposure is used as an artistic technique. If you shoot in RAW, then it forgives small errors in exposure (within 1-2 stops, depending on the camera model), but this is not a reason to keep making mistakes when shooting. In the converter, such imaginary loss of information is easily restored, while the image quality is somewhat improved (see above about the ETTR method).

In order to get a normally exposed photo, if the test one is overexposed, you should do one or two of the following:

1. Close the diaphragm 1-2 stops. This will give you a greater depth of field.
2. If a deeper depth of field is undesirable (for example, you want to blur the background a lot), use a 1-2 stop shutter speed decrease. On the other hand, the risk of getting a blurry shot is reduced.
3. You can also use a 1 or 2 stop ISO reduction if you are shooting at an ISO other than the minimum ISO.

If you are shooting on a bright sunny day fast lens at a wide open aperture, you may not be able to reach the shutter speed limit and ISO of your camera to get a picture with a normal exposure. In this case, it makes sense to use a neutral gray filter that attenuates the luminous flux.

Digital technology makes the photographer's work much more efficient and faster. Today digital cameras can not only show the result of shooting immediately on the camera display, but also analyze these photos - show overexposed areas of the frame and a histogram (common and separate for each of the three RGB channels).

The histogram allows the photographer to analyze the frame and instantly make adjustments to the shot. And thereby save you from unnecessary processing in the RAW converter and Photoshop.

Is a graphical representation of the distribution of halftones in a photograph. Horizontally there is a scale of brightness, and vertically - the relative number of pixels with a given brightness.

The histogram is read from left to right, black to white.

Take a look at the following examples and you will understand how to read a histogram.

The histogram shows that there are no absolutely black areas in the photo. On the right, you can see that there are small overexposed areas in the photo.

The histogram as a whole is evenly distributed over the entire brightness range. There are small areas of over- and under-light, but they are not critical.

In the following example, you can see how to see under- and overexposure on the histogram.

It will not be clear on the display how white the background is white. The histogram shows the total dip on the laptop screen, the light gray tones of the case, and the white background around the subject.

Looking at the camera screen, it is difficult to understand if there are losses on the car body. The histogram clearly shows that there are no completely black areas, but overexposures are clearly visible on white objects.

The histogram also helps when processing in Photoshop in the Levels mode. See how the histogram and photo look after increasing the contrast.

On the left is the original photograph, on the right is the result after a slight increase in contrast. As you can see, the work of contrast stretches the histogram, adding dark and light areas.

Why do you need a Histogram?

All modern cameras are equipped with large enough high-quality displays. Why, then, do we need a histogram?

Displays have their own level of brightness, the perception of which also depends on the ambient light. If you look at the display at night, the picture will seem very bright, and in the daytime, on the contrary, it will be very faded. Due to the fact that the histogram shows the image in the form of a graph, it is independent of any viewing conditions.

The quality of the displays in cameras is really high, but not enough to show the difference between almost white and completely white, as well as the difference between almost black and completely black.

Take a look at the following photo:

http://www.flickr.com/photos/bigfrank/368734607/

This is just the perfect photo for our situation. Of course it was processed in Photoshop, but that doesn't matter.

As you can see in the photo, there are no overexposures or dark areas. The histogram shows us the same thing. There are not tall columns around the edges, which indicate overexposures from lighting lamps and dark areas in the display case. Otherwise, as you can see, the histogram shows that most of the information is in mid-tones.

One glance at the histogram is enough to make sure the exposure is correct and move on.

As you already understood, each image has its own histogram, so there is no correct or incorrect histogram.

The histogram should be considered as a tool for quick analysis of a photo during shooting (or during processing).

When to use a histogram

Night shooting
With absence external sources light, it is especially difficult to determine the brightness and contrast of a photograph.

Studio photography
If you are shooting in a studio and you don't have an exposure meter to measure the power of the devices, you have to work at random, adjusting the camera according to the result on the display. The histogram will more accurately show the situation in the picture.

