Optical aberrations (distortion) of a human visual system. What is the lens distortion and how to remove it

Restoration of a constant component.

Halftone (gradation) distortion.

The quality of tv image. For fully identical transmission of the image of the world around us, a stereo color system with very high quality parameters is necessary. While a similar system is not realized possible, and therefore qualitative parameters of TV images include: the number of rows, the number of frames, the number of flashes into one C, the number of halftone and their distribution in the dynamic range of brightness changes, color coverage, etc. determine nominal quality tv imagereproduced by this system. In addition to these limitations, the image matching is reduced due to distortions arising in almost all elements of the TV system. An objective and subjective assessment of the parameters and distortions of TV systems, the conditions of observation and processing of the results are also regulated.

Consider the main types of distortion and methods for their assessment.

9.1. Geometric (coordinate) distortion.

Geometric distortions occur due to changes in the coordinates of the transmitted elements, appear in the form of a disorder of the geometric similarity of TV images of the original. The geometric similarity is disturbed mainly due to non-identity of the form of a raster and relative speeds of lowercase and frame sweeps when analyzing and synthesizing the image.

Distinguish linear and nonlinear Raster distortions.

Figure 9.1 presents the main types of linear raster distortion,to which: packup, barrel-shaped, trapezoidal.

The assessment is made using a special square or rectangular elements that are part of specialized or universal test tables using geometric distortion coefficients, it is visually easier to produce on test elements in the form of circles and throughout the field of the image.

Fig.9.1. Geometric distortions of the image of the "Check Field", arising due to the distortion of the form of the raster

Panish distortionthe raster arise due to the inconsistency of the linear speed of the deployment ray in the center and peripheral part of the screen due to the projection on the flat screen of electronic rays deviated by radius. With a constant angular velocity of the beam, the beam length increases from the center of the screen, which leads to an increase in its linear speed, and consequently to stretch the image in the edges of the screen ( fig. 9.1A.). To combat patterned distortions, special methods of deflecting form correction are used, slowing the speed of the movement of the peripheral part of the screen or changing the size of the term, increasing the central and squeezing along the edges.

Bochemy distortionarise as a result pushflower recovery(figure 9.1.b.).

Pillow-shaped and barrel distortions are estimated by the geometric distortion coefficient according to the following formulas:

or

Trapestial distortions arise due to disorders of the optical and electrical axis to the image plane ( fig. 9.1.V.).

Distortion of frame format These may occur due to the violation of the ratio of deflecting currents of lowercase and frame sweeps (Fig. 9.1.g, d.).Assessing the values \u200b\u200bof this type of distortion is not an advocacy, as they are easily corrected by the image size adjustments by horizontal and vertical.

Nonlinear geometric distortions (Fig. 9.2)arise due to the impermanence speeds of the ray of the vertical or horizontal, that is, due to the nonlinearity of the current (Fig. 9.2.a)or lowercase scan (Fig. 9.2.b).

Fig.9.2. Geometric image distortions arising due to the nonlinearity of the line and frame sweep

The coefficients of geometric distortion in the vertical and horizontal direction are estimated as follows:

The human eye is weakly noticing nonlinear distortion. So the nonlinearity of the sweep up to 5% in any direction is almost invisible, and at 8 ... 12% the image is perceived as good.

I love to take pictures on a wide angle. If someone had said to choose one lens, which I can take with me on a trip - it would be undoubtedly a wide-roller! As a result - I have a lot of pictures made by a wide angle.

The main problem of all wide-rollers is an optical curvature called distortia(from distorsio. lat. -curvature).

If you look at the photo above, notice that all lines are not even, it is a bright example optical distortion. Now hover over the photo with the mouse and see how it should be. So, distortion is an optical distortion that is characteristic of your lens.

Distsiscy is two species - barrel-shaped (convex distortion) and pillow-shaped (concave distortion):

Restoration is characteristic of a wide angle. You will not notice a distortion on television or on portraight people. Therefore, most often distortion has to be adjusted when you take pictures on a wide-angle lens. Especially the distortion is noticeable if there are many straight lines in the photo through the entire frame, for example, when you take pictures of an architecture of some superwater lens (for example) you will definitely have to correct distortion.

