Sharpness as a criterion for assessing image quality
Sharpness as a criterion for assessing image quality
With the development of photographic technology, the level of requirements for the quality of photographs has also risen. Nowadays, sharpness is one of the most important criteria in assessing the quality of a photo. Fruits in well-sharpened photos taste better than those in poorly sharpened photos. The same is true in all other areas of photography. The equipment seems more reliable, the apartments seem more comfortable, etc.
Sharpness itself is very insidious. You need to know how to use it. So, for example, if you don’t work with sharpness, you can ruin an excellent shot. It will remain excellent, but will not be anything remarkable. Conversely, over-sharpening can make the photo look terrible.
1.1 What is sharpness
What is sharpness? This concept defines contour contrast. The sharpness itself appears in areas of the image with different brightness. Sharpness can depend on a huge number of factors. It can be affected by the quality of the lens, filters, aperture, shutter speed, focusing, processing algorithms, recoding and reducing images, and others.
Graphic editors allow you to correct sharpness.
1.2 Why you need to increase sharpness
Before you take action, you need to understand why you need to increase sharpness. Many photographs, such as portraits or macro photographs, attract the viewer precisely because of their softness. Increasing the sharpness can ruin this atmosphere, so before you start increasing the sharpness, it is worth considering whether it is necessary to do it.
Almost always, when you zoom out your photos, you need to increase the sharpness. This is due to the fact that when the number of pixels is reduced, instead of several, one is created. The algorithm calculates what value this pixel should have, but almost always the picture turns out to be cloudy in terms of sharpness.
To increase sharpness, you need to increase the brightness of light pixels on the contours and darken dark pixels. This enhancement of contrast on edges is part of the sharpening algorithm.
Below are images of the border. The first photo shows the perfect border of the original photo. the second shows the average value of the pixels on the border. The third picture shows the principle of creating a border using Photoshop's Bicubic algorithm. It slightly increases the contrast. The last image shows the border after processing with special filters to enhance sharpness.
2. Sharpening methods
We will demonstrate the work of sharpening using the example of Photoshop.
2.1 Unsharp Mask filter
This is the most common method. The filter has three parameters.
- Amount : intensity of the filter's effect on the image.
- Radius : Specifies the area around the outlines that will be sharpened. If you set the value too high, the filter will increase the sharpness. and the contrast of the photo. For most photos, a value of 0.2 or 0.3 is suitable.
- Threshold : This parameter determines how large the value between the areas must be to define the boundary of the contour. Most often the value should remain zero. If there is a lot of noise, you can add a little value to this parameter.
Another good method for increasing sharpness is the Highpass filter. The method is that a copy of the layer is created and saturation is removed through Hue/Saturation.
Next, the Other->Highpass filter is applied.
Apply a filter with a value so that only light outlines on a gray background are visible on the preview. Next, change the filter blending mode to Overlay. You can reduce the opacity of the layer, but this is done at the discretion of the author. You can also experiment with other blending modes. The advantage of this method is that you can make several sharpening layers with different radii.
2.3. Smart Sharpen
This filter appeared in Photoshop CS2. it works very well for photographs with a lot of fine detail. For rougher shots, it's better to use Unsharp Mask or Highpass.
- The “More Accurate” option gives access to more settings. This will allow you to achieve better results.
- The results of the “Basic” mode are similar to the results of the Unsharp Mask. There is no Threshold parameter. The algorithm works a little differently. The filter looks for areas with blur and eliminates blurriness in them.
- Lens Blur is great for photos with a lot of fine detail. At this value, the halos are not so large.
- The Motion Blur option reduces motion. This will work if the blur is only in one direction.
- The "Advanced" mode allows you to sharpen the dark and light areas. This is done in the Shadow and Highlight tabs, where there are three parameters:
- Fade Amount : sets the maximum sharpening value, that is, you can limit the upper threshold for sharpening.
- Tonal Width : Sets the value of the areas where sharpening will be performed. You can set the value to sharpen only dark tones, or dark and medium tones.
