|DRAWING SKETCHING COURSE: Measuring optically lines in a motif|
This is an online drawing course free of charge.
To measure the length of a motives part „optically“ you take a pencil, brush or stick and hold it with a stretched arm. It is important, that the pencil is in a plane right-angled to the arm. The reason is, that the stretched arm is parallel to the axis of your “optical system”, your eyes and that if you hold it in this way you always hold it in the same pose.
Now shut one eye and hold the pencil in a way, that you see it in this part of the motive, you want to measure. With your thumb you can mark one end of the length, the top of the pencil marks the other one.
When doing so you have to take care, that your arm is stretched and that the pencil (stick, brush, whatever) is right-angled to your arm.
Especially the last case is often made wrong. If you are measuring e.g. the edge of a table, “running away” from you, automatically you try to hold the pencil parallel to the edge. You have to concentrate to the pencil, being aware to the pose.
Holding the pencil in the right pose you measure the length in the motive by marking the end with your thumb. Now you can compare this length with other parts of the motive, holding the pencil in the same way.
The frog is measured, better, the height of the frog. One eye of the drawer is closed. By stretching the arm is guarantied that the pencil (stick, brush) always has the same distance to the eye. The right angle guaranties, that the pencil is always in the same plane, a plane right-angled to the axis of the eye.
With the thumb the seen height of the frog is marked on the pencil. Now you can compare this height e.g. with its length or any other lline in the motive, you are interested in.
The right-angled pencil is important. The reason is, that there is no perspective foreshortening of lines lying in planes being right-angled to the axis of the eye. That is a perspective law. When posing the pencil right-angled, it is lying in such a plane. If the pencil is hold in an different way, not only the line you are measuring, but the pencil itself is foreshortened. That’s no problem if you hold the pencil always in the same angle when comparing lines. But if you are measuring e.g. edges of a table you will instinctively try to hold the pencil parallel to this edges, and than you are changing the angle between arm and pencil. As the effect to the foreshortening of the pencil itself is strong, your measures will be failing. The way to pose the pencil in the photo guaranties nearly automatically, that the pencil is hold in the right way.