Still, distortion is not always bad. If you have ever been photographed on Fishai (Fish eye), then you saw a pronounced example of an optical distortion, only on the fishing point of the distortion is a chip that Like everyone. It looks like this ():

Mouse over the last frame and you will see a photo on which distortion is adjusted.

How to remove distortion.

If you have photoshop, remove distortion easier than simple. You need to go to the Filter menu, go to the Distort tab and select the Lens Correction submenu. Now, move the slider to the left or right, to the desired result:

Naturally, the distortion will take a lot of time on the correction, so not everyone wants to adjust it. And in this case there is a panacea. It is called DXO OPTIC PRO.. With this wise program, you can adjust the distortion (and not only) automatically. All you need is to install the program and load the plugin for your camera and lens, the rest of the program will make automatically. I adjusted the last photograph of the Fish.

Basics of photography. Optical distortions of objects.

I think many readers have noticed more than once that the image in the photo is different from what we see with our own eyes. This is partly due to the peculiarities of the transmission of perspectives with a different focal length. This can be read about it in more detail in the article about. In addition, defects may appear in the image in the form of color halits on contrast areas, donating frames along the edges and changes in the geometry of objects. These shortcomings can be safely attributed to optical distortions of lenses, here they will talk about them in today's article.

Distortia

Reputation is a geometric distortion of straight lines when they look curved. Do not confuse distortion and distortion of perspective, in the latter case, direct parallel lines become converging, but not curved. There are two types of distortion by type of impact on the picture: Pillow-shaped - when the lines are concave and barrel - when they are convex.

Packaging distortion, normal image and barrelous distortion

Of course, in practice, the image rarely takes such ugly forms as in the diagram. A more real example of the effect can serve as a photo at the beginning of an article with a small barrelous distortion.

First of all, the distortion is visible on zoom lenses, and the more the multiplicity of the zoom, the stronger it is noticeable. Usually in a wide-angle position, you can observe a "barrel", and in the body "pillow". Between the extreme positions of the lens, the lack of optics become not as noticeable. In addition, the level of distortion can also change from distance to the object, in some cases a close object can be exposed to them, and the remote will turn out to be normal.

Chromatically aberration

The second type of optical distortion that we will consider are chromatic aberrations, quite often you can meet abbreviated "ha". Chromatic aberrations are caused by the decomposition of white light on the color components, which is why the object in the picture has a little different sizes in different colors and, as a result, color contours appear on its edge. Often invisible in the center of the frame, they become noticeable among objects located closer to the edges of the image. HA does not depend on the value of the focal length, nor from the aperture, but more often and more manifest themselves again in zoom lenses. This is due to the need to introduce additional elements to the optical scheme to eliminate the effect, which for lenses with a variable focal length is noticeably more complicated than for fixes.

On the picture on the left, HA is especially noticeable on the hair (purple contour) and on the windows lattice (turquoise).

It cannot be said that chromatic aberrations strongly spoil the picture, but on the adopted objects, especially in the opposition lighting, they become very noticeable and are quite stronger.

Vignetting

The last item is vignetting, in other words, darkening areas along the edges of the frame. Usually it can be seen on wide-angle lenses at the maximum open diaphragm. This effect is quite rare.

Do not confuse vignetting caused by laptops of optics and appeared due to additional accessories. In the picture above the edge, it turned out black due to several sufficiently thick light filters coated on the lens. A similar effect may turn out when the long blend is screwed.

Initially, all optical distortions directly depend on the class and the type of optics you use. The expensive series of lenses have complex lens location schemes and many additional elements, which minimizes such unwanted effects. Cheaper lenses, especially zoom, due to simplification of the design, are much stronger than similar problems.

I have a hurry to disappoint readers, lenses completely devoid of the above problems simply not. In one degree or another, even expensive optics models with a constant focal length distort the picture, the truth is noticeable, it is mainly on the edges of the frame. The good news is that for the most part these effects do not very much spoil the picture and can be easily eliminated by programmatically (talk about it in the next article). In addition, on the chambers with an informative matrix, and this is all amateur mirrors, the edges of the image in any case are trimmed and when using good optics, visible distortion is minimal.

