Image Distortion In Photography And How To Fix Them
When we capture images in our camera, we sometimes wonder why it looks the way it looks. What I mean by this is the image may look somewhat crooked, warped or at worse distorted. The image turned out different from what you were seeing in the viewfinder of the camera. This is normal, and it happens more often then we like. This is also called image distortion. The thing to know is that not all distortion is actually bad. Sometimes it can be used as a form of effect or a way for the photographer to share their perspective.
Distortion, in digital photography, is the deviation of an observed pixel from its predicted coordinate in a 2D plane. This leads to a more curvilinear appearance that looks unnatural and even unappealing to the observer’s eye. This is why you may often hear people say the camera “added 20 pounds” to my body or “my face looks like an alien, it is too long”. It is caused by the distortion from the camera’s lens.
The lens used in a camera is not a perfect piece of glass, so some form of distortion might be present in the image it captures. There is what we call radial distortion in which the light bends at a certain angle that deviates from a rectilinear plane. These are distortions that are caused by the angle of light and position of the lens when creating the image. Light bends in many ways due to the angle it hits the camera’s lens when composing the shot.
Once the exposure is captured, it will display the final result to the photographer. With digital cameras, the photographer can immediately spot the difference. For example, the photographer can see things are not quite that “straight” or aligned due to how the image turned out when looking at the camera’s LCD viewer. Let’s go over the types of distortion photographers will likely encounter.
Lens Distortion
This is a form of optical aberration, which leads to the deformity of an image due to the curvilinear bending of light. The lines of deviation are compared to the pinhole camera model. It gives the appearance of a curved instead of straight lines. It is a result of the optical design of the camera’s lens. It can be described as a lens error, but that does not mean it is the lens maker’s fault. It just means to say that the lens was not able to project the image accurately or correctly during the capture.
There is really never a perfect image, as there will be a certain level of distortion when trying to create what the eye sees versus what the lens projects and what the camera sensor records. A perfect lens is just not realistic given the many factors that can affect projecting light to create an image. The best lens makers can do is to reduce the distortion to a level that is acceptable to the human eye.
In a simple example of a lens distortion model, the undistorted and distorted radii ru and rd (distances from the image center normalized to the center-to-corner distance (half-diagonal) so that r = 1 at the corner) and the radial distortion coefficient k can be represented by the formula:
The distortion can be taken from the difference between the plotted point in the plane of the radii in a normal (predicted or assumed) position (referring to pixels in digital imaging) and its deviated position. The deviations can either show barreling or a pincushion effect which will be explained further. These are classified as radial distortion.
The fix for radial distortion can be found in the Brown-Conrady Model (more explanation in link).
There are 3 types of radial distortion:
Barrel — The image looks like it is bending outward from the center of the image. When put against a grid of straight lines, the lines will appear to curve toward the edge of the image in the shape of a barrel. This is a typical issue with wide angle and zoom lenses with short focal lengths.
Pincushion — The image appears to be pinched or bending inward to the center of the image. When put against a grid of straight lines, the lines will appear curving away from the edge of the image. This is common to occur with telephoto lenses at longer focal lengths due to increased magnification on the parts of the image that are closest to the edge of the frame.
Complex — This is also called a mustache distortion. It can be described as wavy because the lines show the features of both barrel and pincushion distortion. It can have a sort of skewing effect as the direction changes from lines bending inward to outward and vice versa.
The distortion can occur with wide angle and prime lenses. Although there are lenses that are great for producing high resolution and quality images, they can still suffer from a certain amount of distortion that varies depending on the distance to the subject.
Why We Need Correction
You can look at this in two ways. First is correcting the distortion during the image capture and the second is the easiest, correcting the distortion in post using an image editing application.
In the first method, the photographer will have to take plenty of shots of the subject and inspect the results in-camera until they are able to capture acceptable images that do not appear too distorted. In other words, finding the sweet spot in terms of angle and distance from the subject. This is often how photographers work when taking headshots or close up images (e.g. macro shots).
Correcting in post is considered easier because there are editing programs available that can fix and correct the distortion. Retouchers rely on applications like Adobe Photoshop or LR to fix these problems. It is just a matter of knowing what feature to use to correct the distortion. Unfortunately, this cannot be done directly with film. The photographer would have to develop the film first and scan it to digital before they can do retouching work.
The most obvious answer to correcting distortion is to make the image look better or more accurate. Barreling can make the subject look larger, appearing less flattering and impressionable. This is not good for portraiture, especially if it is going to be used to help promote a public personality.
