Extending a Rectangle

In the field, I often use a little viewfinder to isolate my scene.  The ViewCatcher has markings on it that allow me to set it for a variety of sizes of panels such as an 8x10 and so on.  This way, I can compose to the format I'll be painting in.  Then, once I've found a nice composition, I use the viewfinder as a template to start my thumbnail sketch.  I lay the viewfinder on the paper and run the pencil around the inside of the rectangle lightly.  I remove the viewfinder, and there's my rectangle for sketching in.

Now, most viewfinders will give you a rather small rectangle to work in.  I personally like small sizes because they force me to stick with big, simple shapes.  However,  you may like a larger size.  There's a way to turn that rectangle into a larger one that has the same proportions.

Here's what you do (illustrations follow):

1.  Extend one diagonal of the rectangle to approximately the length you'd like the diagonal of the larger rectangle to be.
2.  Extend the two sides of the rectangle that are adjacent to the starting point of the diagonal far enough so they approximate the length they will be in the larger rectangle.
3.  From the end point of the diagonal, draw a vertical line upward to connect with the top extended side.  From the end point, draw a horizontal line back to connect with the other extended side.  These two lines will give you the larger rectangle, and it'll be scaled to match the original, smaller rectangle.

I show this, among other techniques, in my new mini-video, Composing in the Field.  (Visit my Lulu store to buy this and the other mini-videos.)