- Radius : Determines the size of the area around the border where sharpening will occur.
To better understand how the filter works, you can create a linear gradient, add noise to it, and experiment with the filter parameters.
2.4. Third party plugins for Photoshop
Third party developers create many plugins for Photoshop. Among them there are many that work with the sharpness of images. Let's look at a few plugins that do their job better than others.
FocalBlade. This plugin has very powerful tools. It works in several modes. There aren't that many settings in Simple mode, but if you turn on Expert Mode, you'll be able to adjust sharpness on surfaces and contours, adjust the effect of the filter on light and dark areas of the photo, work with light and dark halos, and also control contour sharpening mask.
Power Retouche Sharpness. This plugin can create results that will differ in appearance from what Photoshop plugins produce. Here you can reduce the influence of the filter on certain areas of the image.
Nik Sharpener Pro. The filter doesn't have too many settings. Its built-in presets are designed for printing on various printers and for viewing on screen.
PhotoKit Sharpener. The filter is not very convenient, but with it you can achieve good results, with proper skill. Here several layers with masks are created. Masks have to be edited manually. There is no preview.
Sharpness in photography - theory, evaluation criteria, ways to improve
Sharpness in photography is an extremely important characteristic on which the overall quality of the image, its perception, and artistic value depend. An out-of-focus frame, unless it was specifically intended by the photographer, can most likely be considered hopelessly damaged, since correcting it even in a photo editor is quite problematic, and most often completely impossible.
I foresee objections that sharpness in photography can be improved, for example, in Photoshop, in which there are several ways to increase sharpness, starting with the “smart sharpening” filter and ending with specific and rather complex techniques. This is true, but only if the picture contains information about small details. In other words, to increase sharpness in a photo editor, the frame must be initially sharp, then the use of sharpening filters can significantly improve the quality of the processed photo.
If the picture is unsharp and blurry, then there will be no effect; it is better to simply delete such a file so as not to upset yourself and others. Therefore, you need to remember about sharpness in photography even at the stage of preparation for shooting, without placing too much hope on further computer processing.
What affects sharpness in photography
First of all, of course, the lens . The sharpness of the image depends very much on the optics. Uncorrected aberrations, poor quality materials, poor assembly and alignment significantly affect the sharpness of a photograph. However, even the most expensive lenses with large zooms are usually inferior in sharpness to cheaper models with a fixed focus.
The camera-lens focusing system is very important If she constantly makes mistakes, then it is very difficult to focus. Cheap lenses sometimes suffer from this, especially at maximum apertures, where the focusing system in some cases cannot work out the shallow depth of field, falling either in front or behind the object.
The characteristics of the matrix also largely determine the sharpness in photography. If the pixel size of the matrix is larger than the lens scattering circle, then the sharpness of the image will depend on the matrix. The resolution of modern matrices has come very close to the capabilities of lenses, so in terms of sharpness we need to consider the lens-matrix system, which is what we will do now.
Sharpness criteria
What does the expression “the thing looks sharp” mean? Only that sees it sharply . Our vision perceives as sharp those objects whose circles of confusion (in other words, circles of confusion or spots of confusion) whose diameter is less than the resolution of the eye. But everyone's vision is different! Therefore, the concept of sharpness in photography is a subjective category.
For definiteness, they usually take the vision of an average person, whose studies show that from a distance of 25 cm, a circle with a diameter of 0.1 mm is perceived as a point. This corresponds to an angular resolution of 1′ and is determined by the average density of light-sensitive rods and cones in the central part of the retina (in the so-called macula).
The same result will be obtained when viewing the line world from the specified distance. If the space between strokes is less than 0.1 mm, then the eye will see a solid gray background. This means that at a distance of 25 cm the resolution of the eye is 10 lines/mm . Taking this value as a basis, it is easy to calculate that in order for individual pixels (or dots) not to be visible in a photograph printed on a printer, their density must be at least 20 Pix/mm, or 20*25=500 Pix/ inch (that is, 2 times the resolution of the eye, since to depict a stroke, you need at least 1 black and 1 white pixel).
