Parallel lines can be projected either as parallel or as converging lines on a 2 dimensional photograph. As converging lines they are considered a distortion of the reality, nevertheless they can be useful to give the illusion of depth. It is your choice as a photographer to represent those lines parallel or not, and this article explains how to control it.
How to influence lines to be parallel or converging ?
The convergence of parallel lines only depends on the orientation of the camera. Contrary to a popular belief, it does not depend at all on the focal length or distance to the subject. A wide angle lens does not accentuate the convergence, actually it helps to keep lines parallel.
The pictures below are all taken with a focal length of 10mm at the same distance from the building. The only change is the orientation of the camera, which causes the lines to be parallel, slightly converging, or strongly converging. When the camera sensor is parallel to the lines, they are projected as parallel.
The next series shall convince you that the focal length has no influence at all. The first photograph is taken with a focal length of 10mm, the second in the same condition with a focal length of 20mm. On both pictures the lines converge with the same angle. On the 3rd one I shrunk the picture taken at 20mm and put it on top of the picture taken at 10mm, they overlap perfectly. This clearly shows that a different focal length does not change the angle of the lines. Generally speaking the focal lens does not influence the perspective.
In order to see the entire tower straight there is no other choice than to go as far as necessary, until the entire tower fits inside the frame when the camera is held horizontal. The longer is your focal lens, the further away you have to go therefore a wide angle lens is an advantage in this situation.
Considerations when using converging lines
So far we only looked at vertical lines, but the principle apply to lines of any direction, when your sensor is parallel to the lines, they look parallel on the picture.
When the lines converge, they converge toward the so called “vanishing point”. Since the orientation of the camera controls the convergence of the lines, it also controls the position of the vanishing point. Furthermore using a wide angle lens makes it easier to fit the vanishing point inside the frame, which can create more impact in your picture.
Consideration when using parallel lines
The professional solution to maintain parallel lines is to use a lens that can rise/fall/shift on a plane parallel to the sensor. It is commonly used in architecture photography but is pretty expensive for the occasional photographer.The picture below describes how a shift lens works compared to an ordinary lens.
There are also a variety of post-processing softwares that can correct the perspective. Just be aware that such correction crops the composition, the stronger the correction, the larger is the crop. It is visible in the example below. On top of that, the perspective correction stretches some parts of the picture (the top of the tower in the example below), which cause a loss of definition or details in the stretched pixels. Here again the stronger the correction, the more loss of definition, but this is generally only visible by inspecting closely the pixels.