What is distortion in photography

Very many photographers and amateurs, at the very beginning of their creative journey, are faced with such an unpleasant thing in photography as distortion. This article will help you understand what distortion is, why it occurs and how you can prevent its occurrence.

It often happens that the image in a photograph differs from what we see with our own eyes. These differences are expressed in changes in the geometry of objects and perspective, darkening of the frame at the edges or the appearance of color halos. Such shortcomings are associated with optical distortions of the lenses, because any glass or mirrors slightly distort the image. Therefore, oddly enough, it is almost impossible to completely eliminate optical distortions of the lens; you can only reduce their appearance in the finished photograph.

Often there is no particular problem with small lens distortions; they are completely invisible in the image. But in some cases, optical distortions seriously change the nature of the image, and here you have to think about how to deal with the distortions of images formed by the optics system.

The type and nature of optical distortion directly depends on the lens used. As you know, all lenses have such important characteristics as focal length, sharpness, viewing angle and depth of field. Optical sharpness is the ability of a lens to distinguish and convey the smallest detail in a scene. If there is no sharpness, they say that the lens is “soapy.” The lens itself is a complex optical system that requires high precision and manufacturing quality. As a result, even lenses from the same model range and with the same characteristics may differ from each other in quality.

Most lenses are characterized by the presence of all kinds of aberrations, that is, optical distortions in the image that are directly related to the optical system. These aberrations can be chromatic or geometric in origin. Many amateur photographers, however, do not notice these distortions, simply because they do not know what to look at to notice the aberrations.

Chromatic aberrations

Chromatic aberration is a fairly common type of optical lens distortion, which is characterized by the appearance of unnecessary color halos and contours at color boundaries. Chromatic aberrations are not caused by the ideality of camera optics, when white light in the lenses is first split into color components and then combined back into a beam. But some inaccuracy of this connection leads to unpleasant distortions in the picture.

Chromatic (color) aberrations at the boundary of contrast media

Bright multi-colored contours framing contrasting objects are chromatic aberration. Such distortions are rarely visible in the center of the frame, but they become noticeable with objects located closer to the edges of the image. Most often, this type of optical distortion can be observed when using zoom lenses. But in fact, to one degree or another, chromatic aberration is inherent in any lens.

In principle, chromatic aberrations do not spoil the image so much, because they appear mainly at the edges of the image. In addition, they are often simply not very noticeable. But sometimes, especially when shooting contrasting objects, such optical distortions in the finished photograph begin to become noticeable.

In order to reduce the possibility of unpleasant chromatic aberrations, special achromatic lenses are used today, consisting of two different varieties glass – crown and flint. If crown is characterized by a low refractive index, then flint, on the contrary, is high. Respectively, smart combination These two materials in the optical system can reduce the likelihood of chromatic aberrations to almost zero.

Geometric aberrations

If chromatic aberrations are associated with color distortions, then geometric aberrations characterize the ability of the lens to distort the geometry of the objects being photographed. Beginning photographers have probably encountered this phenomenon when straight lines in photographs suddenly bend outward, and walls suddenly become crooked. All these are geometric distortions or distortion, which can be combated by adjusting the aperture. By reducing the diameter of the aperture, the photographer thereby reduces the amount of light rays hitting the edges of the lens.

However, if you close the aperture hole too much, another optical effect occurs, called diffraction. Diffraction limits the detail of an image, regardless of the set image resolution. That is, an excessive reduction in the aperture diameter leads to the fact that the achieved sharpness is blocked by the smoothing effect of diffraction, as a result of which problems with image detail already arise.

Geometric aberrations are divided into two individual species- barrel-shaped and pillow-shaped. Barrel distortion is typical for wide angle lenses, when the center of the frame is closer than the edges, as a result of which the central part takes on a more convex shape, and straight lines bend outward.

The opposite of the “barrel” phenomenon is pincushion distortion, when straight lines, on the contrary, bend inward, as a result of which the frame becomes “concave.” Pincushion distortion is characteristic, in particular, of telephoto cameras. The appearance of barrel distortion in a photograph is facilitated by the use of minimum value zoom, and pincushion – maximum zoom. The higher the magnification of the zoom lens, the more noticeable the distortion appears in the image. The visibility of geometric aberrations is also affected by the distance to the object being photographed. In particular, a close subject may be more susceptible to geometric distortion in the frame.

Various manifestations of lens distortion, that is, curvature of straight lines in the image, can be removed or corrected using modern graphic editors. Professional photographers usually used for this purpose special programs for distortion correction, including AdobeCamera RAW, Lightroom, Aperture and PTLens. In addition, in the same Photoshop there is a LensCorrection tool with which you can correct optical distortions.