Thus, if you look at a photograph from a distance of 25 cm, then all details whose size is less than 0.1 mm will be perceived as points. This is the most common and generally accepted sharpness criterion . However, due to the subjectivity of the perception of sharpness, many authors introduce their own criteria, so that in the literature and on the Internet you can find values of the limiting circles of dispersion from 0.07 to 0.3 mm. We will stick to 0.1 mm as the most commonly accepted value.
Sharpness in photography and the circle of scattering on the matrix
We've sorted out the photos, and then the fun begins. Determining the circle of scattering on a photosensor (film, matrix) is fundamental to the concept of sharpness in photography, because we will either view the image obtained from the matrix on a monitor or print it on paper, and with different magnifications . What should the scattering circle on the matrix be so that the photograph obtained from it and printed is perceived by the eye as sharp ?
It is easy to calculate that the 0.1 mm in the photograph should correspond to a diameter of 0.1/ M of the circle of scattering on the matrix, where M is the magnification scale equal to the ratio of the diagonal of the photograph to the diagonal of the matrix while maintaining the proportions of the image. It turns out that the more we want to enlarge the image, the smaller the circle of dispersion on the matrix must be in order to ensure the necessary sharpness in the enlarged photo.
Theoretically, only the image of the object (or rather, even the plane) on which the lens is focused can be perfectly sharp But as we just realized, we don’t need a perfectly sharp image, we only need the circle of dispersion to satisfy the sharpness criterion , i.e. not only the object we are aiming at, but also the objects behind it and in front of it can look sharp. It turns out that sharpness in photography is closely related to the concept of depth of field, and in order for it to have any practical meaning, it is necessary to agree on the scale of magnification of the image at which the circle of dispersion on the matrix will be calculated. In the history of photography, such attempts have been made more than once.
In the days of film, the most popular recommendation from Carl Zeiss was to consider the diameter of the scattering circle on a photosensitive material as 1/1500 of the length of the frame diagonal. For a full frame 36x24 mm (diagonal 43.26 mm) this is 43.26/1500= 0.029 mm . What size (diagonal) of the photograph according to our sharpness criterion (the diameter of the circle of dispersion in the photograph is 0.1 mm) does this recommendation correspond to? Very simple: 43.26*0.1/0.029=149 mm, i.e. less than 15 cm (!). But few people print such photographs today, not to mention enlarging the image on a monitor!
Unfortunately, most of the depth of field tables and scales were calculated using Carl Zeiss assumptions (this standard has been known since pre-war times). Today, all this is hopelessly outdated, so many manufacturers do not indicate this scale at all on their lenses.
In order to correctly calculate the depth of field for a modern digital camera, the value of the diameter of the circle of dispersion on the matrix must be selected based on the criterion of sharpness in the photograph and the required magnification scale . At the same time, keep in mind that the limiting value of the circle of dispersion is equal to the pixel size , a smaller value does not make sense, and knowing the pixel size, you can calculate the maximum print size. Let me give you a specific example.
Let's take the widely used DX of Nikon cameras with a resolution of 12.2 megapixels. and physical size 23.6x15.8 mm (diagonal 28.4 mm, crop factor 1.52). The image on such a matrix consists of 4288x2848 pixels. Let's assume that you want to print photographs in 15x10 (180 mm diagonal) and 30x20 cm (360 mm diagonal) formats.
The scale of increase in the first case: 180/28.4= 6.3 , in the second: 360/28.4= 12.7 . We get scattering circles for 15x10 pictures: 0.1/6.3= 0.016 mm , for 30x20 pictures: 0.1/12.7= 0.0078 mm. It is these values of the diameters of the scattering circles that should be used as a guide when calculating the depth of field if you are going to print photographs of the specified formats.
