The concept of distortion is one of the most confused topics in photography. Countless photographers seem to group lens & perspective distortion together as one entity when in reality they couldn't be more separate. Not only are these two different types of distortion brought about by very different causes but they must also be dealt with individually.
Lens Distortion
Lens distortion is a product of an actual optical aberration (or imperfection) within the elements of a lens. The result is that straight lines in reality will in one way or another appear curved or deformed in the resulting photograph (this has nothing to do with them being vertically oriented, which is perspective). Though this is less than optimal, you usually wouldn't notice it in a portrait or landscape image as there are rarely true straight lines. However, architectural photography is composed largely of straight lines and it is essential to keep their appearance as such. You will typically see more lens distortion in wider local length lenses than you would in medium or telephoto options. Due to their more complex design, wide angle zoom lenses are suspect to heavy lens distortion though prime lenses can also have it. Below are three main types of lens distortion.
Perspective Distortion
Perspective distortion is the direct result of camera placement in relation to your subject. Perspective works the same for the human eye as it does with your camera. Objects closer to you always appear larger than those further away. Try this out - hold your phone 18 inches from your eyes and it may appear larger than a huge TV that’s across the room.
In architectural photography, perspective distortion causes what we call converging verticals. If you stand at the base of a building and point your camera upward to fit the entire structure in frame you'll find that the building appears to be falling backwards in the picture. This is because the foundation of the building is much closer to your lens and therefore exaggerated in size. Likewise when shooting interiors, if you shoot over an island in the kitchen while pointing the camera downward to fill the frame you'll find that your vertical lines are not vertical. Although converging verticals can often be fixed with a tilt-shift lens, within your RAW editing software, or from standalone software such as PT Lens or DxO ViewPoint II, they can also be exaggerated intentionally to create a more artistic effect.
This is also the type of distortion that is known for making facial features appear unattractive. Again, it is not the focal length of a lens but rather your distance from your subject that distorts their appearance. If you use a wide angle lens close to a subject's face, it is going to make the nose appear significantly larger than it should while at the same time shrinking one's ears and even more so the objects in the background. Photographers are often warned early on to avoid wide angle lenses when taking portraits because they "distort facial features", when in reality it is your perspective and how close you must get to your subject to fill the frame that causes facial deformity. I have long been an advocate for creative wide angle portraits as long as your subject does not fill the frame and is a distance away. My 14mm (9mm crop sensor equivalent) was used regularly at weddings when that was more of my focus. Shooting downward on your subject from above can actually give the illusion of slimming your subject down, though be mindful that if overdone you can enlarge their head.
Correcting Distortion (My workflow as of 2014)
I have a two step workflow for distortion that is quick and streamlined. I use PT Lens for lens distortion and then DxO Viewpoint II for perspective changes. Both are available as filters within Photoshop. Once I've finished editing, I can usually remove all lens distortion and straighten all verticals in an interior shot in about a minute.
Below you will see 3 images: Base image with no distortion correction, after PT Lens, and after DxO Viewpoint 2 to straighten. Click to enlarge and notice the left wall & door frame. as you switch through them This was an extreme case as my 14mm lens has an abnormally strong case of lens distortion was used.
I always begin by creating a stamp visible layer (cmd/ctrl + option/alt + shift + E) which merges all current layers onto a new one so that you can work non-destructively. This ensures that corrections can be removed or altered later if desired. Toggling this correction layer on/off also becomes the easiest way to see before & after.
Although lens distortion can be corrected manually in Lightroom (Adobe Camera RAW) or Capture One it is sometimes beneficial with custom designed profiles from standalone programs. Tom Niemann, creator of PT Lens, tests every camera + lens combination and provides the unique fix needed. In my case it has profiles I can’t find in Lightroom. The software literally reads your exif information automatically (or can be chosen manually) and all you have to do is press "OK". PT Lens is almost like a chiropractor visit for your wide angle and realigns everything up with no thinking involved. Click here to read more about PT Lens and how to use it. You must however deal with lens distortion prior to adjusting perspective for these lens profiles to work.
Although I suggest that you perform lens distortion correction every time you use a wide angle lens, perspective distortion can be intentional as I mentioned before. In the commercial world it is industry standard to always fix your converging verticals unless specifically requested by a client or art director. Tilt shift lenses are excellent for avoiding this all together but many don’t have access to them. DxO ViewPoint streamlines this entire process and corrects even the most extreme perspective irregularities. Simply drop anchor points to define lines that should be straight and ViewPoint corrects up to 8 point perspective issues. You can literally turn a warped trapezoid into a perfect square! DxO will allow you to virtually change your angle after the fact when necessary. Click here to read more about DxO ViewPoint II and how to use it.
Hopefully this has helped you establish a basic understanding on some of the different types of distortion and how to address them. If you have questions please leave them in the comments. I am in no way sponsored by PT Lens or DxO, but they are programs that I have really enjoyed using.
If you’re interested in digging deeper into the use of perspective in photography here is another awesome read from Pixpa - Understanding Perspective in Photography