LETS GET STRAIGHT TO THE POINT
Updated: Jan 18, 2019
In Architectural photography, there is a general formality that Lines that are straight – Should appear straight. Nothing screams 'amateur' more, than seeing architectural images with crooked lines or walls that look like they are going to topple over. It's not rocket science – have you ever looked at a beach scene where the horizon line is sloping - as if the water would run out of the frame?
In geometric optics, a distortion is a deviation from a rectilinear projection – a projection in which straight lines in a scene, remain straight in an image. In general, most of the problems associated with rendering straight images are due to either using wide angle lenses, or the camera not being parallel with the subject. To improve the technical look of your images, it is an absolute must that your images are rendered as appearing straight - that is unless it is intentional.
Generally, there are five types of distortions that we need to address.
1. Optical Distortion
2. Geometric Distortion
3. Perspective Distortion
4. Keystoning Distortion
5. Alignment Distortion
1. OPTICAL DISTORTIONS
Optical distortions are those that are caused by the lens itself - all lenses regardless of the cost, brand or focal length, exhibit optical distortions - requiring correction. Naturally different lenses will exhibit different amounts of distortion – some more than others and some that are virtually indistinguishable. The types of distortion that they produce include uneven sharpness, vignetting, chromatic aberrations and curved lines. What is of interest to us, are those that produce curved lines.
Generally, there are two types of optical distortions that will alter the shape of lines.
1. Barrel Distortion
2. Pincushion Distortion
Barrel distortion, has the visual effect in which an image appears to be mapped to a sphere, bowing outwards. This is especially true for lines that do not pass through the center of the image. What this means in real terms, is that lines that are towards the edges of the frame will be curved outwards. The most common cause of barrel distortion are wide angle lenses. In order to get all of the subjects in the scene, a wide angle lens is an essential piece of equipment in interior photography, making this type of distortion the most likely you will encounter. The amount of distortion will vary depending on the focal length of the lens – the wider showing more distortion.
Pincushion distortion gives the visible effect in that lines that do not go through the center of the image are bowed inwards, towards the center of the image. In effect, the opposite of barrel distortion. For the interior photographer, this type of distortion is not as common as barrel distortion as it is commonly caused by telephoto lenses. Apart from detail shots, telephoto lenses are not commonly used in interior photography because in most cases you need to use a wide angle lens in order to capture the whole room.
2. GEOMETRIC DISTORTION
Another type of distortion that affects the shapes of objects towards the edges of the frame is called 'Geometric Distortion' or is sometimes given the name 'Anamorphic Distortion'. A sphere in the image center is normally rendered as a round disk, however, when the sphere is in the image periphery of a lens with a large angle of view, it becomes elliptically elongated.
3. PERSPECTIVE DISTORTION
Depending on how close you are to a subject, the scale of different objects – one that is closer to the camera, as opposed to something that is further away, creates perspective distortion. It can be described by one of two types of distortion - the appearance of a subject larger than its surrounding area, or an apparent lack of distance between objects in the foreground and background. These differences are based on what an object would look like with a normal focal length lens.
For the architectural photographer, gaining an understanding of these types of distortions are important if you want the proportions of a building or objects in an interior to look natural.
Lets imagine you have two identical stone statues one of which is 10 meters in front of you, while the other is 1 meter behind at 11 meters in front of you. The relative distance of the far object to the near object is a ratio of 11/10 or 10% farther away.
If you walk toward the statues until you are only 1 meter away from the closest one, you will be two meters away from the other. However, it will now be twice as far from you as the other, a ratio of 2/1 or 100% farther away. The nearer statue will have gotten bigger in your field of vision, than the further statue and the farther statue will appear to be twice as far away. Linear perspective changes are caused by distance, not by the lens.
There are two types of Perspective distortion.
Extension Distortion – Wide Angle Distortion
Compression Distortion - Telephoto Distortion
Extension or wide-angle distortion can be seen in images shot from a close distance using a wide angle lens. The objects that are closer to the lens appear abnormally large compared to distant objects that are in the background. In portraiture – it is known as the 'Dogs Nose Effect' – the subjects nose will appear too large in respect to the rest of the face.
Compression distortion is caused by photographing from a distance using a telephoto lens. Using a longer focal length lens makes closer and distant objects look approximately the same size – making the distances compressed. Framing the same subject identically while using a moderate telephoto lens (with a narrow angle of view) compresses the image to a more flattering perspective.
4. KEYSTONING DISTORTION
Another example of perspective distortion is where there are converging verticals that occur when a building is photographed with a camera tilted upwards, known as Keystoning. When photographing tall buildings the usual outcome is to look upwards. For interiors it is usually looking downwards. Perspective is not affected by the lens focal length, it depends on the viewpoint only.
When the camera back is parallel to a planar subject (such as the front of a building), all points in the subject are at the same distance from the camera, and are recorded at the same magnification. The shape of the subject is recorded without distortion.
However, when the image plane is not parallel to the subject, as when pointing the camera up at a tall building or from a high viewpoint looking downwards in an interior, parts of the subject are at varying distances from the camera; the more distant parts are recorded at lesser magnification, causing the convergence of parallel lines.
Keystoning also works on a horizontal axis in exactly the same way, in that if the camera angle is skewed, the image plane will not be parallel to the subject. It is not as objectionable as vertical keystoning, because the vertical lines are still straight and aligned to the frames edges.
5 - ALIGNMENT DISTORTION
Alignment distortions are usually caused by the camera not being level horizontally or being rotated slightly. Therefore lines that are referenced to the edges of the frame become skewed. For example an interior scene may have a tiled floor - if the horizontal line running across the image does not line up with the frame edges it becomes instantly noticeable. Like wise vertical lines should also align exactly with the frames edges otherwise the scene may look like it is topling over. Alignment distortion is not the same as keystoning - keystoning is where the camera sensor plane is not parallel with the surface that you are photographing.