The pixel size of the matrix in question is 23.6/4288= 0.0055 mm. Therefore, based on the sharpness criterion , the maximum size of a photograph that can be printed without compromising quality will be horizontally 23.6 * 0.1 / 0.0055 = 429 mm (43 cm), vertically 15.8 * 0 .1/0.0055=287 mm (29 cm), i.e. A 3 . The diagonal of the photograph at this magnification is 52 cm (520 mm), and the magnification scale is 520/28.4 = 18.3 .
How to get maximum sharpness in photography
In the previous example, we calculated the maximum size of an image that satisfies the sharpness criterion based on the matrix resolution , and this is the limit value for this sharpness criterion. If you enlarge a photo further, you need to move further away from it to make it look sharp.
But here a reasonable question arises: will the lens be able to provide such a circle of dispersion that the sharpness in the photograph corresponds to the characteristics of the matrix? After all, 12.2 megapixels. is far from the limit, for example, the matrix of the Nikon D800 camera has 36 megapixels.
The limitation here is imposed by lens aberrations and light diffraction . For each camera, you can calculate the aperture value at which diffraction effects begin to affect sharpness. As shown in the article about light diffraction, in our example it is f /9 , and as the aperture is further closed, the sharpness will only worsen.
As for aberrations , each lens has its own aperture values at which aberrations are best compensated. Manufacturers, as a rule, do not indicate this information, so it can only be obtained experimentally. Typically, sharpness in photography reaches its maximum value in the aperture range from 2 to 3 stops smaller than the maximum open one, to the one calculated according to the diffraction limit.
For example, for a standard 50 mm lens, f: 1.4 and the matrix from our example, the best sharpness will be in the aperture range f / 2 - f / 9 . At the maximum open aperture, aberrations are still quite noticeable and the image turns out to be quite soft, and at smaller apertures than f/9, diffraction begins to affect itself, which also reduces sharpness.
We looked at all the main factors that influence sharpness in photography. If you take them into account, you will be able to maximize the capabilities of your photographic equipment. We must not forget that the artistic value of photographs is largely determined by the skill of the photographer, and in combination with technical knowledge this gives excellent results.
Sharpness as a criterion for assessing image quality
Quality Standards in Commercial Photography
The author of this article is photographer Stanislav Perov
and it was written to determine guidelines
for assessing the quality of photographs
when posting them in various photo banks.
However, the approach to quality assessment and criteria
coincide with the requirements for photography for high-quality printing.
We consider this article useful for photographers and designers
working with images in printing or custom printing.
The style and layout of the article are left to the author,
as they best serve the perception of the text.
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| Commercial photography places particularly high demands on the technical quality of photographs. Photographs intended for commercial use often serve as the face of a product or service, a way to promote it, and they are often printed in large format. It is not surprising that companies that buy photographs want to purchase only the highest quality product, they are not interested in the difficulties that the photographer encountered during filming, the details of computer processing, etc. things, the main thing is an excellent result.
