The job of a photographic lens is to capture light and bend it toward the film or digital sensor. The refraction of light exerts a variety of side effects on the image projected onto the film or digital sensor. Your understanding of how this bending can affect the image may factor into how you compose your images.
Anomalies and Lens Distortion
Glass, crystal, liquid, or plastic lenses made for photography are never “perfect”—just like the lenses in our own eyes. Photographic lenses often comprise multiple lens elements. Because of the way light is transmitted and bent through these elements, many anomalies are introduced, which have an effect on the resulting image.
"Glass, crystal, liquid, or plastic lenses made for photography are never ‘perfect’—just like the lenses in our own eyes."
Most common photographic lenses are rectilinear—meaning straight lines in a scene are reproduced in the image as straight lines. But, because light rays are being refracted, even with a rectilinear lens, distortion appears when the light passes through the elements of the lens. Some rectilinear lenses exhibit barrel distortion, pincushion distortion, or mustache distortion.
Curvilinear lenses, the opposite of rectilinear, will curve straight lines in an image. Wide-angle fisheye lenses are non-rectilinear, or curvilinear, lenses.
In this article, we are going to talk specifically about perspective distortion, but for a more comprehensive look at optical anomalies, click this link.
Perspective Distortion: Focal Length / Subject Distance
There is a common belief in the photography world that the focal length of a lens is the culprit in the crime of distortion. You can find dozens of articles online stating that wide-angle lenses are not acceptable for portraiture because they appear to cause facial distortions. Fact: a rectilinear lens, regardless of its focal length, will not distort objects in the frame, beyond the aforementioned lens distortions. A curvilinear lens will distort the image.
The truth of the matter is that wide-angle lenses do cause facial distortions when the subject-to-lens distance is small. The wide-angle lens does not distort objects on its own. If that were the case, capturing a landscape image of a mountain range with a wide-angle lens would show distorted mountains. As you move an object closer to a lens of any focal length, the resulting image will show signs of distortion due to the changing angles of light entering the lens. The closer the object is, the greater the difference of the angle between objects at the center of the frame and the edges.
This issue comes up most with portraiture, but it applies to any object you are photographing. If you want to make a standard head-and-shoulders portrait of a person, you will have to stand very close to them with a wide-angle lens. The subject’s face will distort due to the angular differential explained above. If you stand farther away from the subject, the distortion will fade, but you will have lost your head-and-shoulders perspective.
At the other end of the focal-length spectrum, a long telephoto lens will optically compress an object because there is a very small angular difference between the light rays entering the lens. When it comes to portraiture, a certain amount of compression is often considered flattering for the subject. Additionally, the longer focal length lens allows a more comfortable working distance between the model and photographer. This is why telephoto lenses are often preferred for portraiture.
Perspective Distortion: Angles
If you stand in the middle of a straight road and look toward the horizon, the road will appear to converge to a point. We know the road doesn’t narrow, but we see it happening visually off in the distance because, the farther away things are, the smaller they appear to our eyes.
We can apply that same principle to a tall building. Stand on the sidewalk and look straight up at a 100-story skyscraper—one that does not taper. The first floor is the width of a city block. The 100th floor is the same width, but, because it is far away from our vantage point, it looks smaller. The building looks as though it is tapering to a point because of the distance from our eyes to the 100th floor.
Photographic lenses emphasize this phenomenon.
If you are photographing a vertical structure (building, home, factory, statue, etc.) and you tilt the camera up to capture the top, you will see the structure decidedly narrow as it extends higher in the frame—this effect is called “keystoning.” Similarly, if the camera is angled right or left off the perpendicular from the building, you will get the same distortion on the horizontal axis.
How do you correct for angular distortion? There are two ways to do it.
1. The common physical method is through the use of a perspective control (PC) lens that allows shift. Most modern PC lenses feature both tilt and shift (TS), but shift is all you need to keep parallel lines from converging in a photograph. Large format view cameras also usually permit tilt and shift movements of the lens.
2. Most popular post-processing software programs allow you to correct digitally for distortion.
The disadvantage of digital correction is that the software virtually creates pixels where pixels did not exist before. Obviously, modern software is very good at doing this and making it look seamless but, in extreme cases, you can see evidence of the digital manipulation. With a shift lens, you are correcting the effect optically, before the capture, so there is no need to manipulate the digital file. Of course, if you are shooting film, the only way to correct is the optical method, unless you scan your negatives into digital format.
Browse your local real estate listings and you will usually see a lot of angular distortion in both interior and exterior images. Did the photographer correct for angular distortion or were the photos provided by a real estate agent who grabbed a few snapshots with a smartphone or point-and-shoot? Look at an interior design or architectural magazine and you will likely not see any distortion, as these photographers generally correct their images before they are published.
|Red lines help emphasize the absence of distortion, and presence of distortion, in the two images.|
Another detail to note: Since our eyes also create angular distortion, it may not always be advantageous to remove all distortion from an architectural image, as this can look unnatural.
There is nothing wrong with angular distortion. It happens all the time and a vast majority of photographers do not correct for this effect. In fact, there are a great many cases where lens distortion—either caused by subject-to-lens distance or angular distortion—adds to the success of the composition of a photograph. When it comes to the elements of composition, success can result from the application of those elements in a scene or from applying the “rules” of composition to your image. When it comes to distortion as a compositional element, knowing what causes distortion and using that knowledge to avoid or embrace it may be another key to a successful composition.