Sighting is measuring with your pencil. It lets you compare sizes on an object across the room, so you can find its proportions.
- Hold the pencil at arm's length, with your elbow straight.
- Close one eye.
- Hold the pencil upright for heights or level for widths, flat to your face (not tipped toward or away from you).
- Line up the tip with one end of a part, and slide your thumb to the other end. That length is your unit.
- Without changing anything, move the pencil to another part and count how many units long it is.
Key idea
Keep the elbow straight and one eye closed, mark a unit with your thumb, and compare other lengths with it. Use the counts, not the sizes, on your paper.
Why it works
A sighting only works if the pencil is the same distance from your eye every time. A straight elbow is an easy way to make sure it is. Closing one eye keeps you from seeing two pencils. The numbers you get are ratios, so you can draw at any size: if the cup is 2 units tall and 3 wide on the pencil, it is 2 by 3 on your paper too, in any unit you like.
Worked example: A window
Sighting a window, you find its width is 2 thumb-lengths and its height is 3 of the same lengths. You draw the window 10 cm wide. How tall should it be?
- Height : width = 3 : 2.
- The drawing is 10 cm wide, so one unit is $10 \div 2 = 5$ cm.
- Height = $3 \times 5 = 15$ cm.
Answer: 15 cm.
Common mistake
People bend the elbow between measurements, which moves the pencil closer or farther and changes the scale, so every comparison is wrong. Lock your elbow every time, as if your arm were a stick.