Object shooting
Subjects are usually shot against a white background. The photo may only show areas of overexposure. And the histogram will help you understand how white really is white.

Outcome

As you can see, the histogram is a very powerful and handy tool for photographers. This is an absolutely essential thing for creating technically high-quality images. And in our next articles we will continue to talk about interesting and effective instruments for working with photographs.

The text of the article was updated: 2.08.2019

Both my previous Nikon D5100 cropped DSLR and my new full-frame Nikon camera D610, viewing pictures on the rotary display by pressing the up button on the joystick can display an incomprehensible graph. For all three years of using the Nikon D5100 camera, I have never dug deeply, did not understand the details, what information it provides. And only after switching to Nikon D610, I decided to understand in detail what it is and why the manufacturer provided photographers this function... It turned out that focusing on this diagram makes it easier to take a correctly exposed photograph. Earlier, I have already noted that when I talk about my discoveries, I myself find it easier to remember and digest new information... Therefore, today I present to your attention a new free photo tutorial for beginners - I share what I learned myself.

Histograms are found in almost every modern graphics editor. It seems to me that all modern cameras, including some point-and-shoot cameras, can display a histogram on the screen. And the fact that these charting tools are so widespread shows how important they are. But many aspiring photographers do not understand what information can be obtained by reading a histogram. There is nothing wrong or shameful if the process seems too complicated at first. In fact, figuring out what they mean is not so difficult. And today we will try to learn how to read histograms, and, hopefully, by the end of the lesson we will understand how they help photographers.

What is a histogram

A histogram is a graphical representation of the tone values ​​in our snapshot. In other words, it shows the number of tones of a particular brightness in a photograph from black (0% brightness) to white (100% brightness) range. As you can see in the picture above, the dark tones are grouped on the left side of the graph. If we move the slider to the right, the tones become lighter. The dot in the middle represents midtones, which are neither dark nor light. The vertical axis of the histogram shows the number of tones for a given specific brightness. The histogram varies with exposure, although it is also influenced by tone curves and other settings.

Overexposures and dips in the shadows on the histogram

Photo 2. Above - clipping in the shadows, below - overexposures (clipping in highlights). How to learn how to take pictures correctly. Photo tutorial on how to learn to read a histogram.

If a certain amount of the histogram "touches" the border, it means a loss of detail, which is called "clipping". Overexposures (areas in the photo where the picture is completely white, details have disappeared) happen if the histogram touches the right side. Under-highlights (areas where all details are lost in black) appear when the histogram touches the left edge of the graph. Each of the options can occur if the exposure settings are incorrectly set. Be that as it may, it must be borne in mind that a lot depends on the scene we are shooting. For example, if there is a sun in the frame, then it will, of course, be completely white - highlights.

We know that overexposures and dips in the shadows, to a certain extent, can be successfully dealt with during the subsequent processing of RAW files in a graphic editor such as Lightroom. If we want to check whether there are "dropped out" areas in our photograph, then we can do this directly on the camera. Different cameras call the histogram differently.

Photo 3. View of the histogram on the screen of the Nikon D5100 camera. To show the graph, I press the "Up" key on the joystick, to hide it again.

For example, with Nikon DSLRs, you need to press the navigation key on the joystick several times. Many modern SLR Cameras Allows you to make exposure adjustments via real-time histogram control. True, for this you need to shoot in Live mode View, not through the viewfinder.

For the very, very beginners, I note that in order to eliminate losses in the shadows, when shooting, you need to introduce exposure compensation a little plus (EV + 0.3 ... + 0.7), and to darken overexposures - decrease EV by -0.3 ... -0.7 units. Typically, exposure is set by pressing the “+” and “-” keys on the camera.

From the tutorial on choosing settings Nikon cameras and Canon we know that another way to change the exposure, say, when shooting in manual mode"M" - set different values ​​for ISO, shutter speed and aperture.