True, the correction of aberrations in graphic editors must be approached carefully and carefully, because with careless correction, some distortions will simply change to others, which can only lead to deterioration of the image. In addition, we should not forget that slight optical distortions of the lens and the associated imperfections in the image can ultimately benefit the photograph.

Sometimes in the practice of photographers there are also such optical distortions as curvature of the image field. This effect is characterized by, for example, the appearance of blurred corners and fuzzy edges. That is, only either the center of the image or its edges can be in focus. Often in photographs one can observe distortions of perspective, which manifest themselves in the convergence of two initially parallel lines in the photograph. Such distortions arise mainly due to the position of the camera, that is, the camera's line of sight is not perpendicular to parallel lines.

Finally, it is quite rare to encounter an effect called vignetting. These are distortions that are characterized by a drop in image brightness from the center to the edges, that is, darkening of the picture at the edges of the frame. A similar effect occurs when using a wide-angle lens and the maximum aperture.

So, optical distortions caused by the characteristics of the optical system, large number. It is important to understand here that there is not a single lens without any aberrations. Even the most expensive lenses from famous manufacturers may exhibit some image distortion. This, in turn, means that when choosing optics you should not focus solely on the high price of the lens, believing that the more expensive it is, the better the quality. You shouldn’t chase new, advertised lens models if you already own optics that completely suit you.

Of course, in expensive lenses, as a rule, more complex design with a unique lens arrangement and multiple additional elements, which should minimize the occurrence of all kinds of optical distortions. However, as we have already noted, none of the lenses can boast that they are completely free from the above-mentioned shortcomings. Therefore, you need to choose optics that only you like and correspond to your feeling. photographic quality. You should not trust the technical characteristics of the lens declared by the manufacturer, but only good, high-quality photographs.

But even if you become the owner of a lens that sometimes exhibits some image distortion, it’s not a problem. After all, if you thoroughly study the features of the “behavior” of a particular lens, then all its shortcomings can be skillfully compensated for by your photographic skill. In any case, most optical distortions of the lens do not spoil the picture that much and can be easily eliminated using software when processing captured photographic images.

In this article with a scary title, we will look into the features of optical distortion of lenses. Have you noticed that when shooting at a wide angle, the edges of the frame are distorted? And when you try to take a photo in backlight, a pink, blue or greenish fringing appears around objects? If you haven't noticed, take another look. In the meantime, let's figure out why this happens.

First, you need to understand and accept the fact that ideal optical systems (i.e., in our case, lenses) do not exist. Each optical system has inherent distortions that it introduces into the projection of reality onto the image (photography). Distortions of optical systems are scientifically called aberrations, i.e. deviations from the norm or from the ideal.

Aberrations of various optical systems can take different shapes and be more noticeable or practically invisible. Typically, the more expensive the lens, the better the quality of its optical system, which means the fewer aberrations it has.

Types of aberrations

Most often, the word “aberration” itself is used in photography in the combination “chromatic aberration.” As you might have already guessed, chromatic aberration- this is one of the types of distortions caused by the characteristics of the optical system of the lens, which is expressed in the form of color deviations. A typical example of chromatic aberration is unnatural colored outlines at the edges of subjects. Chromatic aberrations appear most clearly on contours in high-contrast areas of the image. For example, at the border of tree branches taken against the background bright sky, or along the contour of the hair when shooting a portrait in .

The cause of chromatic aberration is: optical phenomenon as a dispersion of the glass from which lenses are made. Glass dispersion is that light waves different lengths(of different color spectrum) when passing through the lens they are refracted at different angles. White light (which contains a whole spectrum of light waves of different lengths, i.e. different colors), passing through the lens of the lens, first breaks up into a color spectrum, which is then reassembled into a beam for projecting the image onto the camera matrix. As a result, due to the difference in the refraction angles of colored rays, deviations occur during image formation. This is reflected in errors in the color distribution in the image. This is why colored outlines, colored spots or stripes may appear in a photograph that were not present on the subject.

Chromatic aberrations to one degree or another are inherent in almost all lenses. Cheap optics are much more lame than elite series lenses. During the design phase of an optical system, manufacturers can minimize chromatic aberration by using achromatic lenses. Secret achromatic lens is that its design consists of two types of glass: one with a low and the other with a high refractive index of light. Selecting the proportion of a combination of materials with different refractive indices of light makes it possible to reduce the deviations of light waves at the moment of splitting white light.

Don't be too upset if your lens doesn't contain achromatic lenses - chromatic aberration occur mainly when shooting in difficult conditions lighting, and are very noticeable only when viewing a photograph at 80-100% magnification. In addition, no one has canceled processing in graphic editors, which can eliminate such optical errors. To learn how to do this, read the next article, “Correcting Lens Errors” (coming soon).