In this article we will talk specifically about the technical side of photography without analyzing the idea, artistry, composition, etc. In addition to the above goal of “pleasing the buyer,” strict technical control of photographs has another useful function: it disciplines the photographer. Having initially understood what a quality standard is, in the future it is much easier to improve your skills, violating some principles, creating more non-standard things. Another way: initially ignoring the technical quality of photographs, in the expectation that, for example, the artistry will “pull out”, is quite dangerous, as it often leads to the uncontrolled and unreasonable use of any techniques and effects. Ideally, the artistic beauty of a photograph should be combined with its excellent technical quality. Since most modern photographers work with digital cameras, this article discusses digital photography. Among digital cameras, there are devices of completely different classes: costing from hundreds of dollars to tens of thousands. It's no secret that the optimal choice for most commercial photographers is a digital SLR (reflex) camera. Inexpensive compact cameras, also called point-and-shoot cameras, are too limited in functionality and do not allow you to take high-quality photographs. The most advanced medium format digital cameras are too expensive for most photographers, and the circle of specialists working with them is very limited. Therefore, this article will consider the most versatile, quite affordable and capable of obtaining images of very decent quality digital SLR cameras. Thus, in a more comprehensive manner, you can add “. for digital SLR cameras." However, the quality of photographs obtained with the best modern models of SLR cameras is quite suitable for the majority of both viewers and buyers, so the material presented below is quite universal. Below is an overview of the main technical details that a photographer should pay attention to. Almost all photographs are shown without processing and adjustments in a RAW converter, except for some where this was necessary. In those examples where, in addition to the image itself, individual fragments of it are shown, the scale of these fragments is always 100%. Having finished with the introduction, let's move on to the essence of the issue. Sharpness as a criterion for assessing image qualityThe quality of an x-ray , from a technical point of view, is determined by the optical density of blackening, contrast and sharpness of the image. Contrast of two adjacent areas or details of an x-ray image. In practical conditions, the degree of contrast is judged not by the difference in blackening densities of two adjacent sections of the image, but by the difference in the light passed through individual sections of the film and perceived by our eye. Contrast perceived by our eye is called subjective contrast and is only an indirect measure of objective contrast, which is characterized by the difference in blackening densities of two adjacent areas of the image. As already mentioned, the best visibility is achieved at medium optical densities of the parts of the object under study. If the X-ray image is very contrasty, then studying the X-ray image will be difficult and the smallest details will go unnoticed. It is necessary to strive not for the maximum contrast, which is very often observed, but for the optimal one, which ensures the best visibility of the details of the x-ray image.
Maximum contrast is possible only when operating at lower voltages. It should be noted that images with maximum image contrast do not meet modern X-ray diagnostic requirements. The perception of contrast largely depends on the size of the parts. The smaller the size of the details of the object under study, the greater the contrast should be, and, conversely, the larger the size of the details, the lower the contrast of the image they will still be visible to our eyes. The perception of contrast also depends on the sharpness of the image. If the detail of the object under study has sharp boundaries, then high image contrast is not required and vice versa. From what has been said, it follows that the sharper the image , the lower its contrast can be allowed without compromising the quality of the x-ray image, and, conversely, a blurry image requires increasing its contrast; otherwise, the detection of image details will be degraded. Sharpness refers to the clarity of the contours of the X-ray image as a whole or its individual details. The sharper the x-ray image, the easier it is to see fine details in the x-ray. Considering that the initial stages of diseases cause minor changes in organs or tissues, the sharpness of the X-ray image plays a very significant role in X-ray diagnostics. Three components of quality photographyI think that everyone who buys a SLR or just a digital camera initially saves all or almost all of the pictures. I pressed the button and the result seems wonderful, even despite possible serious shortcomings. A person simply does not think about the quality of photographs, because behind them there is some event and the photograph is valuable precisely for these memories. At some point, family recognition of “professionalism” becomes not enough and we go to show tons of pictures to friends and acquaintances. At the same time, they often do not show the seemingly deserved admiration for our photographs. What's wrong and improve the quality of the photos next time? We'll talk about this further. Let's try to highlight the main components of high-quality photography. Now we will not talk