Albrecht Dürer

Albrecht Dürer was a German painter, engraver and printmaker but as well as being a artist  Albrecht Dürer is also know as a mathematician and theorist. Among the many artworks he produced he also published two books, one on geometry called the Four Books on Measurement and the other on human proportions is called the Four Books on Human Proportion. these books included a series of illustrations of drawing frames and perspective machines. The goal of these devices was to enable artists to take accurate measurements of their chosen subject or to trace a scene as it appeared before them in order to create a convincing illusion of the real world.

Many of the techniques used in the 15th and 16th centuries continued to be useful to later artists. The artist John Constable (1776 - 1837) used a glass frame similar to one described in the Treatise on Painting by Leonardo da Vinci (1452 - 1519).

Many artists have used grids to assist them in creating larger or smaller-scale copies. A grid is drawn over the original study, and another grid is marked out at the desired scale on the surface where the image is to be reproduced. The artist can then copy the part of the design that appears in each square at the new size. This technique is still in use today.

PERSPECTIVE MACHINE
String a grid in a frame, (preferred is a grid dividing the frame into thirds or a multiple of three, and a frame in golden proportion or the same proportion as your paper), fix it in place between you and your subject, as shown in Durer's etching. Use it along with a similar grid on your paper to aid in producing an accurate layout of your subject matter. Note that the relationship between your eye, the frame and the subject must remain constant. Useful when dealing with foreshortening or when accuracy of proportion is important to the layout.
A variation of this that works somewhat differently, in that you draw directly on the picture plane, is to place a piece of clear Plexiglas fixed in place between you and your subject, you can now draw your subject on the Plexiglas. Use a suitable maker, one that will clean off, and rough in your sketch. Now transfer the drawing to your paper using tracing and transfer papers, or transfer and enlarge using a grid. Gauze stretched in a frame can be used in place of Plexiglas, and if the size is right, can make it easier to transfer the rough sketch to paper or canvas.

USING A FINDER
 When viewing a subject, especially a landscape, it can be difficult to select what to include and what to exclude from the composition. If you find this to be the case you might try using a "finder" to assist in selecting that portion of the universe to include in your composition, and in its placement on your paper.
  You can make a finder from a piece of cardboard. Cut a rectangular opening approximately 3.5"x 5". (This is approximately the same proportion as a sheet of watercolor paper, if you are using some other size paper adjust the proportions of the rectangular opening to match your paper.) Hold this frame in front of you and closing one eye and moving the finder, study the arrangement of shapes you see in the finder. Move the finder around until a suitable arrangement is found. Once you have developed the "habit of selection" you will find it is no longer necessary to use the finder.
A photographic slide frame can be used as a finder, however, because of its size, you will need to hold it rather close to your eye.
John Pike's "Wonderful Perspective Machine" is such a finder with some extra features. It is a blue plastic frame (blue to aid in judging the values by neutralizing the color) with gird lines marked on it and moveable thin steel strips that can be lined up with building or other angles and are then held in place by the magnetic frame border. This allows you to set the strips and then judge the angle in relation to the grid on the finder and the same grid lightly marked on your paper.