Another type of lens aberration includes geometric distortion, which is commonly called lens distortion. Lens distortion manifests itself in distortion of the proportions of objects located closer to the edges of the frame. Scientifically speaking, with distortion, the linear increase in objects in the field of view occurs unevenly. As a result, objects at the edges of the frame appear unnaturally flattened or elongated.

Based on the nature of the distortions, there are two types: type of distortion: positive ( concave or cushion-shaped) and negative ( convex or barrel-shaped). If no geometric distortions are observed in the frame, then they say that there is no distortion. In this case, the image looks smooth and flat; note the perfectly straight horizon line in the image below. Typically, it is along the horizon line that you can easily notice geometric distortions in landscape photography.

Distortion is most pronounced when used. Moreover, the larger the viewing angle of the lens (the shorter the focal length), the more pronounced geometric aberrations. You've probably noticed that vertical and horizontal lines, when shooting wide, become curved as they approach the edges of the frame. The most striking example lens distortion- these are photographs taken on ultra wide angle lens"fisheye" (fish eye). But in the case of fisheye, distortion is not an error or defect in the optics. Rather, it is its feature that allows you to expand the viewing angle of the lens to 180 degrees (and even more).

When using wide-angle lenses (FR<24 мм) можно наблюдать бочкообразную (вогнутую) дисторсию, при использовании длиннофокусных объективов (ФР>200 mm) pincushion (convex) distortion may appear. Lenses with average focal lengths are usually not characterized by geometric distortions across the frame field.

This is why they say that a wide-angle lens distorts proportions, and lenses with a focal length of 70-200 mm smooth out any distortions. And that is why it is customary to shoot portraits with 70-200 mm lenses, which do not distort the proportions of the face and figure. But portraits shot wide open look comical and are used only to create a special caricature effect. Moreover, the smaller the distance between the shooting point and the subject, the stronger the distortion of proportions. For example, as in the famous portrait of Bill Clinton (photo below) - the head looks disproportionately small compared to big hands and knees. But in in this case This is just a creative idea, the author’s style of the photographer. By using a wide-angle lens, he was able to create a vivid visual image - an association with a person former president USA.

Just like chromatic aberration, distortion can be corrected when designing the lens. For this purpose, a aspherical lens, and lenses with corrected distortion are called aspherical. You may have seen such names (ASP) in the description technical characteristics to the lens. Such lenses are usually more expensive than their spherical counterparts, but when shooting they convey the proportions of objects in the frame without distortion. However, there is a relatively inexpensive Sigma 10-20 mm F4-5.6 EX DC HSM lens, which gives a smooth picture even at a maximum viewing angle of 102 degrees.

If your wide angle lens gives geometric aberrations, so there are two ways to fix this:

1. If you're using a zoom lens, you can simply set it to a longer focal length and take a couple of steps back. So, you will have the same composition in your frame, but by changing the focal length you will get rid of distortions.
2. Geometric aberrations can be corrected using graphic editors (primarily Photoshop). But at the same time, be prepared to lose some of the objects in the photo, because when correcting curvatures, cropping occurs at the edges of the frame. Read the next article to learn how to do this.

Distortion(from distorsio lat. - curvature) is an optical distortion of space. Distortion is most typical when using wide-angle lenses. There are three types of distortion: barrel-shaped (convex), cushion-shaped (concave) and perspective.

Pincushion distortion is typical for wide angles. It is absent on telephoto lenses, but pincushion distortion may appear (usually from a focal length of 200 mm and above). Portrait and standard lenses (for example, 85 mm and 50 mm) are the least susceptible to the distortion effect, where it is practically unnoticeable. So you most often need to correct distortion when shooting with wide-angle lenses.

When is distortion most noticeable?

When the frame contains straight lines throughout its entire area. For example, when shooting architecture with a wide-angle or ultra-wide-angle lens, you will definitely need to correct pincushion distortion. And if the shooting point was low, then hello, perspective distortion!*

However, there are also positive aspects to this. As I said earlier, frames taken with ultra-wide-angle lenses (the so-called “fisheye”) have pronounced optical distortion. But in this case, this is not a minus, but a plus of the lens, its strength, and this is why it is so valued in the photography community. I assure you, no regular camera, portrait lens, and especially a telephoto lens can give such a bright wow effect when the space is covered at all 180 degrees! And this, by the way, is not the limit for fish! There are copies that allow you to take a photo at 270 degrees in one shutter release! Although, of course, each lens has its own purpose and its own strengths, fisheyes still have a certain charm :)

Distortion correction

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

As an example, let’s take a photo with barrel distortion, which has both types of lines (horizontal and vertical), this in the best possible way will demonstrate the effectiveness of Lens Correction tools. If you read the profile in the illustration on the right, then you already know that the photo was taken by a “sweet couple” - full frame camera and a fisheye lens.

First, let's correct the distortion - apply a profile and more accurately correct the result with the slider. As you can see, we waved the barrel distortion knob. All that remains is to align the horizontal and vertical lines.