about the artistic content, this is a separate difficult topic, we will only focus on the technically correct execution of the photo. For myself, I have identified three criteria for assessing the quality of a photo. sharpness comes first , followed by composition and exposure . The photo must be sharpOr rather, not even that, the main object of the photo, its semantic center should be sharp. The reservation is not without reason, for example, when shooting a portrait, the eyes must be in focus, everything else may be somewhat blurred. Therefore, depending on your plan, before pressing the shutter button, you need to determine which object or part of it is the main one in the picture and try to make it as clear as possible. Let's take the two photos above as an illustration. The left one is clearly out of focus and the model turned out blurry, the right one is much better. At the same time, the main thing in the frame is the model. Of course, you can try to pull out the left photo, but it’s better to simply delete it or, if it’s especially dear to you, leave it for “internal” viewing. By the way, it’s quite easy to avoid such embarrassment; just take several identical photographs of the same scene. With the right settings, even if something went wrong on the first photo, the second one is highly likely to be successful. Why did I highlight the sharpness of the photo? Simply, both composition and exposure, within certain limits, of course, can be corrected, but practically nothing can be done about blurriness. Of course, if you want to sharpen the image, you can use the Unsharp Mask filter in Photoshop, but this will not save you from gross focus errors or when shooting with long exposures handheld without a tripod. In our example, the photo on the left has more even light, there are no such sharp shadows as on the right, but even this does not save it. Composition - pay attention to the placement of objects in the photoAs was written above, we will omit the artistic component of the concept of composition, and even temporarily forget about all sorts of “rules of thirds”, etc. Let's talk about this: you selected a shooting object, for example a building, focused on it, and at the moment you pressed the button, a trolleybus passed between you and the building, the result is a ruined photo. This is the simplest example, but it may be that some little thing completely ruins the frame. Therefore, carefully examine the field of the frame through the viewfinder, try to exclude unnecessary details, “extra” passers-by, cigarette butts, bottles, wires, ugly surfaces, etc. even before shooting. It should also be remembered that “cropped” limbs look bad in photographs when feet, fingers or hands are not included in the frame. Under no circumstances should people's heads be left outside the frame. In general, try to compose the frame as tightly as possible so that the main subject fills the entire field of view, and there are as few extraneous objects as possible. Unlike blur, composition can already be corrected. Some things you can crop by cropping the picture on a computer, some things you can retouch, like the wires in the picture of the cathedral in Lviv. But in any case, it is better to try to find the optimal shooting point before pressing the shutter. Exposure - beware of overexposureExposure, like composition, is not a simple concept, but here small errors are even less critical, especially if you shoot in RAW. The most important thing to remember is that a person automatically focuses on bright areas in a photograph, in other words, spots of light distract the eye from the subject. In addition, do not forget that when there is overexposure in the photo (for example, the sky in cloudy weather), large white spots appear and it is almost impossible to correct this. In addition, chiaroscuro from foliage or clear transitions from light to shadow, which at first glance are quite safe, can greatly spoil a photograph, especially a portrait.
The portrait on the left turned out very spotty, although the shadow seemed to have much less contrast. And in the right photo, the sky simply disappeared due to exposure metering on a dark car. It's better to take a darker photo; you can almost always make the shadows lighter than to end up with white spots with information about objects in those areas permanently lost. To summarize, I would like to outline a shooting algorithm that allows you to obtain high-quality images. Assessing the scene for a future photograph proceeds in the reverse order - first we find a place or angle with suitable light, then we tightly frame the scene, highlighting the main subject and discarding unnecessary details, and take a photograph, making sure that it turns out clear. If you remember to follow these tips, you will be able to take really high-quality photos. Three criteria for quality photographyI wrote this article earlier, but decided to publish it only now. Everyone tormented me with doubts: is such an article necessary? Since it will be very short, but informative. That's why it appeared on my blog. In this article, I want to write about three criteria for quality photography . After all, if you bought a DSLR, and even more so visited my blog, then you want to learn how to take photographs , and your photographs should, naturally, be of high quality, right? So let's get down to business. Sharpness comes firstBut this does not mean at all that the entire photograph should be sharp, no, it is not. But what about everyone’s favorite bokeh then? The semantic center of the photo should be sharp! For example, in a portrait these