Pincushion can make the subject look much worse, and even surreal. In landscape photography these effects may not be noticeable until you need to have images display straight lines. Otherwise it would just look unnatural to the viewer, and appear more distracting rather than appealing to the eye.
Correcting Distortion In Post
I will go over how to correct image distortion using Adobe Creative Cloud. This is a part of the digital workflow process, after the image has been captured (not during the actual image capture from camera). There are two applications you can use, Adobe Lightroom Classic or Adobe Photoshop.
Lightroom Classic
The new Lightroom is a cloud-aware version, suitable for various devices including mobiles. The traditional Lightroom that most users may be familiar with is now called Lightroom Classic (LRC). This is the best way to correct image distortion in RAW format.
Go to the Develop module -> Lens Corrections tab. There is a slider control under the Distortion section that allows the user to adjust how much distortion to correct.
Moving the slider to the left corrects the pincushion distortion, while moving to the slider toward the right corrects barrel distortion. Likewise this slider can also be used to create distortion for effects. Retouchers can use it to make adjustments in architectural photos to straighten angles and adjust perspective accordingly.
Photoshop
Most retouchers use Photoshop, in a commercial and production environment. Even the casual photographer can use it to make adjustments to their images, including distortion correction.
From the Photoshop menu, go to Filter -> Lens Correction …
There are two tabs Auto Correction and Custom.
The Auto Correction tab lets users pick a Len Profile, based on the camera they used to capture the image. Photoshop also “ … automatically selects a matching sub-profile for the selected lens based on focal length, f-stop and focus distance.” This might work fine, but some users will not be satisfied with the result. They can use the Custom option.
The Custom option is more specific. This allows users to control more settings, like the Vertical Perspective and Horizontal Perspective. Users can straighten the horizon of an image if it is too slanted using the Angle feature. This can correct certain things that make the image look curved too outward or inward. The best feature is the Remove Distortion slider, which you can adjust. To correct barrel distortion, move the Remove Distortion slider to the right. To correct pincushion distortion, adjust the slider to the left.
Users can also use Filter -> Distort with many more options to choose from (e.g. Displace, Pinch). For correcting barrel distortion or pincushion distortion, I normally select the Pinch feature and adjust the slider until I get the desired result. This usually requires cropping the image though, so it may not be as desirable as correcting the entire frame of the image. In that case, use the Filter -> Lens Correction … feature.
Correcting complex distortion is not very straightforward compared to the other types. It will require more work on the part of the retoucher, and this is usually the most difficult part of image editing. It is not uniform, and can occur randomly within an image. It would require correcting different parts of an image, and cannot be applied as a whole fix.
No Editing
Expert photographers, who don’t edit, can capture near perfect images using their experience and technique. They will still encounter distortion, but their skill is enough to understand how to properly hold the camera and get the best exposure from their subject when composing.
A good example of this are film photographers. Since they shoot directly to film, not to a digital storage media, they have to be much more selective with their shots. Film is very expensive now, and the photographer can only see the result after developing the film.
From their experience, film photographers have a better understanding about how to shoot and compose the shot they want. Whether they are using a 35 mm or medium format camera, this is a much harder thing to do for the average “point-and-click” photographer who uses a digital camera.
Synopsis
We encounter distortion in images, which naturally occurs due to the optics of the lens. Some amount of distortion is acceptable as long as it does not affect the resulting image’s aesthetic look. Too much distortion is unacceptable, and this is where a photographer would need to apply correction. Photographers who use wide angle, telephoto, and zoom lenses are more likely to encounter image distortion.
Distortion can be corrected during the time of shooting. All the photographer needs to do is check how their images are looking in the LCD viewer of the camera and adjust their distance and angle from the subject. Although the viewer itself does not offer the most accurate representation of the final image, it can give an approximation of how the image will look. Use grids as a guide to so if there is any barreling or pincushioning effect occurring in the image.
Some distortion can be done on purpose, but that is more for creative or artistic effect. An example of this is a fisheye lens distortion, used when shooting an extreme wide-angle view of a subject. This can give a more interesting look to macro photography subjects (e.g. insects, flowers, product accessories) and panoramas. There are actual lenses that can create these distorted appearances, so it does not require further work in post. Another type of creative distortion is called perspective distortion. It has nothing to do with the lens at all but more about the position of a subject in relation to the camera lens and angle of view.
Unwanted distortion is not always going to be noticeable until the photographer loads the images from their camera to the computer. This is why many distortion correction takes place during post, or when the photographer is reviewing the images captured by the camera in the storage card. Photographers will use image editing software applications like Lightroom Classic or Photoshop to correct noticeable or unacceptable distortion in the image. The end result is to get the image the photographer intends to display.
