It’s been a while since I wrote anything about the basics of photography. I had already started to write a lot about all the interesting terminology you should know when you begin photography. By the way, a summary of all the photography terminology is available, even if it could probably be expanded. This article goes back to basics and tell you all about distorsion phenomena in photography.
This is surely one of the most well-known words for beginners (and others). In other articles to follow, I will also discuss other optical defects such as chromatic aberrations, diffraction, coma, and others.
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While it is in no way necessary to know all the terminology by heart nor what they all mean, I think it is an important step to take when you want to deepen your photographic techniques. Whether you decide to buy a new lens to diversify into other photographic types, or you want to start processing RAWs with a post-processing software, knowing the distortion phenomenon is, in my opinion, not only useful but also mandatory.
So, I’m starting this “not too technical” article, hoping to share with you what I know, in a simple and efficient manner. However, you should know that even if lenses suffer from this phenomenon, more and more available software corrects it very adequately during the post-processing stage, even if it can lead to loss of image edges, for example, in order to correct the distortion (as is the case with my Tamron 20mm f/2.8). This should generally not be a decisive point when choosing a lens. Having said that, it all depends on the type of photography you’re doing….
To define distortion in a simple way, I would say that it is the deformation phenomenon occurring on an image as a whole and of its constituting elements. Indeed, one might tend to believe that all the elements of an image have the same magnification regardless of their location, but actually, it is not the case. Depending on the type of lens used, an element captured at different locations on an image will not have the same shape. It is therefore a geometric aberration, related solely to the lens, which affects the volume of subjects in an image. The entire image is distorted with the degree of distortion varying according to the location of the element on the image.
Distortions occur when light rays enter and pass through a set of elements (lenses), then exit. Indeed, rays do not all go through the same location on the lens (or lenses). While those closest to the lens, horizontally, will be affected just a little, the further outside they go the more affected they’ll be. Therefore, distortions are often more pronounced on the outside of an image. A perfect lens, in terms of distortion, allows the transmission of light rays at the same point. Some brands, such as Laowa, even produce lenses branded “zero-distortion”, allowing almost zero distortion on an ultra-wide-angle lens (such as the Laowa 12mm f/2.8 ZERO-D). Manufacturers have developed technologies to attempt to control these distortions, including use of aspherical lenses (curved on the outside) for rays of light to converge on a single focal point.
Distortion is measured in percentage points and equals to the difference between the ideal position of an object on the image, and its actual position. Depending on the type of distortion, the percentage points can be positive (pincushion) or negative (barrel).
The best thing to do before buying a lens is to find out its distortion percentage points by type of distortion. You can look at the DXOMark website, or any other website that provides this type of analysis. You can then check, for both zoom and prime lenses, the type of distortion and how it behaves as you move away from the center of the image.
To give you the example of my Tamron 20mm f/2.8, it has a very high barrel distortion, which tends to make subjects in the center of the image clearly stand out, while mostly distorting them around the edges. Tests show an approximate 8% barrel distortion which is still huge for a wide-angle lens (even if it can be corrected quite well with DXO or Lightroom).
- Barrel distortion: it is characterized by an image at the center that looks “inflated” (like a barrel, hence the name), and where the edges of the image tend to curve inward. The magnification of the image’s elements therefore decreases towards the outside. These defects are particularly noticeable on horizontal lines, especially when moving up or down the image.
This type of distortion is mostly found on wide-angle lenses, but also quite often on the shorter focal lengths of zoom lenses (e.g., type 16-35mm). Indeed, the field of view of this type of lens being smaller than sensor size, the image must somehow be curved.
Here is a rather striking example of this type of defect with my Tamron 20mm f/2.8 lens, showing strong barrel distortion. DXO corrections can compensate relatively well for this type of defect.
- Pincushion distortion: it’s just the opposite effect from barrel distortion. One of its main characteristics is the tendency for straight lines to curve from the center of the image to the outside of the frame. It is very common on zoom lenses (at the longest focal lengths) and on telephoto lenses. High-end telephoto lenses manage this optical flaw increasingly well with negligible pincushion distortion with any remaining distortion easily corrected during post-processing.