The material is taken on the site.

Distortia (from distorsio. lat. -curvature) is an optical distortion of space. The most characteristic distortion is manifested when using wide-angle lenses. There is a distortion of three types: bocro-shaped (convex), pillow (concave) and promising.

Pillowing distortion is characteristic of a wide angle. On television objects, it is absent, but it can manifest a pillow-like distortion (usually from the focal length of 200 mm and above). Portrait and regular lenses (for example, 85 mm and 50 mm) are least susceptible to the effect of distortion, it is practically noticed there. So adjusting distortion most often needed when shooting on wide-angle lenses.

When is distortion most noticeable?

When there are straight lines in the frame throughout its area. For example, when shooting Architecture with a wide-angle or over-wide-angle lens, you will certainly need to be engaged in correction of distortion patterned. And if the shooting point was low, then hello, the distortion is promising! *

However, there are positive moments in this. As I said earlier, the frames made by super-wide-angle lenses (so-called "fisheye") have pronounced optical distortions. But in this case it is not a minus, but plus the lens, its strong side and this is so appreciated in the photo community. I assure you, no patenter, a portraight and the more thech of the television will not be able to give such a bright Wow effect when the space is covered with all 180 degrees! And this, by the way, is not the limit for Fisha! There are specimens that allow for one triggering of the shutter to make a photo by 270 degrees! Although, of course, each lens has its own destination and its strengths, a certain charm, the Fishaev still has :)

Correction of distortion

Fortunately, if necessary, each of the above distortions can be corrected. In the landscape photography, by the way, the distortion is less noticeable than, for example, a photo of architecture, replete with vertical lines.

As an example, we take photos with a barrel-shaped distortion, which has both types of lines (horizontal and vertical), which will best demonstrate the effectiveness of the Lens Correction toolor. If you read the profile on the illustration on the right, then you already know that the photo has been removed by the "sweet couple" - a full-frame camera and a fishe lens.

To begin with, I will correct the distortion - apply the profile and more accurately correct the result with the slider. As you can see, we wrapped the handle of the barrel-shaped distortion. It remains to align horizontal and vertical lines.

Also here you can correct promising distortion. To do this, we use sliders responsible for the appropriate correction. And a special grid will help to build a proper geometry (appears by pressing the V key). In the illustration you see that the vertical distortion has already been fixed.

So, all lines are built almost perfect (on the illustrations used sliders are marked). Due to the fact that we corrected the barrelous distortion, we have a minor loss of information at the bottom of the frame in the center. Therefore, the final barcode is cropping (the sixth tool in the upper left corner of the illustration). To automatically learn, leaving all the useful space and eliminate the "eaten" - we put a tick of the function of the Constrain to Image (see illustration).

Disadvantages of distortion correction

As a result of the correction of distortions, it may be necessary to circumcise (cropping) of the curved edges of the corrected frame, it may affect the composition. The correction also redistributes image resolution: when patterned distortion, after correction, the sharpness on the edges of the frame in relation to the center can be climbed. When correcting the barrelous distortion - on the contrary, the sharpness on the edges of the frame may fall.

* Perspective distortion is technically not a distortion, since it is a natural manifestation of three-dimensional space lens. Our brain, in turn, "knows", how to look correctly objects in reality, and therefore, perceives the converging lines in the photo (in cases where they must be parallel), as not corresponding to reality. For the correct display of perspectives, special Tilt / Shift lenses are used in which the properties of tilt / shear lenses make it possible to exclude the appearance of a promising distortion.

So we considered the basic disadvantages of photographic optics and learned how to avoid their appearance. And also mastered the methods of neutralization when appearing in the pictures.

I hope now your photos will become even better. At least, in technical terms, they are just obliged!

Yuri Krvenko, especially for FunPhoto.ua

Aberrations in the photograph call distortions of images, formed by the optics system. Depending on the nature of the origin of the aberration, there are chromatic and geometric. The reason for the occurrence of chromatic (that is, the color) of aberration is the imperfection of the optics of cameras. In fact, this type of distortion can be called the property of the lens, because in one way or another it is inherent in any of them. The lower the quality of the optics used, the greater the color distortion is observed in the pictures. Often in photographs made by cheap "soapboxes", there is a bright colorful border, framing contrasting objects. This is chromatic aberration.