are the eyes. Everything else may be blurry. Disastrous overexposureI have very valuable advice for you: it is better to make a photo darker than light. In a dark photograph you can still draw out the shadows, but a photograph with overexposure is very difficult to correct. Especially if you “killed” the sky. Also, try not to make “sun spots” on your model. Because there is a lot of contrast with the shadows. CompositionYou must provide for EVERYTHING, everything that will be in your picture. After all, even a small detail and the picture is ruined. For example, they often “like” to spoil photographs - power lines. Or the second gross mistake is cutting off limbs. You are a photographer, not a surgeon. But, if it is difficult to deal with overexposure, then photographs can still be saved. Subscribe to new articles on photographs. Good luck 😉 Video course “My first MIRROR”
Buy a courseGet a discount Do you want to receive new free lessons? Or on pages on social networks. Read also:13 comments on the post “Three criteria for quality photography”On my laptop monitor it is problematic to process photos due to the brightness discrepancy like everyone else... On my screen the photo is usually bright and but not saturated, on other monitors it is usually dark and more saturated. Well, yes, in general laptops are not good for processing. Same topic. Then I check on my phone. Modern people have: a “duck”, a mirror, an iPhone :) Thank you, interesting article! Is it possible to make blurry photos sharp? There is a chance to save the photo. It all depends on how “blurry” they are. I'm learning, learning to take photographs. Maybe it's not the lens that's the problem? Not long ago I saw on TV that digital cameras for ordinary users do not need more than 5 pixels. “Stronger” in what way? I didn't write about DSLRs. There are 12 pixel soap dishes. But in general everything is clear. This means that ordinary users who do not need to print large advertising posters will benefit from a 5-pixel soap dish. Well, 5 pixels are unlikely to exist now :) But, in fact, yes, megapixels don’t solve the problem! Sharpness in photographs. Criteria for its perceptionThe word “sharpness” is one of the most used in the vocabulary of photographers. “Sharp lens”, “good sharpness”, “in sharpness”, “depth of field” are just a few examples. It would seem that everything is clear with sharpness, either the object in the photograph is sharp or not, but still this issue requires detailed explanation. Sharpness refers to the clarity of images , i.e. visibility of details in a photograph. To distinguish details in a photograph, they must be there. Photographic equipment is responsible for this: a lens, a camera and a printer (display) on which the photograph is printed (displayed). But besides this, these details must also be distinguished by humans. In this article I want to focus specifically on human perception of sharpness. First of all, it is worth saying that sharpness is a subjective concept . An object may appear sharp to one person, but not to another. The reason for this could be several factors, let's look at them. LightingLighting conditions are important for the perception of sharpness. It is easier to identify flaws in a photograph viewed in a well-lit room than when viewing the same photograph in a dark room. Photo size and viewing distanceA very important factor is the ratio of the viewing distance of a photograph to its size. Obviously, the sharpness requirements for a large photograph viewed from a close distance are higher than for a small photograph viewed from afar. Visual acuityA person with good eyesight may find the photograph unclear, but a person with poor eyesight may perceive it normally. When viewing conditions become worse, such as in poor lighting, visual acuity decreases and the perception of sharpness becomes less critical. In poor lighting, the human eye even stops recognizing colors. Circle of ScatteringLet's imagine a large picture painted by an artist on canvas. When viewing a painting from a close distance, we will not be able to see it properly because we will see details: strokes, strokes, spots, etc. But there is no complete picture. And if we move a little further, then all the details will decrease, and we will see the picture as the artist intended it. The same is true with photography. The image projected by the lens onto the camera matrix is projected in the form of spots - scattering circles. The scattering circle helps to objectively assess sharpness, i.e. describe it in numbers, as a quantity. We can say that up to a certain diameter we perceive circles as points that make up parts, and circles larger than this diameter as blurry spots. Under good lighting conditions, most people can recognize objects as small as 1/3000 of the viewing distance. So for photography. viewed from a distance of 30 cm, the maximum permissible blur diameter (dispersion circle) will correspond to approximately 0.1 mm. For a 35 mm format matrix, provided that the photograph is printed in size 10 × 15 cm and viewed at a distance of 30 cm, the permissible blur diameter on the matrix should be 20 microns. And to view a 20x30 cm photograph from the same distance, it should be 10 microns. Conclusions: How sharp a photo appears depends on its size, distance, and viewing conditions. Based on these criteria, you can select the appropriate photographic equipment for shooting or the corresponding photographs taken.
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Sharpness as a criterion for assessing image quality
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