If you are a designer and use a computer somewhere at some point in your workflow, it is your fate to understand both Photoshop and Illustrator. They are loaded with powerful features and work well with each other; however, there are significant differences between Photoshop and Illustrator. These differences relate to the core purposes of each piece of software. You should be well informed about these differences and how they affect your design process.
Illustrator works with explicit points and lines to generate mostly two dimensional geometries. Because these final geometries are derived from points and lines, which are by definition defined in space, they can be scaled, transformed, rotated, translated, and manipulated without loss of information or quality. This manner of working with and storing graphic information is referred to as vector based graphics. Before reading further, I trust you already see how Illustrator’s features might be very useful in an architect’s design process. Even though vector graphics are derived from explicit data, it all must be represented in pixels on a monitor or dots on paper. The arrangement of different shades and colors of pixels into a grid is another manner of working with and storing graphic information. Most people refer to this manner of handling and storing graphic information as raster graphics. Photoshop mostly works with and generates raster graphics. Because this information is based on a finite amount of pixels with specific values scaling and stretching its content larger will require interpolation of existing information to produce information that doesn’t exist; quality will be lost.
Assume that a typical digital design process begins with some tangible product and returns to such a product after filtering the concept through software. If this product involves planimetric or sectional investigation, Illustrator should be considered for use in the process. Illustrator provides tools for both manipulating and creating the type of information that plans and sections include, i.e. points, lines, shades, tones, line thicknesses, hatches, shapes, transparencies, etc. Illustrator works with this type of data with less accuracy than AutoCAD regarding the explicit coordinates of points and lines; however, Illustrator far surpasses AutoCAD by constantly and fluidly displaying the final result of your product as you work. In Illustrator WYSIWYG, whereas in AutoCAD lines and points are abstracted into infinitely thin elements, which await graphic definition (thickness & color) prior to printing.
Many products of design involve mixing different types of graphic information. Renderings of three dimensional objects, space, and environments often require refinement. Photoshop is often used for this type of graphic manipulation. Because renderings, photos, and scans are stored simply as rasterized pixels (not defines points and lines), they are best refined in Photoshop. Photoshop provides many features for controlled manipulation of the tones and shades of images; however, one of its most significant and used features is Photoshop’s ability to import (copy and paste) and layer graphic content. Layering content is ideal for injecting contextual, programmatic, or tectonic aspects of a concept into a base image. This has the potential to make a rendering more convincing and provocative.
It is extremely useful to understand when, if, and how Illustrator or Photoshop enter your design process. Illustrator is non-exhaustively useful for creating diagrams and refining sections/plans. Photoshop is non-exhaustively useful for adding content to renderings and refining color/contrast. There are many more uses for these two programs, but I trust it is clearer now what the two should be used for in your design process.