Answering the question about what happens when a design is translated into image, I want to start with the Mark Hewitt “Representational forms and modes of Conception.” He states that the most important things are the way we present the image and a subjective perception of it.
The reason the design is being translated can be different as well. On the one hand, it can be an idea of the architect being in the process of development. On the other hand, it can be a form of persuasion, the way Le Corbusier did, or as it is called in “The conventions and rhetoric of architectural drawing” a purpose of “rhetorical exposition.”
Besides, looking at all the variations and types of drawing I can say they all might represent different purposes. Such as perspective gives a three-dimensional space of the future/existing building. Sketches can be some ornamental pattern as work in progress as well as a separate artifact. While plans or section drawings can present how the building or part of it looks inside to show the hidden details.
As Hewitt stated “drawing is said to be the language of architectural design,” so without putting the ideas on paper, it will be just an idea in the head of designer. As I said, everybody’s perception and visual thinking is different, so even the image of linear perspective and axonometric one sometimes cannot be enough. Then the representation comes to a modeling mode.
The medium of representation is important as well. It ranges from pencil to watercolor to computerized graphic. In “The conventions and rhetoric of architectural drawing” it says that “drawing instrument obviously affect not only the appearance of the drawing but also the character of the building they are used to represent.” The color and shadow technique might play essential role as well. The usage of graphite can be used to present a wide variety of textures and shadow. The ink is a useful technique for perspectives, when distinguished lines and sections ar needed an accent. The watercolor definitely gives more of a pictorial effects. Again, on my opinion, the usage of a particular technique, mode or medium is solely depend on the reason of the presentation of the architectural drawing.