Color channels on a histogram

A histogram is commonly used to represent information about the three primary colors (red, green, and blue), which is called RGB. The picture above is an example.

As you can see, the RGB histogram consists of several plots. Three of them represent red, green and blue respectively. Gray - where all three histograms overlap. Yellow, cyan, and magenta are areas where two graphs overlap.

Histogram and exposure

Histograms help graph exposure of the exposure. Few photographers use a graph to estimate exposure values, usually just looking “bad” or “good”.

A "good" histogram contains more information in the mid-tones, and either no tones at all or few tones around the edges. A “bad” histogram has tones at the edges of the graph, which basically means that the image is overexposed (loss of detail in highlights) or underexposure (loss of detail in shadows).

Sometimes it happens that there are under-highlights and overexposures in a photograph at the same time. If we accept this statement, then the histogram at the beginning of the article indicates that the image is slightly overexposed (tones are slightly shifted to the right), but, in general, it is normal.

Let's put our theory into practice by examining the following pictures. I made 3 of them with different exposures.

Underexposed photo

In the first shot, we see an example of an underexposed shot: the photo is too dark. As we can see, most of the photo has lost any neutral and bright details. The histogram clearly shows that all the tones are concentrated in its left part, which is responsible for shadows, and some details are lost (in such an area of ​​the image - a completely black image).

Overexposed photo

Photo 6. Overexposed photo - clipping on the histogram on the right.

In the second photo, we observe overexposure: the picture is too bright. Most of the tones are too light and there are practically no dark areas. Much of the image is “burned out” (completely white) and no detail can be seen here. We can see that the histogram confirms this image flaw: tones are concentrated on the right.

Let's take a look at the third shot. It is exposed correctly and most of the tones are concentrated in the center of the graph.

Correct exposure

Let's analyze this photo. As we can see, on the far left side there are few tones (black colors are displayed here), which means that there is no loss of detail in the shadows.

In the middle of the schedule, everything is fine with us, too. On the right, we see a peak of light tones - this is, almost, an overexposed area of ​​the sky (in this photo, it was covered by the histogram).

Do we need to use a histogram?

As we saw in the examples above, a “good” histogram is one where the tones are concentrated in the middle. This fact testifies to the correct exposure. Does this mean that the histogram can be used to judge the correct exposure? Not really.

Let's see another example with a histogram that is completely different from the “good” one presented earlier. This is a photo hunt for a polar bear in the snows of the Arctic.

As we can see, the histogram is shifted to the right, where we have the highlights. If we were to judge the correctness of the exposure for this particular shot based only on the shape of the graph, we could assume that the shot is overexposed. In the middle tones, we have almost no information, the shadows are almost highlighted. But we can’t say that the image above is not properly exposed? Do we need to change the camera settings to get a “good” histogram, but which will make the photo too dark?

Another example: you decided to shoot a Lenspen (lens cleaning pencil) against a black background in order to place the photo in the catalog of an online store.

We see that the graph is shifted to the left, there are almost no light tones. But the photo, after all, is normally exposed, there is no shadow clipping?

Simple conclusion from this example: The histogram is not necessarily good at determining exposure. “Correctness” depends on many factors, not the least of which is our own vision of the scene we photograph. The histogram simply shows us the number of tones of varying degrees of brightness in our image and nothing more. It can help us determine if a photo has overexposed or underexposed areas at a given exposure setting. We can use the histogram to avoid such loss in highlights or shadows, and it is a great tool for this. When we understand this, we realize that there are no “good” or “bad” histograms.

Conclusion to the photography lesson to explain the meaning of the histogram for beginner photographers

Some photographers are in the habit of looking at the histogram on the camera screen after each shot to determine if there are areas in the photo that are lost in shadows or highlights. Although I, for example, do not always have the time to do such a check. If we use a histogram, we do not become professional photographers, but the ability to read it, in some cases, can be an invaluable skill. Hope, this article will help us learn to read and understand these graphs, apply the histogram in our work.