Binoculars & Monoculars with a built-in compass also may have a ranging reticle..
In the Helios Oceanmaster the reticle is as shown except for the Lighthouse. The vertical scale from 0 to 7 is actually 0 to 70 miles. The horizontal is 0 to 40 miles each side.In the Century Seapro the vertical scale has the numbers 20 , 40 and 60 and the line goes from 0 to 70 miles. The horizontal scale goes from left to right, from 0 to 80 miles. To calculate distance from an object with a known height, such as a light house,
1000 x Object size in mtrs Distance = ------------------------ Reticle Scale ReadingExample: When the Lighthouse is 20 m high and the reticle scale reads 16, as in the drawing, Distance = 1000 x 20 divided by 16 = 1250 m or One and a quarter kilometres.
One nautical mile equals 1852 metres. If you have a Nautical Almanac on board, you may find it has a "Distance Off By Vertical Sextant Angle" page. As there are 17.78 Mils in One degree, the example above, where the object is 16 Mils high, equals 0.9 degrees so the Almanac Page can be used to find the distance off. Your Almanac should explain how to do that.Why go to all this bother? With a compass reading and a distance off, you can quite quickly (yes, I am being serious) determine your position on your chart. Worth practicing it at anchor as one day you might need it for real.NOTE If you have not used a binocular before with a reticle, you will find that the reticle is only vertical when you hold the side with the reticle, horizontal. When you adjust the centre hinge to suit your IPD (the distance between your pupils), the reticle will tilt. You have to hold the binocular so that the reticle is vertical. It is not a fault in the binocular.