To minimize this type of distortion, special achromatic lensesconsisting of two different varieties of glass. One of them - kron., has a low refractive factor, the second - flint, on the contrary, high. The correct combination of these two materials allows you to reduce visible chromatic aberration to almost zero. The same optical phenomenon in which the rays of light with different wavelengths are refracted from different angles, called wind dispersion.

No smaller headache of novice photographers than color, are geometric aberrations.

The distortion at which the points of the object, located outside the optical axis, are displayed in the image in the form of dimming or lines, called astigmatism. Objects in photos with astigmatism look curved, curved and slightly blurred. Thus, astigmatism, along with chromatic aberration, affects the sharpness of the image (albeit to a lesser extent).

If the contours of objects in the photo have an unnaturally concave or convex shape, and this is not an artistic idea, such a type of geometric aberration is called distortion. In the first case (when the lines are concave inside) we are talking about a barrel-like distraction, in the second - about the pillow-shaped.

Distortion arise as a result of a change in linear increase, provided by optics, by field of image. In other words, light rays, passing through the center of the lenses, merge at the point, located on the lens than the rays that pass through its edges. The appearance of a barrelous distortion, as a rule, contributes to the use of the minimum zoom value, sinful - respectively, maximum. The most obvious distortion is manifested when using wide-angle lenses.

Aspherical optics are applied to reduce distortions. Due to the inclusion of the lens lens with an elliptic or parabolic surface, the geometric similarity between the object of the photo and its image is restored. Of course, the cost of producing such lenses significantly exceeds the preparation of spherical optics.

Minor disstraction manifestations are easily corrected by means of a graphic editor.

The type of geometric aberration that prevents the formation of a flat image lens is called curvature field image. With such a distortion in the focus, there may be a center of the image, or its edge.

Adjustment of the curvature of the image field is made by making changes to the lens assembly. In this case, the prerequisite is the observance of the Rules of Peck, which determines the quality of the elements of the lens. If the reverse magnitude of the focal length and the refractive index of one element in the amount with the total number of items gives zero, it means that this element is good. The result of these calculations is called the amount of the pecval.

Interestingly, the technique of correction of curvature fields of photographers did not have owned up to the middle of the XIX century. But it did not bother them to engage in artistic photo. Blurred angles and fuzzy edges were covered with intricate vignettes, and portraits (in order to minimize distortions) were framed in oval frames.

Complex aberration that affects exclusively on light rays passing through the lens at an angle is called comatic (or just a coma). In the pictures of the coma manifests itself in the blurring of individual points of the image in the form of a comet. The "tail" comet can be directed towards the edge of the picture (positive coma) or its center (negative coma). This distortion is noticeable, the closer the point to the edge of the picture. The same rays of lights that pass clearly through the center of the lens, comatic aberration are not subject to.

Most geometric aberrations can be reduced by adjusting the diaphragm. Reduced its diameter, the photographer reduces the number of rays falling on the edges of the lens. But you need to use this opportunity carefully. Because excessive diffragging leads to an increase in diffraction values.

- This is an optical effect that limits the details of the image regardless of the set image resolution. The cause of its occurrence is the dispersion of the light flow when passing through the diaphragm. Many newbies, trying to increase the depth of field, cover the diaphragm hole to such an extent that the sharpness achieved is overlapped by the smoothing effect of diffraction. This effect is called a diffraction limit. Knowing its magnitude allows you to avoid problems with image detail. To calculate the diffraction limit, a special calculator is used, available for free download on most specialized sites.

When choosing a camera, it should be remembered that there are no lenses without aberration. In any case, while. Even the most expensive optics demonstrates some image distortions. Adjusting one type of violations leads to a strengthening of another - and this process has no end. But in order to become a good photographer, it is absolutely necessary to wait for the invention of the ideal lens. It is enough to learn the features of a particular lens - and to level its disadvantages with their own skill.