You can also correct perspective distortions here. To do this, we will use the sliders responsible for the appropriate correction. A special grid will help you build the correct frame geometry (appears by pressing the V key). In the illustration you can see that the vertical distortion has already been corrected.

So, all the lines are lined up almost perfectly (the sliders used are marked in the illustration). Due to the fact that we corrected barrel distortion, we had a slight loss of information in the lower center of the frame. Therefore, the final touch is cropping (the sixth tool in the upper left corner of the illustration). To automatically crop, leaving all the usable space and excluding the “eaten”, check the box next to the Constrain to Image function (see illustration).

Disadvantages of distortion correction

Correcting distortion may require trimming (cropping) the curved edges of the corrected frame, which may affect the composition. The correction also redistributes the image resolution: with pincushion distortion, after correction the sharpness at the edges of the frame may increase relative to the center. When correcting barrel distortion, on the contrary, the sharpness at the edges of the frame may decrease.

*Perspective distortion is not technically a distortion, as it is a natural manifestation of the lens's rendering of three-dimensional space. Our brain, in turn, “knows” how objects look correctly in reality, and therefore perceives converging lines in a photo (in cases where they should be parallel) as not corresponding to reality. To correctly display perspective, special tilt/shift lenses are used, in which the tilt/shift properties of the lenses make it possible to eliminate the appearance of perspective distortion.

So we looked at the main disadvantages of photographic optics and learned how to avoid their occurrence. We also mastered methods of neutralizing them when they appear in photographs.

I hope your photos will be even better now. By at least, in technical terms, they simply must become one!

Yuriy Krivenko, specially for funPhoto.ua

Photoshop CS6 filter " Distortion correction» corrects distortions caused by the camera lens. Go Filter - Distortion Correction. In the dialog box you will see tabs " Automatic correction" And " Custom correction».

If you want to keep things simple, select " Automatic correction" Or go to the " Custom correction" and manually make the necessary changes.

Here is a list of automatic correction settings:

• Correction: Select the problem to be corrected. Find explanations for each issue in the custom corrections tab. Note that if, during correction, the image is stretched or contracted from its original size, select automatic image scaling. Select "Edges" from the drop-down menu ( popup menu on Mac), how you want to fill the edges - black, white, transparent edges or expand the pixels of the image;
• Search Criteria: Select your camera make, model, and lens model. Choosing the right equipment helps Photoshop make more precise adjustments;
• Lens Profiles: Select the appropriate profile. For zoom lenses, right-click (Cmd+click on Mac) and select the most appropriate focal length. If you can't find your lens profile, click the " Internet search" to find profiles uploaded by other photographers. If you want to save the profile for future use, click on the drop-down menu " Lens Profiles» ( popup menu on Mac) and select " Save online profile locally».

Here are the settings on the "tab" Custom correction»:

• Geometric Distortion: Corrects anomalies such as convexity and concavity, in which straight lines (respectively) deviate outward or inward. Select the tool " Eliminate distortion" and drag it onto the image - or you can drag the slider " Eliminate distortion»;
• Chromatic aberration: Are you experiencing a blurred fringe of color around objects? Photographers call this chromatic aberration. Borders, aberrations, or whatever they are called - you can get rid of them using sliders Red/Cyan or Blue/Yellow border. Tools " Moving the grid", "Hand" and "Magnifying glass" will help you set the settings more conveniently;
• Vignette: If you have a vignetting effect, with edges darker than the center, drag the Amount slider to indicate how much you want to lighten or darken the image. Using the “Midpoint” slider, you can specify the width of the effect;
• Transform: Corrects perspective distortions often caused by tilting the camera when shooting. Using the Transform option, you can adjust the perspective horizontally or vertically. Specify the angle at which the image is rotated to compensate for camera tilt or adjust the viewing point. You can also use the Straighten tool to rotate a tilted image:

• Draw a line along the image along which you want to straighten it. Finally, to eliminate empty areas caused by geometric distortion correction, use the Scale settings to crop those areas;
• View / Show Grid: Choose whether to have a grid overlay when viewing the image ( from which you can specify its size). Many problems, such as perspective distortion, are easier to correct with a mesh;
• Tools " Moving the grid", "Color", "Hand", "Magnifier": Help you make adjustments more conveniently. The Color tool changes the color of the grid. Tool " Moving the grid» delineates the image with lines. You can also control magnification using the zoom controls in the lower left corner of the dialog box.

Filter " Distortion correction» works only with 8-bit and 16-bit images. You can edit multiple photos at once by batch processing them using the automated " Distortion corrections" Select File - Automation - Distortion Correction.

Translation of the article " How to Use the Lens Correction Filter in Photoshop CS6"was prepared by the friendly project team.

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