LESSON 1 - KEEP IT STRAIGHT

 

In Architectural photography, there is a general formality that Lines that are straight – Should appear straight. Nothing screams 'amateur' more, than seeing interior 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.

 

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

 

PINCUSHION 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.

Solutions

 

1. Change your lens – If you are working professionally it pays to purchase the highest quality lens available for your camera model. However, considering that each model or brand of a lens has different amounts of distortion, there is no guarantee that this will solve your problem. Also, if you are using a zoom lens, the amount of distortion may vary depending on what focal length you have zoomed to. It is often difficult to test out lenses to obtain a pre-purchasing decision. 

 

2. Use a longer focal length lens – The general rule regarding interior photography is to use as long a lens as possible – mainly to give a more realistic compression of objects in the scene and to avoid perspective distortion. As a lens focal length decreases the amount of distortion increases. The most common problems you will encounter will be with the usage of wide angle lenses. However, when your client asks you to show the whole room – you'll end up using as wide a lens as necessary – making it difficult to use longer focal length lenses.

 

3. Get further away from the subject – Where possible you should try and get the camera as far away from the subject as possible so that you can use a longer focal length lens. This is not always possible in a confined space or when furniture is in the way. Sometimes it only takes just an increase in distance of 30 centimetres from your subject to solve quite a few distortion problems. Moving furniture out of the way or slightly changing your viewpoint can often give you the necessary distance required.

 

4. Use a full frame camera – If you are using a small sensor camera, the equivalent focal length lens compared to a full frame camera is generally wider. For example, a camera with a crop sensor of 1.5 will require a 16mm lens in order to give the equivalent of a 24mm lens on a full frame camera. Considering that the curvature of the 16mm lens will be greater than the curvature of a 24mm lens, it is more than likely that the lens will have more distortion.

 

5. Move objects that are close to the camera further away - The closer an object is towards the camera, the larger it is going to appear relative to the objects behind it. If at all possible, the simple solution to regain a natural looking perspective, is to phsyically move furniture further away from the camera. 

 

6. Use a spirit level on the camera - To avoid image alignment problems, always use a spirit level on the camera and adjust horizontal and vertical lines with the alignment of the frames edges. If you are using a zoom lens, zoom in to the check for consistancy. The spirit level on a tripod head is not adequate enough, as the tripod head may actually be level - however, the camera may be at an angle on the tripod plate.

 

7. Use a Tilt Shift lens - With a Perspective Control lens, the camera back can be kept parallel to the subject while the lens is moved to achieve the desired positioning of the subject in the image area. All points in the subject remain at the same distance from the camera, and the subject shape is preserved. Tilt Shift lenses will be covered in a future lesson.

 

8. Change the tripod height - If you cannot get the camera sensor plane parallel with the surface you are photographing, it sometimes helps to change the height of the tripod. There are many thoughts regarding how high a tripod should be in relation to the subject, but from a theoretical point of view, the tripod height would be half way between the floor and the ceiling. In practice, however, this is not necessarily true and we will discuss this in detail in a later lesson.

 

9. Use software to correct lens distortions – This is by far the best option in correcting lens distortions without having to change your equipment. Barrel and pincushion distortion can be corrected using many different software packages. At this stage we will be leaving post production for later lessons, however, we suggest you research suitable software and test them out for ease of use and accuracy. The software reads the information from the digital file telling it what camera model and lens combination you have used. The manufacturer of the software will have presumably tested out the lenses, creating a profile with the right amount of correction. Where some software fail is when you are using a zoom lens,  by not  accounting for the difference in distortion that changes depending on the zoom focal length – they use the same correction even if the focal length changes and with it the distortion. Some suggested software packages to try include - Dxo, Ptlens, Power Retouche, Perspective Pilot, Lightroom and Adobe Photoshop. 

 

10. Use software to correct Viewpoint distortions - You can manually correct for keystoning and alignment problems using software, however at the trade of loosing image quality. More about this when we cover postprocessing techniques.

 

 

 

Barrel Distortion

BARREL 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

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 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

 

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.

 

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.

 

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.

 

Copyright © 2019 Alan Carville

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