Here’s an example of a fairly large pincushion distortion from my Tamron 70-180mm, at 180mm. The pincushion distortions are given at more than 3.5% at 180mm, which is a lot, really. Fortunately, a software like DXO handles this kind of lens imperfection beautifully so you can easily correct it.
- Mustache distortion: this is the last and most complicated of the three to manage during post-processing. It could be viewed as a combination of the two aforementioned distortions (“pincushion” and “barrel”). Straight lines appear curved inwards towards the center of the image, then curved outwards as you move towards the edges/corners. This type of distortion is mostly found on old lenses, or on very short focal lengths (even recent ones such as the 14mm f/2.8 from Samyang).
A few words to summarize these points. Because of their larger size, know that full frame sensors (24x36mm) suffer the most from distortion phenomena. Indeed, you will use the whole lens when mounted on a full frame sensor, while with a 1.5 times smaller APS-C sensor, you will only “use the center”. If you mount full-frame lenses on an APS-C body, there will be fewer optical flaws. However, lenses designed for APS-C have distortions that are generally the same as the equivalent FF focal lengths. For example, a 16mm APS-C on an APS-C body will have an almost identical distortion than a 24mm FF on an FF body. Here is an article for you to know everything there is to know about sensor size in photography and how it affects your images! Regarding lenses:
- Fish-eye lenses, very strong barrel distortion, but note that on this type of lenses, this a desired effect,
- wide-angle lenses: mostly barrel distortion but with a small caveat. On a fixed focal length like my Tamron 20mm, and on zoom lenses, a distortion of this type can also be found. Moreover, with the outward curvature of the image, the lens will actually frame wider than a 20mm. DXO Mark’s website shows this well with a 20mm prime lens (like the Sigma 20mm f/1.4 DG HSM Art here) framing wider at the image’s edge. This also means that if you tend to crop edges or want to “keep the proportions” during post-processing with a software, you’re not necessarily going to lose any image from your advertised 20mm focal length.
- Standard focal lengths: it all depends on the lens, but barrel distortion is still less pronounced. Note that on zoom lenses such as 24-70mm or 16-35mm, we often find a barrel distortion at the shortest focal lengths (as is the case with wide-angle lenses), and a pincushion distortion at the longest. Here is an example of the Canon 24-70mm f/2.8 II L USM.
- Telephoto lenses: they mostly suffer from more or less pronounced pincushion distortion depending on the lens’s quality and range. On zoom lenses with very wide focal lengths (such as 18-300mm), the pincushion distortion in the long focal lengths will often be quite significant.
We just discussed the three main types of distortions in photography. These are related to lens construction. However, there are other types of distortion, or rather perspective deformations, related to shooting and characteristics. There are mainly two types.
This is a classic distortion found when shooting a subject at very close range. It doesn’t arise from a lens’s optical defect but from the fact that you are very close to a subject with a very short focal length, typically a wide-angle (often being the only lens that allow you to get very close to a subject). This is, for example, the case if you try to shoot someone’s portrait with a wide-angle lens such as an 11mm in APS-C or a 16mm in full frame. The eyes, nose, etc. will be completely distorted and will appear much larger.
A little clarification: we often tend to believe that short focal lengths distort perspectives more than long ones (telephoto lenses), which is actually false. All lenses have and offer the same perspective. It is simply the distance between your camera/lens and the subject that changes its perspective.
You can, however, use this type of stretch distortion to emphasize a part of a subject, by getting very close to that area. This will tend to bring it into sharp focus. It will appear much larger in the image when in fact it is not.
Unlike extension distortion, compression distortion is found when using telephoto lenses. When using a long focal length, perspective appears compressed and background elements (sometimes miles apart) appear much wider and larger than they normally would.
It’s not that hard to figure that out. If shooting JPEG, I recommend you enable the camera’s optical corrections. Generally speaking, they will adequately correct the defects, even though some artifact may remain. If you shoot RAW, you just need enable the correction profile of either the lens or the camera + lens pair, to correct image distortion. And that’s about it. By the way, I have written a comprehensive article explaining the differences between RAW and JPEG.
I’m coming to the end of this article on distortion in photography. I hope you found the article useful and now understand better the point of being familiar with this kind of optical defect.
I will be back soon with a new article on optical defects!