On the Oceana Monocular (#71212), the vertical and horizontal scales on the reticle are graduated in mils (short for milliradians). One mil is a thousandth of the angular measure known as a radian. A degree is equal to approximately 18 mils and it takes about 57.3 degrees to equal one radian.
To measure a distance to an object of known size, first use either scale as appropriate and measure the angle in mils. The vertical scale is marked in numbered intervals of 10 mils for each interval up to a mark of 7, which is 70 mils. Similarly, the horizontal scale is marked to 40 mils to both the left and right.
You should be able to estimate between the marks to the closest mil.
Now multiply the known size of the object by 1000 and then divide by the mil angular measurement to get the distance.
For example, a sailboat is known to have a mast 60 feet high. It is observed out to sea with the Oceana monocular and has a vertical mils angle of 3. What is the distance? 60 x 1000 divided by 3 = 60,000/3 equals 20,000 feet or about 3.8 miles.
The same reasoning can be used to get the size of a boat or other object if you know the distance to it. The size in this case will be the mil angular measurement divided by 1000 then multiplied by the distance.
For example, a whale is seen on the surface near a lighthouse that is known to be 2500 feet away. The whale’s length on the horizontal scale is 24 mils. How big is the whale? 24 divided by 1000 times 2500 = 0.024 times 2500 equals 60 feet.